面向接口编程，你考虑过性能吗？
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 -l000000229e3feda0 00007ff889e40951 *** WARNING: Unable to verify checksum for ConsoleApp2.exeConsoleApp2.Program.Main(System.String[]) [C:\dream\Csharp\ConsoleApp1\ConsoleApp2\Program.cs @ 32] LOCALS: 0x000000229e3fede8 = 0x0000019bf33b9a88 0x000000229e3fede0 = 0x0000019be33b2d90 0x000000229e3fedfc = 0x00000000004c4b400:000> !do 0x0000019be33b2d90Name: System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]]MethodTable: 00007ff8e8d36d18EEClass: 00007ff8e7cf5640Size: 32(0x20) bytesFile: C:\WINDOWS\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dllFields: MT Field Offset Type VT Attr Value Name00007ff8e7a98538 4002ffe 8 System.Int32[] 0 instance 0000019bf33b9a88 _array00007ff8e7a985a0 4002fff 10 System.Int32 1 instance 5000000 _index00007ff8e7a985a0 4003000 14 System.Int32 1 instance 5000000 _endIndex00007ff8e8d36d18 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 00007ff8e8d36d18EEClass: 00007ff8e7cf5640Module: 00007ff8e7a71000Name: System.SZArrayHelper+SZGenericArrayEnumerator`1[[System.Int32, mscorlib]]mdToken: 0000000002000a98File: C:\WINDOWS\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dllBaseSize: 0x20ComponentSize: 0x0Slots in VTable: 11Number of IFaces in IFaceMap: 3--------------------------------------MethodDesc Table Entry MethodDesc JIT Name00007ff8e7ff2450 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::Main2. 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:10i=1,时间:10i=2,时间:10i=3,时间:9i=4,时间:9i=5,时间:9i=6,时间:10i=7,时间:10i=8,时间:12i=9,时间:12---------------i=0,时间:45i=1,时间:37i=2,时间:35i=3,时间:35i=4,时间:37i=5,时间:35i=6,时间:36i=7,时间:37i=8,时间:35i=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。
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