C# String 类 - 润新知

Constructors

	Name	Description
	String(Char*)	Initializes a new instance of the String class to the value indicated by a specified pointer to an array of Unicode characters.
	String(Char[])	Initializes a new instance of the String class to the value indicated by an array of Unicode characters.
	String(SByte*)	Initializes a new instance of the String class to the value indicated by a pointer to an array of 8-bit signed integers.
	String(Char, Int32)	Initializes a new instance of the String class to the value indicated by a specified Unicode character repeated a specified number of times.
	String(Char*, Int32, Int32)	Initializes a new instance of the String class to the value indicated by a specified pointer to an array of Unicode characters, a starting character position within that array, and a length.
	String(Char[], Int32, Int32)	Initializes a new instance of the String class to the value indicated by an array of Unicode characters, a starting character position within that array, and a length.
	String(SByte*, Int32, Int32)	Initializes a new instance of the String class to the value indicated by a specified pointer to an array of 8-bit signed integers, a starting position within that array, and a length.
	String(SByte*, Int32, Int32, Encoding)	Initializes a new instance of the String class to the value indicated by a specified pointer to an array of 8-bit signed integers, a starting position within that array, a length, and an Encoding object.

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Properties

	Name	Description
	Chars	Gets the Char object at a specified position in the current String object.
	Length	Gets the number of characters in the current String object.

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Methods

	Name	Description
	Clone	Returns a reference to this instance of String.
	Compare(String, String)	Compares two specified String objects and returns an integer that indicates their relative position in the sort order.
	Compare(String, String, Boolean)	Compares two specified String objects, ignoring or honoring their case, and returns an integer that indicates their relative position in the sort order.
	Compare(String, String, StringComparison)	Compares two specified String objects using the specified rules, and returns an integer that indicates their relative position in the sort order.
	Compare(String, String, Boolean, CultureInfo)	Compares two specified String objects, ignoring or honoring their case, and using culture-specific information to influence the comparison, and returns an integer that indicates their relative position in the sort order.
	Compare(String, String, CultureInfo, CompareOptions)	Compares two specified String objects using the specified comparison options and culture-specific information to influence the comparison, and returns an integer that indicates the relationship of the two strings to each other in the sort order.
	Compare(String, Int32, String, Int32, Int32)	Compares substrings of two specified String objects and returns an integer that indicates their relative position in the sort order.
	Compare(String, Int32, String, Int32, Int32, Boolean)	Compares substrings of two specified String objects, ignoring or honoring their case, and returns an integer that indicates their relative position in the sort order.
	Compare(String, Int32, String, Int32, Int32, StringComparison)	Compares substrings of two specified String objects using the specified rules, and returns an integer that indicates their relative position in the sort order.
	Compare(String, Int32, String, Int32, Int32, Boolean, CultureInfo)	Compares substrings of two specified String objects, ignoring or honoring their case and using culture-specific information to influence the comparison, and returns an integer that indicates their relative position in the sort order.
	Compare(String, Int32, String, Int32, Int32, CultureInfo, CompareOptions)	Compares substrings of two specified String objects using the specified comparison options and culture-specific information to influence the comparison, and returns an integer that indicates the relationship of the two substrings to each other in the sort order.
	CompareOrdinal(String, String)	Compares two specified String objects by evaluating the numeric values of the corresponding Char objects in each string.
	CompareOrdinal(String, Int32, String, Int32, Int32)	Compares substrings of two specified String objects by evaluating the numeric values of the corresponding Char objects in each substring.
	CompareTo(Object)	Compares this instance with a specified Object and indicates whether this instance precedes, follows, or appears in the same position in the sort order as the specified Object.
	CompareTo(String)	Compares this instance with a specified String object and indicates whether this instance precedes, follows, or appears in the same position in the sort order as the specified String.
	Concat(Object)	Creates the string representation of a specified object.
	Concat(Object[])	Concatenates the string representations of the elements in a specified Object array.
	Concat(IEnumerable<String>)	Concatenates the members of a constructed IEnumerable<T> collection of type String.
	Concat(String[])	Concatenates the elements of a specified String array.
	Concat(Object, Object)	Concatenates the string representations of two specified objects.
	Concat(String, String)	Concatenates two specified instances of String.
	Concat(Object, Object, Object)	Concatenates the string representations of three specified objects.
	Concat(String, String, String)	Concatenates three specified instances of String.
	Concat(Object, Object, Object, Object)	Concatenates the string representations of four specified objects and any objects specified in an optional variable length parameter list.
	Concat(String, String, String, String)	Concatenates four specified instances of String.
	Concat<T>(IEnumerable<T>)	Concatenates the members of an IEnumerable<T> implementation.
	Contains	Returns a value indicating whether the specified String object occurs within this string.
	Copy	Creates a new instance of String with the same value as a specified String.
	CopyTo	Copies a specified number of characters from a specified position in this instance to a specified position in an array of Unicode characters.
	EndsWith(String)	Determines whether the end of this string instance matches the specified string.
	EndsWith(String, StringComparison)	Determines whether the end of this string instance matches the specified string when compared using the specified comparison option.
	EndsWith(String, Boolean, CultureInfo)	Determines whether the end of this string instance matches the specified string when compared using the specified culture.
	Equals(Object)	Determines whether this instance and a specified object, which must also be a String object, have the same value. (Overrides Object.Equals(Object).)
	Equals(String)	Determines whether this instance and another specified String object have the same value.
	Equals(String, String)	Determines whether two specified String objects have the same value.
	Equals(String, StringComparison)	Determines whether this string and a specified String object have the same value. A parameter specifies the culture, case, and sort rules used in the comparison.
	Equals(String, String, StringComparison)	Determines whether two specified String objects have the same value. A parameter specifies the culture, case, and sort rules used in the comparison.
	Format(String, Object)	Replaces one or more format items in a specified string with the string representation of a specified object.
	Format(String, Object[])	Replaces the format item in a specified string with the string representation of a corresponding object in a specified array.
	Format(IFormatProvider, String, Object[])	Replaces the format items in a specified string with the string representations of corresponding objects in a specified array. A parameter supplies culture-specific formatting information.
	Format(String, Object, Object)	Replaces the format items in a specified string with the string representation of two specified objects.
	Format(String, Object, Object, Object)	Replaces the format items in a specified string with the string representation of three specified objects.
	GetEnumerator	Retrieves an object that can iterate through the individual characters in this string.
	GetHashCode	Returns the hash code for this string. (Overrides Object.GetHashCode().)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	GetTypeCode	Returns the TypeCode for class String.
	IndexOf(Char)	Reports the zero-based index of the first occurrence of the specified Unicode character in this string.
	IndexOf(String)	Reports the zero-based index of the first occurrence of the specified string in this instance.
	IndexOf(Char, Int32)	Reports the zero-based index of the first occurrence of the specified Unicode character in this string. The search starts at a specified character position.
	IndexOf(String, Int32)	Reports the zero-based index of the first occurrence of the specified string in this instance. The search starts at a specified character position.
	IndexOf(String, StringComparison)	Reports the zero-based index of the first occurrence of the specified string in the current String object. A parameter specifies the type of search to use for the specified string.
	IndexOf(Char, Int32, Int32)	Reports the zero-based index of the first occurrence of the specified character in this instance. The search starts at a specified character position and examines a specified number of character positions.
	IndexOf(String, Int32, Int32)	Reports the zero-based index of the first occurrence of the specified string in this instance. The search starts at a specified character position and examines a specified number of character positions.
	IndexOf(String, Int32, StringComparison)	Reports the zero-based index of the first occurrence of the specified string in the current String object. Parameters specify the starting search position in the current string and the type of search to use for the specified string.
	IndexOf(String, Int32, Int32, StringComparison)	Reports the zero-based index of the first occurrence of the specified string in the current String object. Parameters specify the starting search position in the current string, the number of characters in the current string to search, and the type of search to use for the specified string.
	IndexOfAny(Char[])	Reports the zero-based index of the first occurrence in this instance of any character in a specified array of Unicode characters.
	IndexOfAny(Char[], Int32)	Reports the zero-based index of the first occurrence in this instance of any character in a specified array of Unicode characters. The search starts at a specified character position.
	IndexOfAny(Char[], Int32, Int32)	Reports the zero-based index of the first occurrence in this instance of any character in a specified array of Unicode characters. The search starts at a specified character position and examines a specified number of character positions.
	Insert	Returns a new string in which a specified string is inserted at a specified index position in this instance.
	Intern	Retrieves the system's reference to the specified String.
	IsInterned	Retrieves a reference to a specified String.
	IsNormalized()	Indicates whether this string is in Unicode normalization form C.
	IsNormalized(NormalizationForm)	Indicates whether this string is in the specified Unicode normalization form.
	IsNullOrEmpty	Indicates whether the specified string is null or an Empty string.
	IsNullOrWhiteSpace	Indicates whether a specified string is null, empty, or consists only of white-space characters.
	Join(String, IEnumerable<String>)	Concatenates the members of a constructed IEnumerable<T> collection of type String, using the specified separator between each member.
	Join(String, Object[])	Concatenates the elements of an object array, using the specified separator between each element.
	Join(String, String[])	Concatenates all the elements of a string array, using the specified separator between each element.
	Join(String, String[], Int32, Int32)	Concatenates the specified elements of a string array, using the specified separator between each element.
	Join<T>(String, IEnumerable<T>)	Concatenates the members of a collection, using the specified separator between each member.
	LastIndexOf(Char)	Reports the zero-based index position of the last occurrence of a specified Unicode character within this instance.
	LastIndexOf(String)	Reports the zero-based index position of the last occurrence of a specified string within this instance.
	LastIndexOf(Char, Int32)	Reports the zero-based index position of the last occurrence of a specified Unicode character within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string.
	LastIndexOf(String, Int32)	Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string.
	LastIndexOf(String, StringComparison)	Reports the zero-based index of the last occurrence of a specified string within the current String object. A parameter specifies the type of search to use for the specified string.
	LastIndexOf(Char, Int32, Int32)	Reports the zero-based index position of the last occurrence of the specified Unicode character in a substring within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.
	LastIndexOf(String, Int32, Int32)	Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.
	LastIndexOf(String, Int32, StringComparison)	Reports the zero-based index of the last occurrence of a specified string within the current String object. The search starts at a specified character position and proceeds backward toward the beginning of the string. A parameter specifies the type of comparison to perform when searching for the specified string.
	LastIndexOf(String, Int32, Int32, StringComparison)	Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for the specified number of character positions. A parameter specifies the type of comparison to perform when searching for the specified string.
	LastIndexOfAny(Char[])	Reports the zero-based index position of the last occurrence in this instance of one or more characters specified in a Unicode array.
	LastIndexOfAny(Char[], Int32)	Reports the zero-based index position of the last occurrence in this instance of one or more characters specified in a Unicode array. The search starts at a specified character position and proceeds backward toward the beginning of the string.
	LastIndexOfAny(Char[], Int32, Int32)	Reports the zero-based index position of the last occurrence in this instance of one or more characters specified in a Unicode array. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.
	Normalize()	Returns a new string whose textual value is the same as this string, but whose binary representation is in Unicode normalization form C.
	Normalize(NormalizationForm)	Returns a new string whose textual value is the same as this string, but whose binary representation is in the specified Unicode normalization form.
	PadLeft(Int32)	Returns a new string that right-aligns the characters in this instance by padding them with spaces on the left, for a specified total length.
	PadLeft(Int32, Char)	Returns a new string that right-aligns the characters in this instance by padding them on the left with a specified Unicode character, for a specified total length.
	PadRight(Int32)	Returns a new string that left-aligns the characters in this string by padding them with spaces on the right, for a specified total length.
	PadRight(Int32, Char)	Returns a new string that left-aligns the characters in this string by padding them on the right with a specified Unicode character, for a specified total length.
	Remove(Int32)	Returns a new string in which all the characters in the current instance, beginning at a specified position and continuing through the last position, have been deleted.
	Remove(Int32, Int32)	Returns a new string in which a specified number of characters in the current this instance beginning at a specified position have been deleted.
	Replace(Char, Char)	Returns a new string in which all occurrences of a specified Unicode character in this instance are replaced with another specified Unicode character.
	Replace(String, String)	Returns a new string in which all occurrences of a specified string in the current instance are replaced with another specified string.
	Split(Char[])	Returns a string array that contains the substrings in this instance that are delimited by elements of a specified Unicode character array.
	Split(Char[], Int32)	Returns a string array that contains the substrings in this instance that are delimited by elements of a specified Unicode character array. A parameter specifies the maximum number of substrings to return.
	Split(Char[], StringSplitOptions)	Returns a string array that contains the substrings in this string that are delimited by elements of a specified Unicode character array. A parameter specifies whether to return empty array elements.
	Split(String[], StringSplitOptions)	Returns a string array that contains the substrings in this string that are delimited by elements of a specified string array. A parameter specifies whether to return empty array elements.
	Split(Char[], Int32, StringSplitOptions)	Returns a string array that contains the substrings in this string that are delimited by elements of a specified Unicode character array. Parameters specify the maximum number of substrings to return and whether to return empty array elements.
	Split(String[], Int32, StringSplitOptions)	Returns a string array that contains the substrings in this string that are delimited by elements of a specified string array. Parameters specify the maximum number of substrings to return and whether to return empty array elements.
	StartsWith(String)	Determines whether the beginning of this string instance matches the specified string.
	StartsWith(String, StringComparison)	Determines whether the beginning of this string instance matches the specified string when compared using the specified comparison option.
	StartsWith(String, Boolean, CultureInfo)	Determines whether the beginning of this string instance matches the specified string when compared using the specified culture.
	Substring(Int32)	Retrieves a substring from this instance. The substring starts at a specified character position.
	Substring(Int32, Int32)	Retrieves a substring from this instance. The substring starts at a specified character position and has a specified length.
	ToCharArray()	Copies the characters in this instance to a Unicode character array.
	ToCharArray(Int32, Int32)	Copies the characters in a specified substring in this instance to a Unicode character array.
	ToLower()	Returns a copy of this string converted to lowercase.
	ToLower(CultureInfo)	Returns a copy of this string converted to lowercase, using the casing rules of the specified culture.
	ToLowerInvariant	Returns a copy of this String object converted to lowercase using the casing rules of the invariant culture.
	ToString()	Returns this instance of String; no actual conversion is performed. (Overrides Object.ToString().)
	ToString(IFormatProvider)	Returns this instance of String; no actual conversion is performed.
	ToUpper()	Returns a copy of this string converted to uppercase.
	ToUpper(CultureInfo)	Returns a copy of this string converted to uppercase, using the casing rules of the specified culture.
	ToUpperInvariant	Returns a copy of this String object converted to uppercase using the casing rules of the invariant culture.
	Trim()	Removes all leading and trailing white-space characters from the current String object.
	Trim(Char[])	Removes all leading and trailing occurrences of a set of characters specified in an array from the current String object.
	TrimEnd	Removes all trailing occurrences of a set of characters specified in an array from the current String object.
	TrimStart	Removes all leading occurrences of a set of characters specified in an array from the current String object.

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Operators

	Name	Description
	Equality	Determines whether two specified strings have the same value.
	Inequality	Determines whether two specified strings have different values.

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Extension Methods

	Name	Description
	Aggregate<Char>(Func<Char, Char, Char>)	Overloaded. Applies an accumulator function over a sequence. (Defined by Enumerable.)
	Aggregate<Char, TAccumulate>(TAccumulate, Func<TAccumulate, Char, TAccumulate>)	Overloaded. Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value. (Defined by Enumerable.)
	Aggregate<Char, TAccumulate, TResult>(TAccumulate, Func<TAccumulate, Char, TAccumulate>, Func<TAccumulate, TResult>)	Overloaded. Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value, and the specified function is used to select the result value. (Defined by Enumerable.)
	All<Char>	Determines whether all elements of a sequence satisfy a condition. (Defined by Enumerable.)
	Any<Char>()	Overloaded. Determines whether a sequence contains any elements. (Defined by Enumerable.)
	Any<Char>(Func<Char, Boolean>)	Overloaded. Determines whether any element of a sequence satisfies a condition. (Defined by Enumerable.)
	AsEnumerable<Char>	Returns the input typed as IEnumerable<T>. (Defined by Enumerable.)
	AsParallel()	Overloaded. Enables parallelization of a query. (Defined by ParallelEnumerable.)
	AsParallel<Char>()	Overloaded. Enables parallelization of a query. (Defined by ParallelEnumerable.)
	AsQueryable()	Overloaded. Converts an IEnumerable to an IQueryable. (Defined by Queryable.)
	AsQueryable<Char>()	Overloaded. Converts a generic IEnumerable<T> to a generic IQueryable<T>. (Defined by Queryable.)
	Average<Char>(Func<Char, Int32>)	Overloaded. Computes the average of a sequence of Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Nullable<Int32>>)	Overloaded. Computes the average of a sequence of nullable Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Int64>)	Overloaded. Computes the average of a sequence of Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Nullable<Int64>>)	Overloaded. Computes the average of a sequence of nullable Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Single>)	Overloaded. Computes the average of a sequence of Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Nullable<Single>>)	Overloaded. Computes the average of a sequence of nullable Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Double>)	Overloaded. Computes the average of a sequence of Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Nullable<Double>>)	Overloaded. Computes the average of a sequence of nullable Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Decimal>)	Overloaded. Computes the average of a sequence of Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<Char>(Func<Char, Nullable<Decimal>>)	Overloaded. Computes the average of a sequence of nullable Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Cast<TResult>	Casts the elements of an IEnumerable to the specified type. (Defined by Enumerable.)
	Concat<Char>	Concatenates two sequences. (Defined by Enumerable.)
	Contains<Char>(Char)	Overloaded. Determines whether a sequence contains a specified element by using the default equality comparer. (Defined by Enumerable.)
	Contains<Char>(Char, IEqualityComparer<Char>)	Overloaded. Determines whether a sequence contains a specified element by using a specified IEqualityComparer<T>. (Defined by Enumerable.)
	Count<Char>()	Overloaded. Returns the number of elements in a sequence. (Defined by Enumerable.)
	Count<Char>(Func<Char, Boolean>)	Overloaded. Returns a number that represents how many elements in the specified sequence satisfy a condition. (Defined by Enumerable.)
	DefaultIfEmpty<Char>()	Overloaded. Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty. (Defined by Enumerable.)
	DefaultIfEmpty<Char>(Char)	Overloaded. Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty. (Defined by Enumerable.)
	Distinct<Char>()	Overloaded. Returns distinct elements from a sequence by using the default equality comparer to compare values. (Defined by Enumerable.)
	Distinct<Char>(IEqualityComparer<Char>)	Overloaded. Returns distinct elements from a sequence by using a specified IEqualityComparer<T> to compare values. (Defined by Enumerable.)
	ElementAt<Char>	Returns the element at a specified index in a sequence. (Defined by Enumerable.)
	ElementAtOrDefault<Char>	Returns the element at a specified index in a sequence or a default value if the index is out of range. (Defined by Enumerable.)
	Except<Char>(IEnumerable<Char>)	Overloaded. Produces the set difference of two sequences by using the default equality comparer to compare values. (Defined by Enumerable.)
	Except<Char>(IEnumerable<Char>, IEqualityComparer<Char>)	Overloaded. Produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values. (Defined by Enumerable.)
	First<Char>()	Overloaded. Returns the first element of a sequence. (Defined by Enumerable.)
	First<Char>(Func<Char, Boolean>)	Overloaded. Returns the first element in a sequence that satisfies a specified condition. (Defined by Enumerable.)
	FirstOrDefault<Char>()	Overloaded. Returns the first element of a sequence, or a default value if the sequence contains no elements. (Defined by Enumerable.)
	FirstOrDefault<Char>(Func<Char, Boolean>)	Overloaded. Returns the first element of the sequence that satisfies a condition or a default value if no such element is found. (Defined by Enumerable.)
	GroupBy<Char, TKey>(Func<Char, TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function. (Defined by Enumerable.)
	GroupBy<Char, TKey>(Func<Char, TKey>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer. (Defined by Enumerable.)
	GroupBy<Char, TKey, TElement>(Func<Char, TKey>, Func<Char, TElement>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function. (Defined by Enumerable.)
	GroupBy<Char, TKey, TResult>(Func<Char, TKey>, Func<TKey, IEnumerable<Char>, TResult>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. (Defined by Enumerable.)
	GroupBy<Char, TKey, TElement>(Func<Char, TKey>, Func<Char, TElement>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a key selector function. The keys are compared by using a comparer and each group's elements are projected by using a specified function. (Defined by Enumerable.)
	GroupBy<Char, TKey, TResult>(Func<Char, TKey>, Func<TKey, IEnumerable<Char>, TResult>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The keys are compared by using a specified comparer. (Defined by Enumerable.)
	GroupBy<Char, TKey, TElement, TResult>(Func<Char, TKey>, Func<Char, TElement>, Func<TKey, IEnumerable<TElement>, TResult>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function. (Defined by Enumerable.)
	GroupBy<Char, TKey, TElement, TResult>(Func<Char, TKey>, Func<Char, TElement>, Func<TKey, IEnumerable<TElement>, TResult>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function. (Defined by Enumerable.)
	GroupJoin<Char, TInner, TKey, TResult>(IEnumerable<TInner>, Func<Char, TKey>, Func<TInner, TKey>, Func<Char, IEnumerable<TInner>, TResult>)	Overloaded. Correlates the elements of two sequences based on equality of keys and groups the results. The default equality comparer is used to compare keys. (Defined by Enumerable.)
	GroupJoin<Char, TInner, TKey, TResult>(IEnumerable<TInner>, Func<Char, TKey>, Func<TInner, TKey>, Func<Char, IEnumerable<TInner>, TResult>, IEqualityComparer<TKey>)	Overloaded. Correlates the elements of two sequences based on key equality and groups the results. A specified IEqualityComparer<T> is used to compare keys. (Defined by Enumerable.)
	Intersect<Char>(IEnumerable<Char>)	Overloaded. Produces the set intersection of two sequences by using the default equality comparer to compare values. (Defined by Enumerable.)
	Intersect<Char>(IEnumerable<Char>, IEqualityComparer<Char>)	Overloaded. Produces the set intersection of two sequences by using the specified IEqualityComparer<T> to compare values. (Defined by Enumerable.)
	Join<Char, TInner, TKey, TResult>(IEnumerable<TInner>, Func<Char, TKey>, Func<TInner, TKey>, Func<Char, TInner, TResult>)	Overloaded. Correlates the elements of two sequences based on matching keys. The default equality comparer is used to compare keys. (Defined by Enumerable.)
	Join<Char, TInner, TKey, TResult>(IEnumerable<TInner>, Func<Char, TKey>, Func<TInner, TKey>, Func<Char, TInner, TResult>, IEqualityComparer<TKey>)	Overloaded. Correlates the elements of two sequences based on matching keys. A specified IEqualityComparer<T> is used to compare keys. (Defined by Enumerable.)
	Last<Char>()	Overloaded. Returns the last element of a sequence. (Defined by Enumerable.)
	Last<Char>(Func<Char, Boolean>)	Overloaded. Returns the last element of a sequence that satisfies a specified condition. (Defined by Enumerable.)
	LastOrDefault<Char>()	Overloaded. Returns the last element of a sequence, or a default value if the sequence contains no elements. (Defined by Enumerable.)
	LastOrDefault<Char>(Func<Char, Boolean>)	Overloaded. Returns the last element of a sequence that satisfies a condition or a default value if no such element is found. (Defined by Enumerable.)
	LongCount<Char>()	Overloaded. Returns an Int64 that represents the total number of elements in a sequence. (Defined by Enumerable.)
	LongCount<Char>(Func<Char, Boolean>)	Overloaded. Returns an Int64 that represents how many elements in a sequence satisfy a condition. (Defined by Enumerable.)
	Max<Char>()	Overloaded. Returns the maximum value in a generic sequence. (Defined by Enumerable.)
	Max<Char>(Func<Char, Int32>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Int32 value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Nullable<Int32>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Int32 value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Int64>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Int64 value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Nullable<Int64>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Int64 value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Single>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Single value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Nullable<Single>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Single value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Double>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Double value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Nullable<Double>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Double value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Decimal>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Decimal value. (Defined by Enumerable.)
	Max<Char>(Func<Char, Nullable<Decimal>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Decimal value. (Defined by Enumerable.)
	Max<Char, TResult>(Func<Char, TResult>)	Overloaded. Invokes a transform function on each element of a generic sequence and returns the maximum resulting value. (Defined by Enumerable.)
	Min<Char>()	Overloaded. Returns the minimum value in a generic sequence. (Defined by Enumerable.)
	Min<Char>(Func<Char, Int32>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Int32 value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Nullable<Int32>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Int32 value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Int64>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Int64 value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Nullable<Int64>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Int64 value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Single>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Single value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Nullable<Single>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Single value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Double>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Double value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Nullable<Double>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Double value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Decimal>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Decimal value. (Defined by Enumerable.)
	Min<Char>(Func<Char, Nullable<Decimal>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Decimal value. (Defined by Enumerable.)
	Min<Char, TResult>(Func<Char, TResult>)	Overloaded. Invokes a transform function on each element of a generic sequence and returns the minimum resulting value. (Defined by Enumerable.)
	OfType<TResult>	Filters the elements of an IEnumerable based on a specified type. (Defined by Enumerable.)
	OrderBy<Char, TKey>(Func<Char, TKey>)	Overloaded. Sorts the elements of a sequence in ascending order according to a key. (Defined by Enumerable.)
	OrderBy<Char, TKey>(Func<Char, TKey>, IComparer<TKey>)	Overloaded. Sorts the elements of a sequence in ascending order by using a specified comparer. (Defined by Enumerable.)
	OrderByDescending<Char, TKey>(Func<Char, TKey>)	Overloaded. Sorts the elements of a sequence in descending order according to a key. (Defined by Enumerable.)
	OrderByDescending<Char, TKey>(Func<Char, TKey>, IComparer<TKey>)	Overloaded. Sorts the elements of a sequence in descending order by using a specified comparer. (Defined by Enumerable.)
	Reverse<Char>	Inverts the order of the elements in a sequence. (Defined by Enumerable.)
	Select<Char, TResult>(Func<Char, TResult>)	Overloaded. Projects each element of a sequence into a new form. (Defined by Enumerable.)
	Select<Char, TResult>(Func<Char, Int32, TResult>)	Overloaded. Projects each element of a sequence into a new form by incorporating the element's index. (Defined by Enumerable.)
	SelectMany<Char, TResult>(Func<Char, IEnumerable<TResult>>)	Overloaded. Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence. (Defined by Enumerable.)
	SelectMany<Char, TResult>(Func<Char, Int32, IEnumerable<TResult>>)	Overloaded. Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. The index of each source element is used in the projected form of that element. (Defined by Enumerable.)
	SelectMany<Char, TCollection, TResult>(Func<Char, Int32, IEnumerable<TCollection>>, Func<Char, TCollection, TResult>)	Overloaded. Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein. The index of each source element is used in the intermediate projected form of that element. (Defined by Enumerable.)
	SelectMany<Char, TCollection, TResult>(Func<Char, IEnumerable<TCollection>>, Func<Char, TCollection, TResult>)	Overloaded. Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein. (Defined by Enumerable.)
	SequenceEqual<Char>(IEnumerable<Char>)	Overloaded. Determines whether two sequences are equal by comparing the elements by using the default equality comparer for their type. (Defined by Enumerable.)
	SequenceEqual<Char>(IEnumerable<Char>, IEqualityComparer<Char>)	Overloaded. Determines whether two sequences are equal by comparing their elements by using a specified IEqualityComparer<T>. (Defined by Enumerable.)
	Single<Char>()	Overloaded. Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence. (Defined by Enumerable.)
	Single<Char>(Func<Char, Boolean>)	Overloaded. Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists. (Defined by Enumerable.)
	SingleOrDefault<Char>()	Overloaded. Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence. (Defined by Enumerable.)
	SingleOrDefault<Char>(Func<Char, Boolean>)	Overloaded. Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition. (Defined by Enumerable.)
	Skip<Char>	Bypasses a specified number of elements in a sequence and then returns the remaining elements. (Defined by Enumerable.)
	SkipWhile<Char>(Func<Char, Boolean>)	Overloaded. Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements. (Defined by Enumerable.)
	SkipWhile<Char>(Func<Char, Int32, Boolean>)	Overloaded. Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements. The element's index is used in the logic of the predicate function. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Int32>)	Overloaded. Computes the sum of the sequence of Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Nullable<Int32>>)	Overloaded. Computes the sum of the sequence of nullable Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Int64>)	Overloaded. Computes the sum of the sequence of Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Nullable<Int64>>)	Overloaded. Computes the sum of the sequence of nullable Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Single>)	Overloaded. Computes the sum of the sequence of Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Nullable<Single>>)	Overloaded. Computes the sum of the sequence of nullable Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Double>)	Overloaded. Computes the sum of the sequence of Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Nullable<Double>>)	Overloaded. Computes the sum of the sequence of nullable Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Decimal>)	Overloaded. Computes the sum of the sequence of Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<Char>(Func<Char, Nullable<Decimal>>)	Overloaded. Computes the sum of the sequence of nullable Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Take<Char>	Returns a specified number of contiguous elements from the start of a sequence. (Defined by Enumerable.)
	TakeWhile<Char>(Func<Char, Boolean>)	Overloaded. Returns elements from a sequence as long as a specified condition is true. (Defined by Enumerable.)
	TakeWhile<Char>(Func<Char, Int32, Boolean>)	Overloaded. Returns elements from a sequence as long as a specified condition is true. The element's index is used in the logic of the predicate function. (Defined by Enumerable.)
	ToArray<Char>	Creates an array from a IEnumerable<T>. (Defined by Enumerable.)
	ToDictionary<Char, TKey>(Func<Char, TKey>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function. (Defined by Enumerable.)
	ToDictionary<Char, TKey>(Func<Char, TKey>, IEqualityComparer<TKey>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function and key comparer. (Defined by Enumerable.)
	ToDictionary<Char, TKey, TElement>(Func<Char, TKey>, Func<Char, TElement>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to specified key selector and element selector functions. (Defined by Enumerable.)
	ToDictionary<Char, TKey, TElement>(Func<Char, TKey>, Func<Char, TElement>, IEqualityComparer<TKey>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function, a comparer, and an element selector function. (Defined by Enumerable.)
	ToList<Char>	Creates a List<T> from an IEnumerable<T>. (Defined by Enumerable.)
	ToLookup<Char, TKey>(Func<Char, TKey>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function. (Defined by Enumerable.)
	ToLookup<Char, TKey>(Func<Char, TKey>, IEqualityComparer<TKey>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function and key comparer. (Defined by Enumerable.)
	ToLookup<Char, TKey, TElement>(Func<Char, TKey>, Func<Char, TElement>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to specified key selector and element selector functions. (Defined by Enumerable.)
	ToLookup<Char, TKey, TElement>(Func<Char, TKey>, Func<Char, TElement>, IEqualityComparer<TKey>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function, a comparer and an element selector function. (Defined by Enumerable.)
	Union<Char>(IEnumerable<Char>)	Overloaded. Produces the set union of two sequences by using the default equality comparer. (Defined by Enumerable.)
	Union<Char>(IEnumerable<Char>, IEqualityComparer<Char>)	Overloaded. Produces the set union of two sequences by using a specified IEqualityComparer<T>. (Defined by Enumerable.)
	Where<Char>(Func<Char, Boolean>)	Overloaded. Filters a sequence of values based on a predicate. (Defined by Enumerable.)
	Where<Char>(Func<Char, Int32, Boolean>)	Overloaded. Filters a sequence of values based on a predicate. Each element's index is used in the logic of the predicate function. (Defined by Enumerable.)
	Zip<Char, TSecond, TResult>	Merges two sequences by using the specified predicate function. (Defined by Enumerable.)

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Fields

	Name	Description
	Empty	Represents the empty string. This field is read-only.

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Explicit Interface Implementations

	Name	Description
	IComparable.CompareTo	Compares the current instance with another object of the same type and returns an integer that indicates whether the current instance precedes, follows, or occurs in the same position in the sort order as the other object.
	IConvertible.ToBoolean	Infrastructure. For a description of this member, see ToBoolean.
	IConvertible.ToByte	Infrastructure. For a description of this member, see ToByte.
	IConvertible.ToChar	Infrastructure. For a description of this member, see ToChar.
	IConvertible.ToDateTime	Infrastructure. For a description of this member, see ToDateTime.
	IConvertible.ToDecimal	Infrastructure. For a description of this member, see ToDecimal.
	IConvertible.ToDouble	Infrastructure. For a description of this member, see ToDouble.
	IConvertible.ToInt16	Infrastructure. For a description of this member, see ToInt16.
	IConvertible.ToInt32	Infrastructure. For a description of this member, see ToInt32.
	IConvertible.ToInt64	Infrastructure. For a description of this member, see ToInt64.
	IConvertible.ToSByte	Infrastructure. For a description of this member, see ToSByte.
	IConvertible.ToSingle	Infrastructure. For a description of this member, see ToSingle.
	IConvertible.ToType	Infrastructure. For a description of this member, see ToType.
	IConvertible.ToUInt16	Infrastructure. For a description of this member, see ToUInt16.
	IConvertible.ToUInt32	Infrastructure. For a description of this member, see ToUInt32.
	IConvertible.ToUInt64	Infrastructure. For a description of this member, see ToUInt64.
	IEnumerable<Char>.GetEnumerator	Returns an enumerator that iterates through the current String object.
	IEnumerable.GetEnumerator	Returns an enumerator that iterates through the current String object.

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Remarks

A string is a sequential collection of Unicode characters that is used to represent text. A String object is a sequential collection of System.Char objects that represent a string. The value of the String object is the content of the sequential collection, and that value is immutable (that is, it is read-only). For more information about the immutability of strings, see the Immutability and the StringBuilder class section later in this topic. The maximum size of a String object in memory is 2 GB, or about 1 billion characters.

In this section:

Instantiating a String object Char objects and Unicode characters Strings and embedded null characters Strings and indexes Null strings and empty strings Immutability and the StringBuilder class Ordinal vs. culture-sensitive operations Normalization String operations by category

Instantiating a String object

You can instantiate a String object in the following ways:

By assigning a string literal to a String variable. This is the most commonly used method for creating a string. The following example uses assignment to create several strings. Note that in C#, because the backslash (\) is an escape character, literal backslashes in a string must be escaped or the entire string must be @-quoted.

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string string1 = "This is a string created by assignment.";
Console.WriteLine(string1);
string string2a = "The path is C:\\PublicDocuments\\Report1.doc";
Console.WriteLine(string2a);
string string2b = @"The path is C:\PublicDocuments\Report1.doc";
Console.WriteLine(string2b);
// The example displays the following output: 
//       This is a string created by assignment. 
//       The path is C:\PublicDocuments\Report1.doc 
//       The path is C:\PublicDocuments\Report1.doc

By calling a String class constructor. The following example instantiates strings by calling several class constructors. Note that some of the constructors include pointers to character arrays or signed byte arrays as parameters. Visual Basic does not support calls to these constructors. For detailed information about String constructors, see the String constructor summary.

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char[] chars = { 'w', 'o', 'r', 'd' };
sbyte[] bytes = { 0x41, 0x42, 0x43, 0x44, 0x45, 0x00 };

// Create a string from a character array. 
string string1 = new string(chars);
Console.WriteLine(string1);

// Create a string that consists of a character repeated 20 times. 
string string2 = new string('c', 20);
Console.WriteLine(string2);

string stringFromBytes = null;
string stringFromChars = null;
unsafe
{
   fixed (sbyte* pbytes = bytes)
   {
      // Create a string from a pointer to a signed byte array.
      stringFromBytes = new string(pbytes);
   }
   fixed (char* pchars = chars)
   {
      // Create a string from a pointer to a character array.
      stringFromChars = new string(pchars);
   }
}
Console.WriteLine(stringFromBytes);
Console.WriteLine(stringFromChars);
// The example displays the following output: 
//       word 
//       cccccccccccccccccccc 
//       ABCDE 
//       word

By using the string concatenation operator (+ in C# and & or + in Visual Basic) to create a single string from any combination of String instances and string literals. The following example illustrates the use of the string concatenation operator.

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string string1 = "Today is " + DateTime.Now.ToString("D") + ".";
Console.WriteLine(string1);

string string2 = "This is one sentence. " + "This is a second. ";
string2 += "This is a third sentence.";
Console.WriteLine(string2);
// The example displays output like the following: 
//    Today is Tuesday, July 06, 2011. 
//    This is one sentence. This is a second. This is a third sentence.

By retrieving a property or calling a method that returns a string. The following example uses the methods of the String class to extract a substring from a larger string.

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string sentence = "This sentence has five words.";
// Extract the second word. 
int startPosition = sentence.IndexOf(" ") + 1;
string word2 = sentence.Substring(startPosition,
                                  sentence.IndexOf(" ", startPosition) - startPosition);
Console.WriteLine("Second word: " + word2);
// The example displays the following output: 
//       Second word: sentence

By calling a formatting method to convert a value or object to its string representation. The following example uses the composite formatting feature to embed the string representation of two objects into a string.

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DateTime dateAndTime = new DateTime(2011, 7, 6, 7, 32, 0);
double temperature = 68.3;
string result = String.Format("At {0:t} on {0:D}, the temperature was {1:F1} degrees Fahrenheit.",
                              dateAndTime, temperature);
Console.WriteLine(result);
// The example displays the following output: 
//       At 7:32 AM on Wednesday, July 06, 2011, the temperature was 68.3 degrees Fahrenheit.

Char objects and Unicode characters

Each character in a string is defined by a Unicode scalar value, also called a Unicode code point or the ordinal (numeric) value of the Unicode character. Each code point is encoded by using UTF-16 encoding, and the numeric value of each element of the encoding is represented by a Char object.

A single Char object usually represents a single code point; that is, the numeric value of the Char equals the code point. For example, the code point for the character "a" is U+0061. However, a code point might require more than one encoded element (more than one Char object). The Unicode standard defines three types of characters that correspond to multiple Char objects: graphemes, Unicode supplementary code points, and characters in the supplementary planes.

A grapheme is represented by a base character followed by one or more combining characters. For example, the character ä is represented by a Char object whose code point is U+0061 followed by a Char object whose code point is U+0308. This character can also be defined by a single Char object that has a code point of U+00E4. As the following example shows, a culture-sensitive comparison for equality indicates that these two representations are equal, although an ordinary ordinal comparison does not. However, if the two strings are normalized, an ordinal comparison also indicates that they are equal. (For more information on normalizing strings, see the Normalization section.)

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using System;
using System.Globalization;
using System.IO;

public class Example
{
   public static void Main()
   {
      StreamWriter sw = new StreamWriter(@".\graphemes.txt");
      string grapheme = "\u0061\u0308";
      sw.WriteLine(grapheme);

      string singleChar = "\u00e4";
      sw.WriteLine(singleChar);

      sw.WriteLine("{0} = {1} (Culture-sensitive): {2}", grapheme, singleChar, 
                   String.Equals(grapheme, singleChar, 
                                 StringComparison.CurrentCulture));
      sw.WriteLine("{0} = {1} (Ordinal): {2}", grapheme, singleChar, 
                   String.Equals(grapheme, singleChar, 
                                 StringComparison.Ordinal));
      sw.WriteLine("{0} = {1} (Normalized Ordinal): {2}", grapheme, singleChar, 
                   String.Equals(grapheme.Normalize(), 
                                 singleChar.Normalize(), 
                                 StringComparison.Ordinal));
      sw.Close(); 
   }
}
// The example produces the following output: 
//       ä 
//       ä 
//       ä = ä (Culture-sensitive): True 
//       ä = ä (Ordinal): False 
//       ä = ä (Normalized Ordinal): True

A Unicode supplementary code point (a surrogate pair) is represented by a Char object whose code point is a high surrogate followed by a Char object whose code point is a low surrogate. The code points of high surrogates range from U+D800 to U+DBFF. The code points of low surrogates range from U+DC00 to U+DFFF. Surrogate pairs are used to represent characters in the 16 Unicode supplementary planes. The following example creates a surrogate character and passes it to the Char.IsSurrogatePair(Char, Char) method to determine whether it is a surrogate pair.
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using System; public class Example { public static void Main() { string surrogate = "\uD800\uDC03"; for (int ctr = 0; ctr < surrogate.Length; ctr++) Console.Write("U+{0:X2} ", Convert.ToUInt16(surrogate[ctr])); Console.WriteLine(); Console.WriteLine(" Is Surrogate Pair: {0}", Char.IsSurrogatePair(surrogate[0], surrogate[1])); } } // The example displays the following output: // U+D800 U+DC03 // Is Surrogate Pair: True

Strings and embedded null characters

In the .NET Framework, a String object can include embedded null characters, which count as a part of the string's length. However, in some languages such as C and C++, a null character indicates the end of a string; it is not considered a part of the string and is not counted as part of the string's length. This means that the following common assumptions that C and C++ programmers or libraries written in C or C++ might make about strings are not necessarily valid when applied to String objects:

The value returned by the strlen or wcslen functions does not necessarily equal String.Length.
The string created by the strcpy_s or wcscpy_s functions is not necessarily identical to the string created by the String.Copy method.

You should ensure that native C and C++ code that instantiates String objects, and code that is passed String objects through platform invoke, do not assume that an embedded null character marks the end of the string.

Embedded null characters in a string are also treated differently when a string is sorted (or compared) and when a string is searched. Null characters are ignored when performing culture-sensitive comparisons between two strings, including comparisons using the invariant culture. They are considered only for ordinal or case-insensitive ordinal comparisons. On the other hand, embedded null characters are always considered when searching a string with methods such as Contains, StartsWith, and IndexOf.

Strings and indexes

An index is the position of a Char object (not a Unicode character) in a String. An index is a zero-based, nonnegative number that starts from the first position in the string, which is index position zero. A number of search methods, such as IndexOf and LastIndexOf, return the index of a character or substring in the string instance.

The Chars property lets you access individual Char objects by their index position in the string. Because the Chars property is the default property (in Visual Basic) or the indexer (in C#), you can access the individual Char objects in a string by using code such as the following. This code looks for white space or punctuation characters in a string to determine how many words the string contains.

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using System;

public class Example
{
   public static void Main()
   {
      string s1 = "This string consists of a single short sentence.";
      int nWords = 0;

      s1 = s1.Trim();      
      for (int ctr = 0; ctr < s1.Length; ctr++) {
         if (Char.IsPunctuation(s1[ctr]) | Char.IsWhiteSpace(s1[ctr]))
            nWords++;              
      }
      Console.WriteLine("The sentence\n   {0}\nhas {1} words.",
                        s1, nWords);                                                                     
   }
}
// The example displays the following output: 
//       The sentence 
//          This string consists of a single short sentence. 
//       has 8 words.

Because the String class implements the IEnumerable interface, you can also iterate through the Char objects in a string by using a foreach construct, as the following example shows.

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using System;

public class Example
{
   public static void Main()
   {
      string s1 = "This string consists of a single short sentence.";
      int nWords = 0;

      s1 = s1.Trim();      
      foreach (var ch in s1) {
         if (Char.IsPunctuation(ch) | Char.IsWhiteSpace(ch))
            nWords++;              
      }
      Console.WriteLine("The sentence\n   {0}\nhas {1} words.",
                        s1, nWords);                                                                     
   }
}
// The example displays the following output: 
//       The sentence 
//          This string consists of a single short sentence. 
//       has 8 words.

Consecutive index values might not correspond to consecutive Unicode characters, because a Unicode character might be encoded as more than one Char object. To work with Unicode characters instead of Char objects, use the System.Globalization.StringInfo and TextElementEnumerator classes. The following example illustrates the difference between code that works with Char objects and code that works with Unicode characters. It compares the number of characters or text elements in each word of a sentence. The string includes two sequences of a base character followed by a combining character.

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using System;
using System.Collections.Generic;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      // First sentence of The Mystery of the Yellow Room, by Leroux. 
      string opening = "Ce n'est pas sans une certaine émotion que "+
                       "je commence à raconter ici les aventures " +
                       "extraordinaires de Joseph Rouletabille."; 
      // Character counters. 
      int nChars = 0;
      // Objects to store word count.
      List<int> chars = new List<int>();
      List<int> elements = new List<int>();

      foreach (var ch in opening) {
         // Skip the ' character. 
         if (ch == '\u0027') continue;

         if (Char.IsWhiteSpace(ch) | (Char.IsPunctuation(ch))) {
            chars.Add(nChars);
            nChars = 0;
         }
         else {
            nChars++;
         }
      }

      TextElementEnumerator te = StringInfo.GetTextElementEnumerator(opening);
      while (te.MoveNext()) {
         string s = te.GetTextElement();   
         // Skip the ' character. 
         if (s == "\u0027") continue;
         if ( String.IsNullOrEmpty(s.Trim()) | (s.Length == 1 && Char.IsPunctuation(Convert.ToChar(s)))) {
            elements.Add(nChars);         
            nChars = 0;
         }
         else {
            nChars++;
         }
      }

      // Display character counts.
      Console.WriteLine("{0,6} {1,20} {2,20}",
                        "Word #", "Char Objects", "Characters"); 
      for (int ctr = 0; ctr < chars.Count; ctr++) 
         Console.WriteLine("{0,6} {1,20} {2,20}",
                           ctr, chars[ctr], elements[ctr]); 
   }
}
// The example displays the following output: 
//       Word #         Char Objects           Characters 
//            0                    2                    2 
//            1                    4                    4 
//            2                    3                    3 
//            3                    4                    4 
//            4                    3                    3 
//            5                    8                    8 
//            6                    8                    7 
//            7                    3                    3 
//            8                    2                    2 
//            9                    8                    8 
//           10                    2                    1 
//           11                    8                    8 
//           12                    3                    3 
//           13                    3                    3 
//           14                    9                    9 
//           15                   15                   15 
//           16                    2                    2 
//           17                    6                    6 
//           18                   12                   12

Null strings and empty strings

A string that has been declared but has not been assigned a value is null. Attempting to call methods on that string throws a NullReferenceException. A null string is different from an empty string, which is a string whose value is "" or String.Empty. In some cases, passing either a null string or an empty string as an argument in a method call throws an exception. For example, passing a null string to the Int32.Parse method throws an ArgumentNullException, and passing an empty string throws a FormatException. In other cases, a method argument can be either a null string or an empty string. For example, if you are providing an IFormattable implementation for a class, you want to equate both a null string and an empty string with the general ("G") format specifier.

The String class includes the following two convenience methods that enable you to test whether a string is null or empty:

IsNullOrEmpty, which indicates whether a string is either null or is equal to String.Empty. This method eliminates the need to use code such as the following:
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if (str == null || str.Equals(String.Empty))
IsNullOrWhiteSpace, which indicates whether a string is null, equals String.Empty, or consists exclusively of white-space characters. This method eliminates the need to use code such as the following:
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if (str == null || str.Equals(String.Empty) || str.Trim().Equals(String.Empty))

The following example uses the IsNullOrEmpty method in the IFormattable.ToString implementation of a custom Temperature class. The method supports the "G", "C", "F", and "K" format strings. If an empty format string or a format string whose value is null is passed to the method, its value is changed to the "G" format string.

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public string ToString(string format, IFormatProvider provider) 
{
   if (String.IsNullOrEmpty(format)) format = "G";  
   if (provider == null) provider = CultureInfo.CurrentCulture;

   switch (format.ToUpperInvariant())
   {
      // Return degrees in Celsius.     
      case "G":
      case "C":
         return temp.ToString("F2", provider) + "°C";
      // Return degrees in Fahrenheit. 
      case "F": 
         return (temp * 9 / 5 + 32).ToString("F2", provider) + "°F";
      // Return degrees in Kelvin. 
      case "K":   
         return (temp + 273.15).ToString();
      default:
         throw new FormatException(
               String.Format("The {0} format string is not supported.", 
                             format));
   }                                   
}

Immutability and the StringBuilder class

A String object is called immutable (read-only), because its value cannot be modified after it has been created. Methods that appear to modify a String object actually return a new String object that contains the modification.

Because strings are immutable, string manipulation routines that perform repeated additions or deletions to what appears to be a single string can exact a significant performance penalty. For example, the following code uses a random number generator to create a string with 1000 characters in the range 0x0001 to 0x052F. Although the code appears to use string concatenation to append a new character to the existing string named str, it actually creates a new String object for each concatenation operation.

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using System;
using System.IO;
using System.Text;

public class Example
{
   public static void Main()
   {
      Random rnd = new Random();

      string str = String.Empty;
      StreamWriter sw = new StreamWriter(@".\StringFile.txt", 
                           false, Encoding.Unicode);

      for (int ctr = 0; ctr <= 1000; ctr++) {
         str += Convert.ToChar(rnd.Next(1, 0x0530)); 
         if (str.Length % 60 == 0)
            str += Environment.NewLine;          
      }                    
      sw.Write(str);
      sw.Close();
   }
}

You can use the StringBuilder class instead of the String class for operations that make multiple changes to the value of a string. Unlike instances of the String class, StringBuilder objects are mutable; when you concatenate, append, or delete substrings from a string, the operations are performed on a single string. When you have finished modifying the value of a StringBuilder object, you can call its StringBuilder.ToString method to convert it to a string. The following example replaces the String used in the previous example to concatenate 1000 random characters in the range to 0x0001 to 0x052F with a StringBuilder object.

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using System;
using System.IO;
using System.Text;

public class Example
{
   public static void Main()
   {
      Random rnd = new Random();
      StringBuilder sb = new StringBuilder();
      StreamWriter sw = new StreamWriter(@".\StringFile.txt", 
                                         false, Encoding.Unicode);

      for (int ctr = 0; ctr <= 1000; ctr++) {
         sb.Append(Convert.ToChar(rnd.Next(1, 0x0530))); 
         if (sb.Length % 60 == 0)
            sb.AppendLine();          
      }                    
      sw.Write(sb.ToString());
      sw.Close();
   }
}

Ordinal vs. culture-sensitive operations

Members of the String class perform either ordinal or culture-sensitive (linguistic) operations on a String object. An ordinal operation acts on the numeric value of each Char object. A culture-sensitive operation acts on the value of the String object, and takes culture-specific casing, sorting, formatting, and parsing rules into account. Culture-sensitive operations execute in the context of an explicitly declared culture or the implicit current culture. The two kinds of operations can produce very different results when they are performed on the same string.

Security Note
If your application makes a security decision about a symbolic identifier such as a file name or named pipe, or about persisted data such as the text-based data in an XML file, the operation should use an ordinal comparison instead of a culture-sensitive comparison. This is because a culture-sensitive comparison can yield different results depending on the culture in effect, whereas an ordinal comparison depends solely on the binary value of the compared characters.

Security Note

If your application makes a security decision about a symbolic identifier such as a file name or named pipe, or about persisted data such as the text-based data in an XML file, the operation should use an ordinal comparison instead of a culture-sensitive comparison. This is because a culture-sensitive comparison can yield different results depending on the culture in effect, whereas an ordinal comparison depends solely on the binary value of the compared characters.

Important
Most methods that perform string operations include an overload that has a parameter of type StringComparison, which enables you to specify whether the method performs an ordinal or culture-sensitive operation. In general, you should call this overload to make the intent of your method call clear. For best practices and guidance for using ordinal and culture-sensitive operations on strings, see Best Practices for Using Strings in the .NET Framework.

Important

Most methods that perform string operations include an overload that has a parameter of type StringComparison, which enables you to specify whether the method performs an ordinal or culture-sensitive operation. In general, you should call this overload to make the intent of your method call clear. For best practices and guidance for using ordinal and culture-sensitive operations on strings, see Best Practices for Using Strings in the .NET Framework.

Operations for casing, parsing and formatting, comparison and sorting, and testing for equality can be either ordinal or culture-sensitive. The following sections discuss each category of operation.

Casing

Casing rules determine how to change the capitalization of a Unicode character; for example, from lowercase to uppercase. Often, a casing operation is performed before a string comparison. For example, a string might be converted to uppercase so that it can be compared with another uppercase string. You can convert the characters in a string to lowercase by calling the ToLower or ToLowerInvariant method, and you can convert them to uppercase by calling the ToUpper or ToUpperInvariant method. In addition, you can use the TextInfo.ToTitleCase method to convert a string to title case.

Casing operations can be based on the rules of the current culture, a specified culture, or the invariant culture. The following example illustrates some of the differences in casing rules between cultures when converting strings to uppercase.

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using System;
using System.Globalization;
using System.IO;

public class Example
{
   public static void Main()
   {
      StreamWriter sw = new StreamWriter(@".\case.txt");   
      string[] words = { "file", "sıfır", "ǅenana" };
      CultureInfo[] cultures = { CultureInfo.InvariantCulture, 
                                 new CultureInfo("en-US"),  
                                 new CultureInfo("tr-TR") };

      foreach (var word in words) {
         sw.WriteLine("{0}:", word);
         foreach (var culture in cultures) {
            string name = String.IsNullOrEmpty(culture.Name) ? 
                                 "Invariant" : culture.Name;
            string upperWord = word.ToUpper(culture);
            sw.WriteLine("   {0,10}: {1,7} {2, 38}", name, 
                         upperWord, ShowHexValue(upperWord));

         }
         sw.WriteLine();  
      }
      sw.Close();
   }

   private static string ShowHexValue(string s)
   {
      string retval = null;
      foreach (var ch in s) {
         byte[] bytes = BitConverter.GetBytes(ch);
         retval += String.Format("{0:X2} {1:X2} ", bytes[1], bytes[0]);     
      }
      return retval;
   } 
}
// The example displays the following output: 
//    file: 
//        Invariant:    FILE               00 46 00 49 00 4C 00 45  
//            en-US:    FILE               00 46 00 49 00 4C 00 45  
//            tr-TR:    FİLE               00 46 01 30 00 4C 00 45  
//     
//    sıfır: 
//        Invariant:   SıFıR         00 53 01 31 00 46 01 31 00 52  
//            en-US:   SIFIR         00 53 00 49 00 46 00 49 00 52  
//            tr-TR:   SIFIR         00 53 00 49 00 46 00 49 00 52  
//     
//    ǅenana: 
//        Invariant:  ǅENANA   01 C5 00 45 00 4E 00 41 00 4E 00 41  
//            en-US:  ǄENANA   01 C4 00 45 00 4E 00 41 00 4E 00 41  
//            tr-TR:  ǄENANA   01 C4 00 45 00 4E 00 41 00 4E 00 41

Parsing and formatting

Formatting and parsing are inverse operations. Formatting rules determine how to convert a value, such as a date and time or a number, to its string representation, whereas parsing rules determine how to convert a string representation to a value such as a date and time. Both formatting and parsing rules are dependent on cultural conventions. The following example illustrates the ambiguity that can arise when interpreting a culture-specific date string. Without knowing the conventions of the culture that was used to produce a date string, it is not possible to know whether 03/01/2011, 3/1/2011, and 01/03/2011 represent January 3, 2011 or March 1, 2011.

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using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      DateTime date = new DateTime(2011, 3, 1);
      CultureInfo[] cultures = { CultureInfo.InvariantCulture, 
                                 new CultureInfo("en-US"), 
                                 new CultureInfo("fr-FR") };

      foreach (var culture in cultures)
         Console.WriteLine("{0,-12} {1}", String.IsNullOrEmpty(culture.Name) ?
                           "Invariant" : culture.Name, 
                           date.ToString("d", culture));                                    
   }
}
// The example displays the following output: 
//       Invariant    03/01/2011 
//       en-US        3/1/2011 
//       fr-FR        01/03/2011

Similarly, as the following example shows, a single string can produce different dates depending on the culture whose conventions are used in the parsing operation.

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using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      string dateString = "07/10/2011";
      CultureInfo[] cultures = { CultureInfo.InvariantCulture, 
                                 CultureInfo.CreateSpecificCulture("en-GB"), 
                                 CultureInfo.CreateSpecificCulture("en-US") };
      Console.WriteLine("{0,-12} {1,10} {2,8} {3,8}\n", "Date String", "Culture", 
                                                 "Month", "Day");
      foreach (var culture in cultures) {
         DateTime date = DateTime.Parse(dateString, culture);
         Console.WriteLine("{0,-12} {1,10} {2,8} {3,8}", dateString, 
                           String.IsNullOrEmpty(culture.Name) ?
                           "Invariant" : culture.Name, 
                           date.Month, date.Day);
      }                      
   }
}
// The example displays the following output: 
//       Date String     Culture    Month      Day 
//        
//       07/10/2011    Invariant        7       10 
//       07/10/2011        en-GB       10        7 
//       07/10/2011        en-US        7       10

String comparison and sorting

Sort rules determine the alphabetic order of Unicode characters and how two strings compare to each other. For example, the String.Compare(String, String, StringComparison) method compares two strings based on the StringComparison parameter. If the parameter value is StringComparison.CurrentCulture, the method performs a linguistic comparison that uses the conventions of the current culture; if the parameter value is StringComparison.Ordinal, the method performs an ordinal comparison. Consequently, as the following example shows, if the current culture is U.S. English, the first call to the String.Compare(String, String, StringComparison) method (using culture-sensitive comparison) considers "a" less than "A", but the second call to the same method (using ordinal comparison) considers "a" greater than "A".

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using System;
using System.Globalization;
using System.Threading;

public class Example
{
   public static void Main()
   {
      Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("en-US");
      Console.WriteLine(String.Compare("A", "a", StringComparison.CurrentCulture));
      Console.WriteLine(String.Compare("A", "a", StringComparison.Ordinal));
   }
}
// The example displays the following output: 
//       1 
//       -32

The .NET Framework supports word, string, and ordinal sort rules:

A word sort performs a culture-sensitive comparison of strings in which certain nonalphanumeric Unicode characters might have special weights assigned to them. For example, the hyphen (-) might have a very small weight assigned to it so that "coop" and "co-op" appear next to each other in a sorted list. For a list of the String methods that compare two strings using word sort rules, see the String operations by category section.
A string sort also performs a culture-sensitive comparison. It is similar to a word sort, except that there are no special cases, and all nonalphanumeric symbols come before all alphanumeric Unicode characters. Two strings can be compared using string sort rules by calling the CompareInfo.Compare method overloads that have an options parameter that is supplied a value of CompareOptions.StringSort. Note that this is the only method that the .NET Framework provides to compare two strings using string sort rules.
An ordinal sort compares strings based on the numeric value of each Char object in the string. An ordinal comparison is automatically case-sensitive because the lowercase and uppercase versions of a character have different code points. However, if case is not important, you can specify an ordinal comparison that ignores case. This is equivalent to converting the string to uppercase by using the invariant culture and then performing an ordinal comparison on the result. For a list of the String methods that compare two strings using ordinal sort rules, see the String operations by category section.

A culture-sensitive comparison is any comparison that explicitly or implicitly uses a CultureInfo object, including the invariant culture that is specified by the CultureInfo.InvariantCulture property. The implicit culture is the current culture, which is specified by the Thread.CurrentCulture and CultureInfo.CurrentCulture properties. A culture-sensitive comparison is generally appropriate for sorting, whereas an ordinal comparison is not. An ordinal comparison is generally appropriate for determining whether two strings are equal (that is, for determining identity), whereas a culture-sensitive comparison is not.

Note
The culture-sensitive sorting and casing rules used in string comparison depend on the version of the .NET Framework. In the .NET Framework 4.5 running on the Windows 8 operating system, sorting, casing, normalization, and Unicode character information conforms to the Unicode 6.0 standard. On other operating systems, it conforms to the Unicode 5.0 standard.

For more information about word, string, and ordinal sort rules, see the System.Globalization.CompareOptions topic. For recommendations on when to use each rule, see Best Practices for Using Strings in the .NET Framework.

Ordinarily, you do not call string comparison methods such as Compare directly to determine the sort order of strings. Instead, comparison methods are called by sorting methods such as Array.Sort or List<T>.Sort. The following example performs four different sorting operations (word sort using the current culture, word sort using the invariant culture, ordinal sort, and string sort using the invariant culture) without explicitly calling a string comparison method. Note that each type of sort produces a unique ordering of strings in its array.

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using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      string[] strings = { "coop", "co-op", "cooperative", 
                           "co\u00ADoperative", "cœur", "coeur" };

      // Perform a word sort using the current (en-US) culture. 
      string[] current = new string[strings.Length]; 
      strings.CopyTo(current, 0); 
      Array.Sort(current, StringComparer.CurrentCulture);

      // Perform a word sort using the invariant culture. 
      string[] invariant = new string[strings.Length];
      strings.CopyTo(invariant, 0); 
      Array.Sort(invariant, StringComparer.InvariantCulture);

      // Perform an ordinal sort. 
      string[] ordinal = new string[strings.Length];
      strings.CopyTo(ordinal, 0); 
      Array.Sort(ordinal, StringComparer.Ordinal);

      // Perform a string sort using the current culture. 
      string[] stringSort = new string[strings.Length];
      strings.CopyTo(stringSort, 0); 
      Array.Sort(stringSort, new SCompare());

      // Display array values
      Console.WriteLine("{0,13} {1,13} {2,15} {3,13} {4,13}\n", 
                        "Original", "Word Sort", "Invariant Word", 
                        "Ordinal Sort", "String Sort");
      for (int ctr = 0; ctr < strings.Length; ctr++)
         Console.WriteLine("{0,13} {1,13} {2,15} {3,13} {4,13}", 
                           strings[ctr], current[ctr], invariant[ctr], 
                           ordinal[ctr], stringSort[ctr] );          
   }
}

// IComparer<String> implementation to perform string sort. 
internal class SCompare : IComparer<String>
{
   public int Compare(string x, string y)
   {
      return CultureInfo.CurrentCulture.CompareInfo.Compare(x, y, CompareOptions.StringSort);
   }
}
// The example displays the following output: 
//         Original     Word Sort  Invariant Word  Ordinal Sort   String Sort 
//     
//             coop          cœur            cœur         co-op         co-op 
//            co-op         coeur           coeur         coeur          cœur 
//      cooperative          coop            coop          coop         coeur 
//     cooperative         co-op           co-op   cooperative          coop 
//             cœur   cooperative     cooperative  cooperative   cooperative 
//            coeur  cooperative    cooperative          cœur  cooperative

Caution
If your primary purpose in comparing strings is to determine whether they are equal, you should call the String.Equals method. Typically, you should use Equals to perform an ordinal comparison. The String.Compare method is intended primarily to sort strings.

String search methods, such as String.StartsWith and String.IndexOf, also can perform culture-sensitive or ordinal string comparisons. The following example illustrates the differences between ordinal and culture-sensitive comparisons using the IndexOf method. A culture-sensitive search in which the current culture is English (United States) considers the substring "oe" to match the ligature "œ". Because a soft hyphen (U+00AD) is a zero-width character, the search treats the soft hyphen as equivalent to Empty and finds a match at the beginning of the string. An ordinal search, on the other hand, does not find a match in either case.

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using System;

public class Example
{
   public static void Main()
   {
      // Search for "oe" and "œu" in "œufs" and "oeufs".
      string s1 = "œufs";
      string s2 = "oeufs";
      FindInString(s1, "oe", StringComparison.CurrentCulture);
      FindInString(s1, "oe", StringComparison.Ordinal);
      FindInString(s2, "œu", StringComparison.CurrentCulture);
      FindInString(s2, "œu", StringComparison.Ordinal);
      Console.WriteLine();

      string s3 = "co\u00ADoperative";
      FindInString(s3, "\u00AD", StringComparison.CurrentCulture);
      FindInString(s3, "\u00AD", StringComparison.Ordinal);
   }

   private static void FindInString(string s, string substring, StringComparison options)
   {
      int result = s.IndexOf(substring, options);
      if (result != -1)
         Console.WriteLine("'{0}' found in {1} at position {2}", 
                           substring, s, result);
      else
         Console.WriteLine("'{0}' not found in {1}", 
                           substring, s);                                                  
   }
}
// The example displays the following output: 
//       'oe' found in œufs at position 0 
//       'oe' not found in œufs 
//       'œu' found in oeufs at position 0 
//       'œu' not found in oeufs 
//        
//       '' found in cooperative at position 0 
//       '' found in cooperative at position 2

Testing for equality

Use the String.Compare method to determine the relationship of two strings in the sort order. Typically, this is a culture-sensitive operation. In contrast, call the String.Equals method to test for equality. Because the test for equality usually compares user input with some known string, such as a valid user name, a password, or a file system path, it is typically an ordinal operation.

Caution
It is possible to test for equality by calling the String.Compare method and determining whether the return value is zero. However, this practice is not recommended. To determine whether two strings are equal, you should call one of the overloads of the String.Equals method. The preferred overload to call is either the instance Equals(String, StringComparison) method or the static Equals(String, String, StringComparison) method, because both methods include a System.StringComparison parameter that explicitly specifies the type of comparison.

Caution

It is possible to test for equality by calling the String.Compare method and determining whether the return value is zero. However, this practice is not recommended. To determine whether two strings are equal, you should call one of the overloads of the String.Equals method. The preferred overload to call is either the instance Equals(String, StringComparison) method or the static Equals(String, String, StringComparison) method, because both methods include a System.StringComparison parameter that explicitly specifies the type of comparison.

The following example illustrates the danger of performing a culture-sensitive comparison for equality when an ordinal one should be used instead. In this case, the intent of the code is to prohibit file system access from URLs that begin with "FILE://" or "file://" by performing a case-insensitive comparison of the beginning of a URL with the string "FILE://". However, if a culture-sensitive comparison is performed using the Turkish (Turkey) culture on a URL that begins with "file://", the comparison for equality fails, because the Turkish uppercase equivalent of the lowercase "i" is "İ" instead of "I". As a result, file system access is inadvertently permitted. On the other hand, if an ordinal comparison is performed, the comparison for equality succeeds, and file system access is denied.

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using System;
using System.Globalization;
using System.Threading;

public class Example
{
   public static void Main()
   {
      Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("tr-TR");      

      string filePath = "file://c:/notes.txt";

      Console.WriteLine("Culture-sensitive test for equality:");
      if (! TestForEquality(filePath, StringComparison.CurrentCultureIgnoreCase))
         Console.WriteLine("Access to {0} is allowed.", filePath);
      else
         Console.WriteLine("Access to {0} is not allowed.", filePath);

      Console.WriteLine("\nOrdinal test for equality:");
      if (! TestForEquality(filePath, StringComparison.OrdinalIgnoreCase))
         Console.WriteLine("Access to {0} is allowed.", filePath);
      else
         Console.WriteLine("Access to {0} is not allowed.", filePath);
   }

   private static bool TestForEquality(string str, StringComparison cmp)
   {
      int position = str.IndexOf("://");
      if (position < 0) return false;

      string substring = str.Substring(0, position);  
      return substring.Equals("FILE", cmp);
   }
}
// The example displays the following output: 
//       Culture-sensitive test for equality: 
//       Access to file://c:/notes.txt is allowed. 
//        
//       Ordinal test for equality: 
//       Access to file://c:/notes.txt is not allowed.

Normalization

Some Unicode characters have multiple representations. For example, any of the following code points can represent the letter "ắ":

U+1EAF
U+0103 U+0301
U+0061 U+0306 U+0301

Multiple representations for a single character complicate searching, sorting, matching, and other string operations.

The Unicode standard defines a process called normalization that returns one binary representation of a Unicode character for any of its equivalent binary representations. Normalization can use several algorithms, called normalization forms, that follow different rules. The .NET Framework supports Unicode normalization forms C, D, KC, and KD. When strings have been normalized to the same normalization form, they can be compared by using ordinal comparison. For more information about normalization and normalization forms, see System.Text.NormalizationForm.

You can determine whether a string is normalized to normalization form C by calling the String.IsNormalized() method, or you can call the String.IsNormalized(NormalizationForm) method to determine whether a string is normalized to a specified normalization form. You can also call the String.Normalize() method to convert a string to normalization form C, or you can call the String.Normalize(NormalizationForm) method to convert a string to a specified normalization form.

The following example illustrates string normalization. It defines the letter "ố" in three different ways in three different strings, and uses an ordinal comparison for equality to determine that each string differs from the other two strings. It then converts each string to the supported normalization forms, and again performs an ordinal comparison of each string in a specified normalization form. In each case, the second test for equality shows that the strings are equal.

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using System;
using System.Globalization;
using System.IO;
using System.Text;

public class Example
{
   private static StreamWriter sw;

   public static void Main()
   {
      sw = new StreamWriter(@".\TestNorm1.txt");

      // Define three versions of the same word.  
      string s1 = "sống";        // create word with U+1ED1 
      string s2 = "s\u00F4\u0301ng";
      string s3 = "so\u0302\u0301ng";

      TestForEquality(s1, s2, s3);      
      sw.WriteLine();

      // Normalize and compare strings using each normalization form. 
      foreach (string formName in Enum.GetNames(typeof(NormalizationForm)))
      {
         sw.WriteLine("Normalization {0}:\n", formName); 
         NormalizationForm nf = (NormalizationForm) Enum.Parse(typeof(NormalizationForm), formName);
         string[] sn = NormalizeStrings(nf, s1, s2, s3);
         TestForEquality(sn);           
         sw.WriteLine("\n");                                        
      }

      sw.Close();   
   }

   private static void TestForEquality(params string[] words)
   {
      for (int ctr = 0; ctr <= words.Length - 2; ctr++)
         for (int ctr2 = ctr + 1; ctr2 <= words.Length - 1; ctr2++) 
            sw.WriteLine("{0} ({1}) = {2} ({3}): {4}", 
                         words[ctr], ShowBytes(words[ctr]),
                         words[ctr2], ShowBytes(words[ctr2]),
                         words[ctr].Equals(words[ctr2], StringComparison.Ordinal));
   }

   private static string ShowBytes(string str)
   {
      string result = null;
      foreach (var ch in str)
         result += String.Format("{0} ", Convert.ToUInt16(ch).ToString("X4")); 
      return result.Trim();            
   } 

   private static string[] NormalizeStrings(NormalizationForm nf, params string[] words)
   {
      for (int ctr = 0; ctr < words.Length; ctr++)
         if (! words[ctr].IsNormalized(nf))
            words[ctr] = words[ctr].Normalize(nf); 
      return words;   
   }
}
// The example displays the following output: 
//       sống (0073 1ED1 006E 0067) = sống (0073 00F4 0301 006E 0067): False 
//       sống (0073 1ED1 006E 0067) = sống (0073 006F 0302 0301 006E 0067): False 
//       sống (0073 00F4 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): False 
//        
//       Normalization FormC: 
//        
//       sống (0073 1ED1 006E 0067) = sống (0073 1ED1 006E 0067): True 
//       sống (0073 1ED1 006E 0067) = sống (0073 1ED1 006E 0067): True 
//       sống (0073 1ED1 006E 0067) = sống (0073 1ED1 006E 0067): True 
//        
//        
//       Normalization FormD: 
//        
//       sống (0073 006F 0302 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): True 
//       sống (0073 006F 0302 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): True 
//       sống (0073 006F 0302 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): True 
//        
//        
//       Normalization FormKC: 
//        
//       sống (0073 1ED1 006E 0067) = sống (0073 1ED1 006E 0067): True 
//       sống (0073 1ED1 006E 0067) = sống (0073 1ED1 006E 0067): True 
//       sống (0073 1ED1 006E 0067) = sống (0073 1ED1 006E 0067): True 
//        
//        
//       Normalization FormKD: 
//        
//       sống (0073 006F 0302 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): True 
//       sống (0073 006F 0302 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): True 
//       sống (0073 006F 0302 0301 006E 0067) = sống (0073 006F 0302 0301 006E 0067): True

String operations by category

The String class provides members for comparing strings, testing strings for equality, finding characters or substrings in a string, modifying a string, extracting substrings from a string, combining strings, formatting values, copying a string, and normalizing a string.

Comparing strings

You can compare strings to determine their relative position in the sort order by using the following String methods:

Compare returns an integer that indicates the relationship of one string to a second string in the sort order.
CompareOrdinal returns an integer that indicates the relationship of one string to a second string based on a comparison of their code points.
CompareTo returns an integer that indicates the relationship of the current string instance to a second string in the sort order. The CompareTo(String) method provides the IComparable and IComparable<T> implementations for the String class.

Testing strings for equality

You call the Equals method to determine whether two strings are equal. The instance Equals(String, String, StringComparison) and the static Equals(String, StringComparison) overloads let you specify whether the comparison is culture-sensitive or ordinal, and whether case is considered or ignored. Most tests for equality are ordinal, and comparisons for equality that determine access to a system resource (such as a file system object) should always be ordinal.

Finding characters in a string

The String class includes two kinds of search methods:

Methods that return a Boolean value to indicate whether a particular substring is present in a string instance. These include the Contains, EndsWith, and StartsWith methods.
Methods that indicate the starting position of a substring in a string instance. These include the IndexOf, IndexOfAny, LastIndexOf, and LastIndexOfAny methods.

Caution
If you want to search a string for a particular pattern rather than a specific substring, you should use regular expressions. For more information, see .NET Framework Regular Expressions.

Modifying a string

The String class includes the following methods that appear to modify the value of a string:

Insert inserts a string into the current String instance.
PadLeft inserts one or more occurrences of a specified character at the beginning of a string.
PadRight inserts one or more occurrences of a specified character at the beginning of a string.
Remove deletes a substring from the current String instance.
Replace replaces a substring with another substring in the current String instance.
ToLower and ToLowerInvariant convert all the characters in a string to lowercase.
ToUpper and ToUpperInvariant convert all the characters in a string to uppercase.
Trim removes all occurrences of a character from the beginning and end of a string.
TrimEnd removes all occurrences of a character from the end of a string.
TrimStart removes all occurrences of a character from the beginning of a string.

Important
All string modification methods return a new String object. They do not modify the value of the current instance.

Extracting substrings from a string

The String.Split method separates a single string into multiple strings. Overloads of the method allow you to specify multiple delimiters, to determine the maximum number of substrings that the method extracts, and to determine whether empty strings (which occur when delimiters are adjacent) are included among the returned strings.

Combining strings

The following String methods can be used for string concatenation:

Concat combines one or more substrings into a single string.
Join concatenates one or more substrings into a single element and adds a separator between each substring.

Formatting values

The String.Format method uses the composite formatting feature to replace one or more placeholders in a string with the string representation of some object or value. The Format method is often used to do the following:

To embed the string representation of a numeric value in a string.
To embed the string representation of a date and time value in a string.
To embed the string representation of an enumeration value in a string.
To embed the string representation of some object that supports the IFormattable interface in a string.
To right-justify or left-justify a substring in a field within a larger string.

For detailed information about formatting operations and examples, see the Format overload summary.

Copying a string

You can call the following String methods to make a copy of a string:

Clone returns a reference to an existing String object.
Copy creates a copy of an existing string.
CopyTo copies a portion of a string to a character array.

Normalizing a string

In Unicode, a single character can have multiple code points. Normalization converts these equivalent characters into the same binary representation. The String.Normalize method performs the normalization, and the String.IsNormalized method determines whether a string is normalized.

Version Information

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

Platforms

Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Thread Safety

This type is thread safe.

Reference

System.Text.StringBuilder

CultureInfo

Other Resources

Formatting Types

Best Practices for Using Strings in the .NET Framework