C#中的DateTime

一、DateTime是值类型还是引用类型的探索

二、了解DateTime结构体

三、DateTime.Now和DateTime.UtcNow是怎么计算出来的

一、DateTime是值类型还是引用类型的探索

     DateTime是值类型，因为DateTime是结构体，而结构体继承自System.ValueType，属于值类型

     

[Serializable, __DynamicallyInvokable]
public struct DateTime : IComparable, IFormattable, IConvertible, ISerializable, IComparable<DateTime>, IEquatable<DateTime>
{
    // Fields
    private ulong dateData;
    private const string DateDataField = "dateData";
    private const int DatePartDay = 3;
    private const int DatePartDayOfYear = 1;
    private const int DatePartMonth = 2;
    private const int DatePartYear = 0;
    private const int DaysPer100Years = 0x8eac;
    private const int DaysPer400Years = 0x23ab1;
    private const int DaysPer4Years = 0x5b5;
    private const int DaysPerYear = 0x16d;
    private const int DaysTo10000 = 0x37b9db;
    private const int DaysTo1601 = 0x8eac4;
    private const int DaysTo1899 = 0xa9559;
    private static readonly int[] DaysToMonth365;
    private static readonly int[] DaysToMonth366;
    private const long DoubleDateOffset = 0x85103c0cb83c000L;
    private const long FileTimeOffset = 0x701ce1722770000L;
    private const ulong FlagsMask = 13835058055282163712L;
    private const ulong KindLocal = 9223372036854775808L;
    private const ulong KindLocalAmbiguousDst = 13835058055282163712L;
    private const int KindShift = 0x3e;
    private const ulong KindUnspecified = 0L;
    private const ulong KindUtc = 0x4000000000000000L;
    private const ulong LocalMask = 9223372036854775808L;
    private const long MaxMillis = 0x11efae44cb400L;
    internal const long MaxTicks = 0x2bca2875f4373fffL;
    [__DynamicallyInvokable]
    public static readonly DateTime MaxValue;
    private const int MillisPerDay = 0x5265c00;
    private const int MillisPerHour = 0x36ee80;
    private const int MillisPerMinute = 0xea60;
    private const int MillisPerSecond = 0x3e8;
    internal const long MinTicks = 0L;
    [__DynamicallyInvokable]
    public static readonly DateTime MinValue;
    private const double OADateMaxAsDouble = 2958466.0;
    private const double OADateMinAsDouble = -657435.0;
    private const long OADateMinAsTicks = 0x6efdddaec64000L;
    private const long TicksCeiling = 0x4000000000000000L;
    private const string TicksField = "ticks";
    private const ulong TicksMask = 0x3fffffffffffffffL;
    private const long TicksPerDay = 0xc92a69c000L;
    private const long TicksPerHour = 0x861c46800L;
    private const long TicksPerMillisecond = 0x2710L;
    private const long TicksPerMinute = 0x23c34600L;
    private const long TicksPerSecond = 0x989680L;

    // Methods
    static DateTime();
    [__DynamicallyInvokable]
    public DateTime(long ticks);
    [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    private DateTime(ulong dateData);
    [__DynamicallyInvokable]
    public DateTime(long ticks, DateTimeKind kind);
    private DateTime(SerializationInfo info, StreamingContext context);
    [__DynamicallyInvokable]
    public DateTime(int year, int month, int day);
    internal DateTime(long ticks, DateTimeKind kind, bool isAmbiguousDst);
    [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime(int year, int month, int day, Calendar calendar);
    [__DynamicallyInvokable]
    public DateTime(int year, int month, int day, int hour, int minute, int second);
    [__DynamicallyInvokable]
    public DateTime(int year, int month, int day, int hour, int minute, int second, DateTimeKind kind);
    public DateTime(int year, int month, int day, int hour, int minute, int second, Calendar calendar);
    [__DynamicallyInvokable]
    public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond);
    [__DynamicallyInvokable]
    public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, DateTimeKind kind);
    public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, Calendar calendar);
    public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, Calendar calendar, DateTimeKind kind);
    [__DynamicallyInvokable]
    public DateTime Add(TimeSpan value);
    private DateTime Add(double value, int scale);
    [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime AddDays(double value);
    [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime AddHours(double value);
    [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime AddMilliseconds(double value);
    [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime AddMinutes(double value);
    [__DynamicallyInvokable]
    public DateTime AddMonths(int months);
    [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime AddSeconds(double value);
    [__DynamicallyInvokable]
    public DateTime AddTicks(long value);
    [__DynamicallyInvokable]
    public DateTime AddYears(int value);
    [__DynamicallyInvokable]
    public static int Compare(DateTime t1, DateTime t2);
    [__DynamicallyInvokable]
    public int CompareTo(DateTime value);
    public int CompareTo(object value);
    private static long DateToTicks(int year, int month, int day);
    [__DynamicallyInvokable]
    public static int DaysInMonth(int year, int month);
    internal static long DoubleDateToTicks(double value);
    [return: MarshalAs(UnmanagedType.Bool)]
    [SecurityCritical, SuppressUnmanagedCodeSecurity, DllImport("QCall", CharSet=CharSet.Unicode)]
    internal static extern bool EnableAmPmParseAdjustment();
    [__DynamicallyInvokable]
    public bool Equals(DateTime value);
    [__DynamicallyInvokable]
    public override bool Equals(object value);
    [__DynamicallyInvokable]
    public static bool Equals(DateTime t1, DateTime t2);
    [__DynamicallyInvokable]
    public static DateTime FromBinary(long dateData);
    internal static DateTime FromBinaryRaw(long dateData);
    [__DynamicallyInvokable]
    public static DateTime FromFileTime(long fileTime);
    [__DynamicallyInvokable]
    public static DateTime FromFileTimeUtc(long fileTime);
    [__DynamicallyInvokable]
    public static DateTime FromOADate(double d);
    private int GetDatePart(int part);
    [__DynamicallyInvokable]
    public string[] GetDateTimeFormats();
    [__DynamicallyInvokable]
    public string[] GetDateTimeFormats(char format);
    [__DynamicallyInvokable]
    public string[] GetDateTimeFormats(IFormatProvider provider);
    [__DynamicallyInvokable]
    public string[] GetDateTimeFormats(char format, IFormatProvider provider);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public override int GetHashCode();
    [MethodImpl(MethodImplOptions.InternalCall), SecurityCritical]
    internal static extern long GetSystemTimeAsFileTime();
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    public TypeCode GetTypeCode();
    internal bool IsAmbiguousDaylightSavingTime();
    [__DynamicallyInvokable]
    public bool IsDaylightSavingTime();
    [__DynamicallyInvokable]
    public static bool IsLeapYear(int year);
    [__DynamicallyInvokable]
    public static DateTime operator +(DateTime d, TimeSpan t);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static bool operator ==(DateTime d1, DateTime d2);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static bool operator >(DateTime t1, DateTime t2);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static bool operator >=(DateTime t1, DateTime t2);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static bool operator !=(DateTime d1, DateTime d2);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static bool operator <(DateTime t1, DateTime t2);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static bool operator <=(DateTime t1, DateTime t2);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static TimeSpan operator -(DateTime d1, DateTime d2);
    [__DynamicallyInvokable]
    public static DateTime operator -(DateTime d, TimeSpan t);
    [__DynamicallyInvokable]
    public static DateTime Parse(string s);
    [__DynamicallyInvokable]
    public static DateTime Parse(string s, IFormatProvider provider);
    [__DynamicallyInvokable]
    public static DateTime Parse(string s, IFormatProvider provider, DateTimeStyles styles);
    [__DynamicallyInvokable]
    public static DateTime ParseExact(string s, string format, IFormatProvider provider);
    [__DynamicallyInvokable]
    public static DateTime ParseExact(string s, string format, IFormatProvider provider, DateTimeStyles style);
    [__DynamicallyInvokable]
    public static DateTime ParseExact(string s, string[] formats, IFormatProvider provider, DateTimeStyles style);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public static DateTime SpecifyKind(DateTime value, DateTimeKind kind);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public TimeSpan Subtract(DateTime value);
    [__DynamicallyInvokable]
    public DateTime Subtract(TimeSpan value);
    bool IConvertible.ToBoolean(IFormatProvider provider);
    byte IConvertible.ToByte(IFormatProvider provider);
    char IConvertible.ToChar(IFormatProvider provider);
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    DateTime IConvertible.ToDateTime(IFormatProvider provider);
    decimal IConvertible.ToDecimal(IFormatProvider provider);
    double IConvertible.ToDouble(IFormatProvider provider);
    short IConvertible.ToInt16(IFormatProvider provider);
    int IConvertible.ToInt32(IFormatProvider provider);
    long IConvertible.ToInt64(IFormatProvider provider);
    sbyte IConvertible.ToSByte(IFormatProvider provider);
    float IConvertible.ToSingle(IFormatProvider provider);
    object IConvertible.ToType(Type type, IFormatProvider provider);
    ushort IConvertible.ToUInt16(IFormatProvider provider);
    uint IConvertible.ToUInt32(IFormatProvider provider);
    ulong IConvertible.ToUInt64(IFormatProvider provider);
    [SecurityCritical]
    void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context);
    private static double TicksToOADate(long value);
    private static long TimeToTicks(int hour, int minute, int second);
    [__DynamicallyInvokable]
    public long ToBinary();
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    internal long ToBinaryRaw();
    [__DynamicallyInvokable]
    public long ToFileTime();
    [__DynamicallyInvokable]
    public long ToFileTimeUtc();
    [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public DateTime ToLocalTime();
    internal DateTime ToLocalTime(bool throwOnOverflow);
    public string ToLongDateString();
    public string ToLongTimeString();
    [__DynamicallyInvokable]
    public double ToOADate();
    public string ToShortDateString();
    public string ToShortTimeString();
    [__DynamicallyInvokable]
    public override string ToString();
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
    public string ToString(IFormatProvider provider);
    [__DynamicallyInvokable]
    public string ToString(string format);
    [__DynamicallyInvokable]
    public string ToString(string format, IFormatProvider provider);
    [__DynamicallyInvokable]
    public DateTime ToUniversalTime();
    internal static bool TryCreate(int year, int month, int day, int hour, int minute, int second, int millisecond, out DateTime result);
    [__DynamicallyInvokable]
    public static bool TryParse(string s, out DateTime result);
    [__DynamicallyInvokable]
    public static bool TryParse(string s, IFormatProvider provider, DateTimeStyles styles, out DateTime result);
    [__DynamicallyInvokable]
    public static bool TryParseExact(string s, string format, IFormatProvider provider, DateTimeStyles style, out DateTime result);
    [__DynamicallyInvokable]
    public static bool TryParseExact(string s, string[] formats, IFormatProvider provider, DateTimeStyles style, out DateTime result);

    // Properties
    [__DynamicallyInvokable]
    public DateTime Date { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int Day { [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] get; }
    [__DynamicallyInvokable]
    public DayOfWeek DayOfWeek { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int DayOfYear { [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] get; }
    [__DynamicallyInvokable]
    public int Hour { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    private ulong InternalKind { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")] get; }
    internal long InternalTicks { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")] get; }
    [__DynamicallyInvokable]
    public DateTimeKind Kind { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int Millisecond { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int Minute { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int Month { [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] get; }
    [__DynamicallyInvokable]
    public static DateTime Now { [__DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int Second { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public long Ticks { [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] get; }
    [__DynamicallyInvokable]
    public TimeSpan TimeOfDay { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public static DateTime Today { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public static DateTime UtcNow { [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), SecuritySafeCritical, __DynamicallyInvokable] get; }
    [__DynamicallyInvokable]
    public int Year { [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] get; }
}

 

 

二、了解DateTime结构体

转到DateTime的定义，也可以看到DateTime确实是struct类型，

 

看来前面的试验都是无用功了，但DateTime到底是怎样的结构体呢，主要有如下成员

 

这些成员里面最关键的是Ticks

Ticks的时间值以 100 毫微秒为单位，它的64位中，前两位表示Kind，后面62位表示有多少个毫微秒。Kind用来指示 DateTime 结构是表示本地时间、协调通用时间 (UTC) 还是 UTC 和本地时间都未指定。Kind 字段用于处理本地时间和 UTC 时间之间的转换。

DateTime的值范围在0001/1/1 00:00:00到9999/12/31 23:59:59之间

三、DateTime.Now和DateTime.UtcNow是怎么计算出来的

反编译DateTime.Now如下：

[__DynamicallyInvokable]
public static DateTime Now
{
    [__DynamicallyInvokable]
    get
    {
        DateTime utcNow = UtcNow;
        bool isAmbiguousLocalDst = false;
        long ticks = TimeZoneInfo.GetDateTimeNowUtcOffsetFromUtc(utcNow, out isAmbiguousLocalDst).Ticks;
        long num2 = utcNow.Ticks + ticks;
        if (num2 > 0x2bca2875f4373fffL)
        {
            return new DateTime(0x2bca2875f4373fffL, DateTimeKind.Local);
        }
        if (num2 < 0L)
        {
            return new DateTime(0L, DateTimeKind.Local);
        }
        return new DateTime(num2, DateTimeKind.Local, isAmbiguousLocalDst);
    }
}
 

 

大致的意思是先获取DateTime.UtcNow，然后转换为本地时间

再反编译DateTime.UtcNow：

[__DynamicallyInvokable]
public static DateTime UtcNow =>
    new DateTime((ulong) ((GetSystemTimeAsFileTime() + 0x701ce1722770000L) | 0x4000000000000000L));
 

 

最后查阅GetSystemTimeAsFileTime()是Windows的API，作用便是获取当前的UTC时间