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
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