大家在平时开发中大多都会遵循接口编程，这样就可以方便实现依赖注入也方便实现多态等各种小技巧，但这种是以牺牲性能为代价换取代码的灵活性，万物皆有阴阳，看你的应用场景进行取舍。
一：背景
1. 缘由
在项目的性能改造中，发现很多方法签名的返回值都是采用IEnumerable接口，比如下面这段代码:
public static void Main(string[] args)
{
var list = GetHasEmailCustomerIDList();
foreach (var item in list){}
Console.ReadLine();
}
public static IEnumerable<int> GetHasEmailCustomerIDList()
{
return Enumerable.Range(1, 5000000).ToArray();
}
2. 有什么问题
这段代码乍一看也没啥什么性能问题，foreach迭代天经地义，这个还能怎么优化？？？
<1> 从MSIL中寻找问题
首先我们尽可能把原貌还原出来，简化后的MSIL如下。
.method public hidebysig static
void Main (
string[] args
) cil managed
{
IL_0009: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<!0> class [mscorlib]System.Collections.Generic.IEnumerable`1<int32>::GetEnumerator()
IL_000e: stloc.1
.try
{
IL_000f: br.s IL_001a
// loop start (head: IL_001a)
IL_0011: ldloc.1
IL_0012: callvirt instance !0 class [mscorlib]System.Collections.Generic.IEnumerator`1<int32>::get_Current()
IL_0017: stloc.2
IL_0018: nop
IL_0019: nop
IL_001a: ldloc.1
IL_001b: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
IL_0020: brtrue.s IL_0011
// end loop
IL_0022: leave.s IL_002f
} // end .try
finally
{
IL_0024: ldloc.1
IL_0025: brfalse.s IL_002e
IL_0027: ldloc.1
IL_0028: callvirt instance void [mscorlib]System.IDisposable::Dispose()
IL_002d: nop
IL_002e: endfinally
} // end handler
IL_002f: ret
} // end of method Program::Main
从IL中看到了标准的get_Current,MoveNext,Dispose 还有一个try,finally，一下子多了这么多方法和关键词，不就是一个简单的foreach迭代数组嘛？至于搞的这么复杂嘛？这样在大数据下怎么快的起来？
还有一个奇葩的事，如果你仔细观察IL代码，比如这句：[mscorlib]System.Collections.Generic.IEnumerable``1<int32>::GetEnumerator(), 这个GetEnumerator前面是接口IEnumerable，正常情况下应该是具体迭代类吧，按理说应该会调用Array的GetEnumerator方法，如下所示。
[Serializable]
[ComVisible(true)]
[__DynamicallyInvokable]
public abstract class Array : ICloneable, IList, ICollection, IEnumerable, IStructuralComparable, IStructuralEquatable
{
[__DynamicallyInvokable]
public IEnumerator GetEnumerator()
{
int lowerBound = GetLowerBound(0);
if (Rank == 1 && lowerBound == 0)
{
return new SZArrayEnumerator(this);
}
return new ArrayEnumerator(this, lowerBound, Length);
}
}
<2> 从windbg中寻找问题
IL中发现的第二个问题我特别好奇，我们到托管堆上去看下到底是哪一个具体类调用了GetEnumerator()方法。
!clrstack -l > !do xx 到线程栈上抓list变量
0:000> !clrstack -l
000000229e3feda0 00007ff889e40951 *** WARNING: Unable to verify checksum for ConsoleApp2.exe
ConsoleApp2.Program.Main(System.String[]) [C:\dream\Csharp\ConsoleApp1\ConsoleApp2\Program.cs @ 32]
LOCALS:
0x000000229e3fede8 = 0x0000019bf33b9a88
0x000000229e3fede0 = 0x0000019be33b2d90
0x000000229e3fedfc = 0x00000000004c4b40
0:000> !do 0x0000019be33b2d90
Name: System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]]
MethodTable: 00007ff8e8d36d18
EEClass: 00007ff8e7cf5640
Size: 32(0x20) bytes
File: C:\WINDOWS\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll
Fields:
MT Field Offset Type VT Attr Value Name
00007ff8e7a98538 4002ffe 8 System.Int32[] 0 instance 0000019bf33b9a88 _array
00007ff8e7a985a0 4002fff 10 System.Int32 1 instance 5000000 _index
00007ff8e7a985a0 4003000 14 System.Int32 1 instance 5000000 _endIndex
00007ff8e8d36d18 4003001 0 ...Int32, mscorlib]] 0 shared static Empty
>> Domain:Value dynamic statics NYI 0000019be1893a80:NotInit <<
居然有这么一个类型 Name: System.SZArrayHelper+SZGenericArrayEnumerator，然来是JIT捣的鬼，生成了这么一个SZGenericArrayEnumerator类型，接下来把它的方法表打出来看看里面都有啥方法。
0:000> !dumpmt -md 00007ff8e8d36d18
EEClass: 00007ff8e7cf5640
Module: 00007ff8e7a71000
Name: System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]]
mdToken: 0000000002000a98
File: C:\WINDOWS\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll
BaseSize: 0x20
ComponentSize: 0x0
Slots in VTable: 11
Number of IFaces in IFaceMap: 3
--------------------------------------
MethodDesc Table
Entry MethodDesc JIT Name
00007ff8e7ff2450 00007ff8e7a78de8 PreJIT System.Object.ToString()
00007ff8e800cc60 00007ff8e7c3b9b0 PreJIT System.Object.Equals(System.Object)
00007ff8e7ff2090 00007ff8e7c3b9d8 PreJIT System.Object.GetHashCode()
00007ff8e7fef420 00007ff8e7c3b9e0 PreJIT System.Object.Finalize()
00007ff8e8b99fd0 00007ff8e7ebf388 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]].MoveNext()
00007ff8e8b99f90 00007ff8e7ebf390 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]].get_Current()
00007ff8e8b99f60 00007ff8e7ebf398 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]].System.Collections.IEnumerator.get_Current()
00007ff8e8b99f50 00007ff8e7ebf3a0 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]].System.Collections.IEnumerator.Reset()
00007ff8e8b99f40 00007ff8e7ebf3a8 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]].Dispose()
00007ff8e8b99ef0 00007ff8e7ebf3b0 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]]..cctor()
00007ff8e8b99ff0 00007ff8e7ebf380 PreJIT System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]]..ctor(Int32[], Int32)
可以看到这是一个标准的迭代类，这性能又被拖累了。。。
二：优化性能
综合上面分析，貌似问题出在了 foreach 和 IEnumerable<int>这两个方面。
1. IEnumerable 替换 int[]， foreach改成for
知道了这两点，接下来把代码修改如下：
public static void Main(string[] args)
{
var list = GetHasEmailCustomerIDList();
for (int i = 0; i < list.Length; i++) { }
Console.ReadLine();
}
public static int[] GetHasEmailCustomerIDList()
{
return Enumerable.Range(1, 5000000).ToArray();
}
.method public hidebysig static
void Main (
string[] args
) cil managed
{
// (no C# code)
IL_0000: nop
// int[] hasEmailCustomerIDList = GetHasEmailCustomerIDList();
IL_0001: call int32[] ConsoleApp2.Program::GetHasEmailCustomerIDList()
IL_0006: stloc.0
// for (int i = 0; i < hasEmailCustomerIDList.Length; i++)
IL_0007: ldc.i4.0
IL_0008: stloc.1
// (no C# code)
IL_0009: br.s IL_0011
// loop start (head: IL_0011)
IL_000b: nop
IL_000c: nop
// for (int i = 0; i < hasEmailCustomerIDList.Length; i++)
IL_000d: ldloc.1
IL_000e: ldc.i4.1
IL_000f: add
IL_0010: stloc.1
// for (int i = 0; i < hasEmailCustomerIDList.Length; i++)
IL_0011: ldloc.1
IL_0012: ldloc.0
IL_0013: ldlen
IL_0014: conv.i4
IL_0015: clt
IL_0017: stloc.2
IL_0018: ldloc.2
// (no C# code)
IL_0019: brtrue.s IL_000b
// end loop
// Console.ReadLine();
IL_001b: call string [mscorlib]System.Console::ReadLine()
// (no C# code)
IL_0020: pop
// }
IL_0021: ret
} // end of method Program::Main
可以看到上面的IL指令都是非常基础的指令，大多都有CPU指令直接提供支持，非常简洁，大爱~~~
这里有一点要注意：我后来观察foreach不需要改成for，vs编辑器在底层帮我们转换了，看的出来foreach在迭代数组类型的时候还是非常智能的，知道怎么帮助我们优化。。。修改代码如下：
public static void Main(string[] args)
{
var list = GetHasEmailCustomerIDList();
//for (int i = 0; i < list.Length; i++) { }
foreach (var item in list) { }
Console.ReadLine();
}
.method public hidebysig static
void Main (
string[] args
) cil managed
{
// (no C# code)
IL_0000: nop
// int[] hasEmailCustomerIDList = GetHasEmailCustomerIDList();
IL_0001: call int32[] ConsoleApp2.Program::GetHasEmailCustomerIDList()
IL_0006: stloc.0
// (no C# code)
IL_0007: nop
// int[] array = hasEmailCustomerIDList;
IL_0008: ldloc.0
IL_0009: stloc.1
// for (int i = 0; i < array.Length; i++)
IL_000a: ldc.i4.0
IL_000b: stloc.2
// (no C# code)
IL_000c: br.s IL_0018
// loop start (head: IL_0018)
// int num = array[i];
IL_000e: ldloc.1
IL_000f: ldloc.2
IL_0010: ldelem.i4
// (no C# code)
IL_0011: stloc.3
IL_0012: nop
IL_0013: nop
// for (int i = 0; i < array.Length; i++)
IL_0014: ldloc.2
IL_0015: ldc.i4.1
IL_0016: add
IL_0017: stloc.2
// for (int i = 0; i < array.Length; i++)
IL_0018: ldloc.2
IL_0019: ldloc.1
IL_001a: ldlen
IL_001b: conv.i4
IL_001c: blt.s IL_000e
// end loop
// Console.ReadLine();
IL_001e: call string [mscorlib]System.Console::ReadLine()
// (no C# code)
IL_0023: pop
// }
IL_0024: ret
} // end of method Program::Main
2. 代码测试
微观方面已经带大家分析过了，接下来宏观测试两种方式的性能到底相差多少，每一个方法我都做10次性能对比。
public static void Main(string[] args)
{
var arr = GetHasEmailCustomerIDArray();
for (int i = 0; i < 10; i++)
{
var watch = Stopwatch.StartNew();
foreach (var item in arr) { }
watch.Stop();
Console.WriteLine($"i={i},时间:{watch.ElapsedMilliseconds}");
}
Console.WriteLine("---------------");
var list = arr as IEnumerable<int>;
for (int i = 0; i < 10; i++)
{
var watch = Stopwatch.StartNew();
foreach (var item in list) { }
watch.Stop();
Console.WriteLine($"i={i},时间:{watch.ElapsedMilliseconds}");
}
Console.ReadLine();
}
public static int[] GetHasEmailCustomerIDArray()
{
return Enumerable.Range(1, 5000000).ToArray();
}
i=0,时间:10
i=1,时间:10
i=2,时间:10
i=3,时间:9
i=4,时间:9
i=5,时间:9
i=6,时间:10
i=7,时间:10
i=8,时间:12
i=9,时间:12
---------------
i=0,时间:45
i=1,时间:37
i=2,时间:35
i=3,时间:35
i=4,时间:37
i=5,时间:35
i=6,时间:36
i=7,时间:37
i=8,时间:35
i=9,时间:36
难以置信的是居然有3-4倍的差距。。。这就是用灵活性换取性能的代价好了，本篇就说到这里，希望对你有帮助。
