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 = ;
     private const int DatePartDayOfYear = ;
     private const int DatePartMonth = ;
     private const int DatePartYear = ;
     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