c# – 调用Windows API时CLR的速度比我快

使用P / Invoke调用Windows的GetSystemTimeAsFileTime比调用DateTime.UtcNow慢约3倍,内部使用CLR的包装器来获取相同的GetSystemTimeAsFileTime.

怎么可能？

这是DateTime.UtcNow‘s implementation：

public static DateTime UtcNow {
    get {
        long ticks = 0;
        ticks = GetSystemTimeAsFileTime();
        return new DateTime( ((UInt64)(ticks + FileTimeOffset)) | KindUtc);
    }
}

[MethodImplAttribute(MethodImplOptions.InternalCall)] // Implemented by the CLR
internal static extern long GetSystemTimeAsFileTime();

核心CLR的wrapper for GetSystemTimeAsFileTime：

FCIMPL0(INT64,SystemNative::__GetSystemTimeAsFileTime)
{
    FCALL_CONTRACT;

    INT64 timestamp;

    ::GetSystemTimeAsFileTime((FILETIME*)&timestamp);

#if BIGENDIAN
    timestamp = (INT64)(((UINT64)timestamp >> 32) | ((UINT64)timestamp << 32));
#endif

    return timestamp;
}
FCIMPLEND;

我的测试代码利用BenchmarkDotNet：

public class Program
{
    static void Main() => BenchmarkRunner.Run<Program>();

    [Benchmark]
    public DateTime UtcNow() => DateTime.UtcNow;

    [Benchmark]
    public long GetSystemTimeAsFileTime()
    {
        long fileTime;
        GetSystemTimeAsFileTime(out fileTime);
        return fileTime;
    }

    [DllImport("kernel32.dll")]
    public static extern void GetSystemTimeAsFileTime(out long systemTimeAsFileTime);
}

结果：

Method |     Median |    StdDev |
------------------------ |----------- |---------- |
 GetSystemTimeAsFileTime | 14.9161 ns | 1.0890 ns |
                  UtcNow |  4.9967 ns | 0.2788 ns |