有没有办法确定DateTime.Now的两个刻度之间的最小经过时间？我可以用秒表计时,但是有一些方法可以让操作系统在.NET中报告这个吗？
换句话说 - DateTime.Now有多准确？
更新
这很重要,因为我正在编写一个DateTimePrecision类,它存储DateTime.Now,来自StopWatch的滴答,以及秒表每秒钟的常数.在两个DateTimePrecision值之间计算TimeSpan时,如果差异小于DateTime.Now的量程,则使用秒表,否则使用DateTime.Now.
更新
DateTime如果你要比较两次,有几个人已经质疑为什么必须使用它.
原因是它StopWatch会在数小时或数天内缓慢漂移,直到你的时间为秒.
我通过存储两个校正该DateTime和Stopwatch中的结构,并且使用度量来返回一个这是目前或者最精确的,或最准确的.
这样我有:
在我的计时器类中,如果比较两次以计算增量,则切换点由考虑的度量确定System.Runtime.IOThreadTimer.GetSystemTimeResolution().
A和B两次之间的增量绝对误差:
System.Runtime.IOThreadTimer.GetSystemTimeResolution()是几十毫秒.再次,Stopwatch如果你在很多天里测量,依靠它本身将导致几秒钟的增量.
更新
如果您自己实现这一点,请确保您确定也存储Stopwatch.Frequency在结构中.如果您更换硬件或更改操作系统,这可能会改变.如果您持久保存DateTimePrecision实例,则无法知道刻度线的进入速度,您将无法在另一台计算机上读取它们.
这是我到目前为止的测试代码:
using System;
using System.Diagnostics;
using System.IO;
using MyLogType;
using ProtoBuf;
namespace DateTimePrecisionNamespace
{
/// <summary>
/// This class returns a precision time.
/// This class combines the best of both worlds, both precision and accuracy.
/// - It contains a DateTime.Now, which gives us good absolute time during the day.
/// - It contains the output from a stopwatch, in ticks, which gives us good relative time during the day.
/// - It contains the ticks per second of the stopwatch, which means this time record is portable across multiple PC architectures,
/// and we can easily tell how accurate the original time was.
/// Class always deals in non-UTC, this is a design decision as we are working with Eurex and we want to reduce the possibility of errors.
/// Class is serialized using Google Protocol Buffers, so any files created by this serializer are highly compatible, and can be read by:
/// - Future versions of .NET (ISerializable is only guaranteed to support the current version of .NET).
/// - Other .NET languages such as Python .NET, etc.
/// - Other non-.NET languages such as C++, Java, Python, etc.
/// - Other hardware platforms such as Linux, Mac, etc.
/// - Other endians (what if the platform is BigEndian or LittleEndian?).
/// - Future versions of the struct which may add additional fields or change existing fields
/// (the numbering of elements means its backwards and fowards compatible without necessarily breaking anything).
/// </summary>
[ProtoContract] // Serializable with Google Protocol Buffers, see protobuf-net.
public struct MyDateTimePrecision : IEquatable<MyDateTimePrecision>
{
[ProtoMember(1)]
public DateTime MyDateTime;
// Debug: display the expiration date as a string.
public string MyDateTimeAsString { get { return MyDateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffffff"); } }
public long MyDateTimeAsTicks { get { return MyDateTime.Ticks; } }
[ProtoMember(2)]
public int StopwatchTicksPerSecondConstant;
[ProtoMember(3)]
public long StopwatchTicksSinceProgramStart;
public MyDateTimePrecision(DateTime myDateTime, Int64 stopwatchTicksSinceProgramStart)
{
MyDateTime = myDateTime;
// This is always a constant. We need to embed this metric in the timestamp so this is portable to different PC hardware in the future.
StopwatchTicksPerSecondConstant = MyDateTimePrecisionStatic.MyGetStopwatchTicksPerSecondConstant;
StopwatchTicksSinceProgramStart = stopwatchTicksSinceProgramStart;
}
public MyDateTimePrecision(DateTime myDateTime, Int32 stopwatchTicksPerSecondConstant, Int64 stopwatchTicksSinceProgramStart)
{
MyDateTime = myDateTime;
// This is always a constant. We need to embed this metric in the timestamp so this is portable to different PC hardware in the future.
StopwatchTicksPerSecondConstant = stopwatchTicksPerSecondConstant;
StopwatchTicksSinceProgramStart = stopwatchTicksSinceProgramStart;
}
/// <summary>
/// Returns the current precision time.
/// </summary>
public static MyDateTimePrecision Now
{
get
{
return new MyDateTimePrecision(
MyDateTimePrecisionStatic.MyGetDateTime,
MyDateTimePrecisionStatic.MyGetStopwatchTicksPerSecondConstant,
MyDateTimePrecisionStatic.MyGetStopwatchTicksSinceProgramStart);
}
}
/// <summary>
/// Returns the current time, in ticks, since the program has started.
/// </summary>
public static long NowTicksSinceProgramStart
{
get { return MyDateTimePrecisionStatic.MyGetStopwatchTicksSinceProgramStart; }
}
/// <summary>
/// Returns the the amount of ticks per second, as a constant.
/// </summary>
public static long NowTicksPerSecondConstant
{
get { return MyDateTimePrecisionStatic.MyGetStopwatchTicksPerSecondConstant; }
}
/// <summary>
/// Returns the relative time, in seconds since the class was instantiated.
/// This method is only used to gauge the difference between two points in time, accurate to 300ns.
/// To get the absolute time, use DateTimeUtcAbsolute.
/// </summary>
public double SecondsRelative
{
get
{
return ((double)StopwatchTicksSinceProgramStart/StopwatchTicksPerSecondConstant);
}
}
#region Google Protocol Buffers serializer.
/// <summary>
/// Serialize using Google Protocol Buffers.
/// </summary>
public byte[] SerializeUsingProtobuf()
{
byte[] data;
using (var ms = new MemoryStream())
{
Serializer.Serialize(ms, this);
data = ms.ToArray();
}
return data;
}
#endregion
#region Google Protocol Buffers deserializer.
/// <summary>
/// Deserialize using Google Protocol Buffers.
/// </summary>
public static MyDateTimePrecision DeserializeUsingProtobuf(byte[] data)
{
MyDateTimePrecision result;
using (var ms = new MemoryStream(data))
{
result = Serializer.Deserialize<MyDateTimePrecision>(ms);
}
return result;
}
#endregion
#region SerializeUsingPointers
/// <summary>
/// Serialize using pointers, and raw binary format.
/// This method is blindingly fast, but not guaranteed to be compatible with anything other than the current version of the .NET runtime.
/// </summary>
public byte[] SerializeUsingPointers()
{
unsafe
{
const int bufferLength = 8+4+8;
byte[] buffer = new byte[bufferLength];
fixed (byte* constPointerToBufferStart = buffer)
{
byte* pointerToBuffer = constPointerToBufferStart;
(*(Int64*)pointerToBuffer) = this.MyDateTime.ToBinary();
pointerToBuffer += sizeof(Int64);
(*(Int32*) pointerToBuffer) = this.StopwatchTicksPerSecondConstant;
pointerToBuffer += sizeof(Int32);
(*(Int64*)pointerToBuffer) = this.StopwatchTicksSinceProgramStart;
#if UNITTEST
pointerToBuffer += sizeof(Int64);
if (pointerToBuffer - constPointerToBufferStart != bufferLength)
{
MyLog.LogFatalAndThrowAndExit("Error E20111004-1731. Buffer is not the expected length within SerializeUsingPointers.\n");
}
#endif
}
return buffer;
}
}
#endregion
/// <summary>
/// Deserialize using pointers.
/// This method is blindingly fast, but not guaranteed to be compatible with anything other than the current version of the .NET runtime.
/// </summary>
public static MyDateTimePrecision DeserializeUsingPointers(byte[] buffer)
{
MyDateTimePrecision result;
#if UNITTEST
const int bufferLength = 8 + 4 + 8;
#endif
unsafe
{
fixed (byte* constPointerToBufferStart = buffer)
{
byte* pointerToBuffer = constPointerToBufferStart;
result.MyDateTime = DateTime.FromBinary((*(Int64*)pointerToBuffer));
pointerToBuffer += sizeof(Int64);
result.StopwatchTicksPerSecondConstant = (*(Int32*)pointerToBuffer);
pointerToBuffer += sizeof(Int32);
result.StopwatchTicksSinceProgramStart = (*(Int64*)pointerToBuffer);
#if UNITTEST
pointerToBuffer += sizeof(Int64);
if ((pointerToBuffer - constPointerToBufferStart != buffer.Length) || (buffer.Length != bufferLength))
{
MyLog.LogFatalAndThrowAndExit("Error E20111004-1732. Buffer is not the expected length within DeserializeUsingPointers.\n");
}
#endif
}
}
return result;
}
/// <summary>
/// Checksum for the data contained in this structure, based on SerializeUsingPointers.
/// </summary>
/// <returns>Checksum.</returns>
public long ChecksumFromProtobuf()
{
return SerializeUsingProtobuf().MyToChecksum();
}
/// <summary>
/// Checksum for the data contained in this structure, based on XORing the contents of this structure.
/// </summary>
/// <returns>Checksum.</returns>
public long ChecksumFromXor()
{
return this.MyDateTime.Ticks
^ this.StopwatchTicksPerSecondConstant
^ this.StopwatchTicksSinceProgramStart;
}
/// <summary>
/// Indicates whether the current object is equal to another object of the same type.
/// </summary>
/// <returns>
/// True if the current object is equal to the <paramref name="other"/> parameter; otherwise, false.
/// </returns>
/// <param name="other">An object to compare with this object.</param>
public bool Equals(MyDateTimePrecision other)
{
return other.MyDateTime.Equals(MyDateTime) && other.StopwatchTicksPerSecondConstant == StopwatchTicksPerSecondConstant && other.StopwatchTicksSinceProgramStart == StopwatchTicksSinceProgramStart;
}
/// <summary>
/// Override operator == to compare two MyDateTimePrecision variables.
/// </summary>
/// <param name="c1">First MyDateTimePrecision.</param>
/// <param name="c2">Second MyDateTimePrecision.</param>
/// <returns>True if equal, false if not.</returns>
public static bool operator ==(MyDateTimePrecision c1, MyDateTimePrecision c2)
{
return c1.Equals(c2);
}
/// <summary>
/// Override operator != to compare two MyDateTimePrecision variables.
/// </summary>
/// <param name="c1">First MyDateTimePrecision.</param>
/// <param name="c2">Second MyDateTimePrecision.</param>
/// <returns>True if not equal, false if equal.</returns>
public static bool operator !=(MyDateTimePrecision c1, MyDateTimePrecision c2)
{
return !c1.Equals(c2);
}
/// <summary>
/// Print out both the absolute and the relative time.
/// Useful, as the debugger in Visual Studio displays this string when you observe the variable.
/// </summary>
/// <returns>The string.</returns>
public new string ToString()
{
return String.Format("Abs:{0:yyyy-MM-dd HH:mm:ss},Rel:{1:0.000000}sec", MyDateTime, SecondsRelative);
}
#region Unit test.
/// <summary>
/// Unit test for this entire class.
/// </summary>
/// <returns>False if there is no errors (false is the default for all new unit tests; saves typing).</returns>
public static bool Unit()
{
// Check serialization using pointers.
{
MyDateTimePrecision first = MyDateTimePrecision.Now;
MyDateTimePrecision second = first;
Debug.Assert(first == second);
{
byte[] sFirst = first.SerializeUsingPointers();
MyDateTimePrecision third = MyDateTimePrecision.DeserializeUsingPointers(sFirst);
Debug.Assert(first == third);
Debug.Assert(first.ChecksumFromProtobuf() == third.ChecksumFromProtobuf());
}
{
byte[] sFirst = first.SerializeUsingProtobuf();
MyDateTimePrecision third = MyDateTimePrecision.DeserializeUsingProtobuf(sFirst);
Debug.Assert(first == third);
Debug.Assert(first.ChecksumFromProtobuf() == third.ChecksumFromProtobuf());
}
{
try
{
byte[] sFirst = first.SerializeUsingProtobuf();
MyDateTimePrecision third = MyDateTimePrecision.DeserializeUsingPointers(sFirst);
// Program should never get to here as this should throw an error for an unknown buffer length.
Debug.Assert(true == false);
}
catch (Exception)
{
// Program should get to here.
Debug.Assert(true);
}
}
{
MyDateTimePrecision third = MyDateTimePrecision.Now;
Debug.Assert(first != third);
Debug.Assert(first.ChecksumFromProtobuf() != third.ChecksumFromProtobuf());
}
}
return false;
}
#endregion
#region Windows serializer.
/*
/// <summary>
/// Serialize this object into a string.
/// Observe that this method creates binary code that is only portable within the same version of .NET.
/// Recommend using a faster serializer that is language, hardware, and .NET version independent, such as Google Protocol Buffers (see protobuf-net).
/// </summary>
/// <returns></returns>
public string SerializeToString()
{
MyDateTimePrecision obj = this;
string result;
IFormatter formatter = new BinaryFormatter();
using (Stream stream = new MemoryStream())
{
formatter.Serialize(stream, obj);
result = stream.ToString();
}
return result;
}
/// <summary>
/// Serialize this object into a byte array.
/// Observe that this method creates binary code that is only portable within the same version of .NET.
/// Recommend using a faster that is language, hardware, and .NET version independent, such as Google Protocol Buffers (see protobuf-net).
/// </summary>
/// <returns></returns>
public byte[] SerializeToByteArray()
{
MyDateTimePrecision obj = this;
byte[] bytes;
IFormatter formatter = new BinaryFormatter();
using (MemoryStream stream = new MemoryStream())
{
formatter.Serialize(stream, obj);
bytes = stream.ToArray();
}
return bytes;
}
*/
#endregion
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj)) return false;
if (obj.GetType() != typeof (MyDateTimePrecision)) return false;
return Equals((MyDateTimePrecision) obj);
}
public override int GetHashCode()
{
unchecked
{
int result = MyDateTime.GetHashCode();
result = (result*397) ^ StopwatchTicksPerSecondConstant;
result = (result*397) ^ StopwatchTicksSinceProgramStart.GetHashCode();
return result;
}
}
}
/// <summary>
/// This class starts a stopwatch when the program starts. We can query this value in MyDateTimePrecision.
/// </summary>
static public class MyDateTimePrecisionStatic
{
/// <summary>
/// When this static class is instantiated for the first time (once on program start), start the stopwatch.
/// This stopwatch is accurate to 300ns, unlike DateTime which is accurate to only 30ms.
/// </summary>
private static readonly Stopwatch stopwatchSinceClassInstantiated;
static MyDateTimePrecisionStatic()
{
stopwatchSinceClassInstantiated = new Stopwatch();
stopwatchSinceClassInstantiated.Start();
}
/// <summary>
/// Return current time, non-UTC.
/// </summary>
public static DateTime MyGetDateTime
{
get { return DateTime.Now; }
}
/// <summary>
/// Return the number of ticks per second in the stopwatch.
/// </summary>
public static int MyGetStopwatchTicksPerSecondConstant
{
// We can safely downcast this to int. Typically its ~3.3 million on an Intel i7, its unlikely to get beyond int.Max on PC hardware anytime soon.
get { return (int)Stopwatch.Frequency; }
}
/// <summary>
/// Return the number of ticks since the program has started (or this static class has been instantiated).
/// </summary>
public static long MyGetStopwatchTicksSinceProgramStart
{
get { return stopwatchSinceClassInstantiated.ElapsedTicks; }
}
/// <summary>
/// Return timespan since the program has started (or this static class has been instantied).
/// </summary>
public static TimeSpan MyGetTimespanSinceProgramStart
{
get { return stopwatchSinceClassInstantiated.Elapsed; }
}
}
}
如果您关心精度等级,那么您不应该首先使用DateTime.Now.它对于人类规模的时间安排非常有用,例如"是时候回家看神秘博士吗？" 并且对于诸如"是否该重新同步此CRT上的栅格线？"这样的事情没有用处.
一个保守的假设是精确到不超过四分之一秒.它是否准确取决于您设置系统时钟的准确程度,以及您根据已知的准确时间来检查它的频率; 这与DateTime.Now的精度无关.
要真正回答您的问题:DateTime.Now的精度通常约为16毫秒,但这可能因机器而异.