我写了两个等价的方法：
static bool F<T>(T a, T b) where T : class
{
return a == b;
}
static bool F2(A a, A b)
{
return a == b;
}
时间差：
00:00:00.0380022
00:00:00.0170009
测试代码：
var a = new A();
for (int i = 0; i < 100000000; i++)
F<A>(a, a);
Console.WriteLine(DateTime.Now - dt);
dt = DateTime.Now;
for (int i = 0; i < 100000000; i++)
F2(a, a);
Console.WriteLine(DateTime.Now - dt);
有人知道为什么吗？
在下面的评论中，dtb* 展示了 CIL：
IL for F2: ldarg.0, ldarg.1, ceq, ret. IL for F<T>: ldarg.0, box !!T, ldarg.1, box !!T, ceq, ret.
我认为这是我的问题的答案，但是我可以使用什么魔法来拒绝装箱？
接下来我使用来自Psilon的代码：
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
namespace ConsoleApplication58
{
internal class Program
{
private class A
{
}
private static bool F<T>(T a, T b) where T : class
{
return a == b;
}
private static bool F2(A a, A b)
{
return a == b;
}
private static void Main()
{
const int rounds = 100, n = 10000000;
var a = new A();
var fList = new List<TimeSpan>();
var f2List = new List<TimeSpan>();
for (int i = 0; i < rounds; i++)
{
// Test generic
GCClear();
bool res;
var sw = new Stopwatch();
sw.Start();
for (int j = 0; j < n; j++)
{
res = F(a, a);
}
sw.Stop();
fList.Add(sw.Elapsed);
// Test not-generic
GCClear();
bool res2;
var sw2 = new Stopwatch();
sw2.Start();
for (int j = 0; j < n; j++)
{
res2 = F2(a, a);
}
sw2.Stop();
f2List.Add(sw2.Elapsed);
}
double f1AverageTicks = fList.Average(ts => ts.Ticks);
Console.WriteLine("Elapsed for F = {0} \t ticks = {1}", fList.Average(ts => ts.TotalMilliseconds),
f1AverageTicks);
double f2AverageTicks = f2List.Average(ts => ts.Ticks);
Console.WriteLine("Elapsed for F2 = {0} \t ticks = {1}", f2List.Average(ts => ts.TotalMilliseconds),
f2AverageTicks);
Console.WriteLine("Not-generic method is {0} times faster, or on {1}%", f1AverageTicks/f2AverageTicks,
(f1AverageTicks/f2AverageTicks - 1)*100);
Console.ReadKey();
}
private static void GCClear()
{
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
}
}
}
不附加发布优化版，使用 Windows 7、.NET 4.5 和 Visual Studio 2012。
x64
Elapsed for F = 23.68157 ticks = 236815.7
Elapsed for F2 = 1.701638 ticks = 17016.38
Not-generic method is 13.916925926666 times faster, or on 1291.6925926666%
x86
Elapsed for F = 6.713223 ticks = 67132.23
Elapsed for F2 = 6.729897 ticks = 67298.97
Not-generic method is 0.997522398931217 times faster, or on -0.247760106878314%
我有了新的魔法：x64比之前快三倍...
附注：我的目标平台是x64。
ldarg.0, ldarg.1, ceq, ret。F<T>的指令码为：ldarg.0, box !!T, ldarg.1, box !!T, ceq, ret。那么答案就是因为参数被装箱了。但是为什么会被装箱呢？ - dtb