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将一个复杂对象的构件与它的表示分离，使得同样的构建过程可以创建不同的表述。
建造者（Builder）角色：给出一个抽象接口，以规范产品对象的各个组成成分的建造。一般而言，此接口独立于应用程序的商业逻辑。模式中直接创建产品对象的是具体建造者（ConcreteBuilder）角色。具体建造者类必须实现这个接口所要求的方法：一个是建造方法，另一个是结果返还方法。
具体建造者（Concrete Builder）角色：担任这个角色的是于应用程序紧密相关的类，它们在应用程序调用下创建产品实例。这个角色主要完成的任务包括：实现Builder角色提供的接口，一步一步完成创建产品实例的过程；在建造过程完成后，提供产品的实例。
指导者（Director）角色：担任这个角色的类调用具体建造者角色以创建产品对象。导演者并没有产品类的具体知识，真正拥有产品类的具体知识的是具体建造者对象。
产品（Product）角色：产品便是建造中的复杂对象。
以以.NET Framework 2.0 System.Text. StringBuilder为例
System.Text. StringBuilder
Code
public sealed class StringBuilder : ISerializable
{
//
internal IntPtr m_currentThread = Thread.InternalGetCurrentThread();
internal int m_MaxCapacity = 0;
internal volatile String m_StringValue = null;
//
public StringBuilder() : this(DefaultCapacity)
{
}
//
public StringBuilder Append(String value)
{
//If the value being added is null, eat the null
//and return.
if (value == null)
{
return this;
}
IntPtr tid;
// hand inlining of GetThreadSafeString
String currentString = m_StringValue;
tid = Thread.InternalGetCurrentThread();
if (m_currentThread != tid)
currentString = String.GetStringForStringBuilder(currentString, currentString.Capacity);
int currentLength = currentString.Length;
int requiredLength = currentLength + value.Length;
if (NeedsAllocation(currentString, requiredLength))
{
String newString = GetNewString(currentString, requiredLength);
newString.AppendInPlace(value, currentLength);
ReplaceString(tid, newString);
}
else
{
currentString.AppendInPlace(value, currentLength);
ReplaceString(tid, currentString);
}
return this;
}
//
public StringBuilder Append(int value)
{
return Append(value.ToString(CultureInfo.CurrentCulture));
}
//
public override String ToString()
{
//
// We assume that their read of m_currentThread will always occur after read of m_StringValue.
// If these reads get re-ordered then it is possible to get a currentString owned by some other
// (mutating) thread and yet think, according to currentThread, that such was not the case.
// This is acheived by marking m_StringValue as volatile.
//
String currentString = m_StringValue;
IntPtr currentThread = m_currentThread;
//
// Note calling ToString the second time or from a different thread will cause allocation of a new string.
// If we do not make a copy if currentThread is IntPtr.Zero, we will have following race:
//
// (1) Thread T1 completes a mutation of the string and will become the owner.
// T1 then starts another mutation operation and
// A thread interleaving happens at this point.
// (2) Thread T2 starts a ToString operation. T2 reads m_StringValue into its local currentString variable.
// A thread interleaving happens at this point.
// (3) Thread T3 finshes a mutation of the string in the StringBuilder , performing the ReplaceString call.
// Thread T3 then starts a ToString operation. Assuming the string is not wasting excessive space,
// T3 will proceeds to call ClearPostNullChar, registers NOBODY as the owner, and returns the string.
// A thread interleaving happens at this point.
// (4) Thread T2 resumes execution. T2 reads m_currentThread and sees that NOBODY is the registered owner
// Assuming its currentString is not wasting excessive space, T2 will return the same string that thread T1 is
// in the middle of mutating.
//
if (currentThread != Thread.InternalGetCurrentThread())
{
return String.InternalCopy(currentString);
}
if ((2 * currentString.Length) < currentString.ArrayLength)
{
return String.InternalCopy(currentString);
}
currentString.ClearPostNullChar();
m_currentThread = IntPtr.Zero;
return currentString;
}
}
System.String
Code
public sealed class String : IComparable, ICloneable, IConvertible, IEnumerable
{
//
// Creates a new string with the characters copied in from ptr. If
// ptr is null, a string initialized to ";<;No Object>;"; (i.e.,
// String.NullString) is created.
//
[CLSCompliant(false), MethodImplAttribute(MethodImplOptions.InternalCall)]
unsafe public extern String(char* value);
[CLSCompliant(false), MethodImplAttribute(MethodImplOptions.InternalCall)]
unsafe public extern String(char* value, int startIndex, int length);
[CLSCompliant(false), MethodImplAttribute(MethodImplOptions.InternalCall)]
unsafe public extern String(sbyte* value);
[CLSCompliant(false), MethodImplAttribute(MethodImplOptions.InternalCall)]
unsafe public extern String(sbyte* value, int startIndex, int length);
[CLSCompliant(false), MethodImplAttribute(MethodImplOptions.InternalCall)]
unsafe public extern String(sbyte* value, int startIndex, int length, Encoding enc);
//
// Creates a new string from the characters in a subarray. The new string will
// be created from the characters in value between startIndex and
// startIndex + length - 1.
//
[MethodImplAttribute(MethodImplOptions.InternalCall)]
public extern String(char[] value, int startIndex, int length);
// Creates a new string from the characters in a subarray. The new string will be
// created from the characters in value.
//
[MethodImplAttribute(MethodImplOptions.InternalCall)]
public extern String(char[] value);
//
internal unsafe void AppendInPlace(String value, int currentLength)
{
int count = value.Length;
int newLength = currentLength + count;
//
fixed (char* dest = &this.m_firstChar)
{
fixed (char* src = &value.m_firstChar)
{
wstrcpy(dest + currentLength, src, count);
}
dest[newLength] = '';
}
this.m_stringLength = newLength;
}
//
}
调用代码
Code
public class Client
{
public static void Main()
{
StringBuilder sb = new StringBuilder();
sb.Append("Hello");
sb.Append(" World");
sb.Append("!");
String s = sb.ToString();
Console.Write(s);
Console.ReadLine();
}
}
由于String类型代表的是一个不可变的字符串，所以BCL提供了另一个名为System.Text.StringBuilder的类型，它允许我们有效的对字符串的字符执行动态操作，以创建一个String。
从逻辑上说，StringBuilder对象中包含一个字段，它引用由Char结构构成的一个数组。StringBuilder的成员允许我们操作这个字符，有效的缩减字符串的大小或者更改字符串中的字符。如果字符串变大，超过已经分配的字符的大小，StringBuilder就会自动的分配一个全新的、更大的数组，并开始使用新的数组，前一个数组会被垃圾回收器回收。用StringBuilder对象构建好字符串之后，为了将StringBuilder的字符“转换”成一个String，只需调用StringBuilder的ToString方法，在内部，该方法只是返回对StringBuilder内部维护的字符串的字段的一个引用，执行效率非常快，因为它不需要进行字符数组复制。
C#中volatile 关键字指示一个字段可以由多个同时执行的线程修改。声明为 volatile 的字段不受编译器优化（假定由单个线程访问）的限制。这样可以确保该字段在任何时间呈现的都是最新的值。可变关键字仅可应用于类或结构字段。不能将局部变量声明为 volatile。
StringBuilder既是具体建造者（Builder）又是指导者（Director），最终生成一个复杂的String对象作为产品（Product）。在具体建造者只有一个的情况下，如果抽象建造者角色已经被省略掉，那么还可以省略掉指导者角色。让Builder角色自己扮演指导者与建造者双重角色。
以下情况应当使用建造者模式：
1．需要生成的产品对象有复杂的内部结构。
2．需要生成的产品对象的属性相互依赖，建造者模式可以强迫生成顺序。
3．在对象创建过程中会使用到系统中的一些其它对象，这些对象在产品对
象的创建过程中不易得到。
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