C＃TimeSpan给出0作为TotalMilliseconds

C# TimeSpan gives 0 as TotalMilliseconds

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我试图"基准"两个几乎相同的循环。当我一个接一个地检查它们时，减法就不会打嗝，TimeSpan会返回正确的数字作为总毫秒数。然而，当我将两个循环放在一起进行基准测试时，第一个减法返回一个介于3-4之间的数字(应该是正确的)，第二个总是返回0。我做错什么了？谢谢！

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class Program
{
static void Main(string[] args)
{
Console.WriteLine(":: Simple for ::");

var a = 0;
var start = DateTime.Now;

for (int i = 0; i < 10; i++)
{
a += i;
}

Console.WriteLine("Elapsed time: {0} ms", (DateTime.Now - start).TotalMilliseconds); // 3 - 4 ms
Console.WriteLine(":: Fancy for ::");

a = 0;
start = DateTime.Now;

foreach (var i in Enumerable.Range(0, 9))
{
a += i;
}

Console.WriteLine("Elapsed time: {0} ms", (DateTime.Now - start).TotalMilliseconds); // 0 ms
}
}

相关讨论

从文档：

The resolution of this property depends on the system timer, which is approximately 15 milliseconds on Windows systems.As a result, repeated calls to the Now property in a short time interval, such as in a loop, may return the same value.

The Now property is frequently used to measure performance. However, because of its low resolution, it is not suitable for use as a benchmarking tool. A better alternative is to use the Stopwatch class.

尝试使用秒表类是专门设计为基准：

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Stopwatch timer = new Stopwatch();

timer.Start();

for (int i = 0; i < 10; i++)
{
a += i;
}

timer.Stop();

Console.WriteLine($"Elapsed time: {timer.Elapsed}");

结果(。NET 4.6 IA-64，酷睿i7 3.2 GHz)

1	00:00:00.0000003

请注意，这是约300纳米秒的时间