C#中Foreach循环遍历的本质与枚举器详解 C#中Foreach循环遍历的本质与枚

前言
1、创建一个控制台应用程序
2、编写测试代码并分析
3、总结

前言对于C#里面的Foreach学过语言的人都知道怎么用，但是其原理相信很多人和我一样都没有去深究。刚回顾泛型讲到枚举器让我联想到了Foreach的实现，所以进行一番探究，有什么不对或者错误的地方大家多多斧正。

1、创建一个控制台应用程序

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2、编写测试代码并分析
在Program类中写一个foreach循环

class Program{static void Main(string[] args){List peopleList = new List() { "张三", "李四", "王五" }; foreach (string people in peopleList){Console.WriteLine(people); }Console.ReadKey(); }}

生成项目将项目编译后在debug目录下用Reflection反编译ForeachTest.exe程序集后查看Program类的IL代码，IL代码如下：

.class private auto ansi beforefieldinit Programextends [mscorlib]System.Object{.method public hidebysig specialname rtspecialname instance void .ctor() cil managed{.maxstack 8L_0000: ldarg.0L_0001: call instance void [mscorlib]System.Object::.ctor()L_0006: ret}.method private hidebysig static void Main(string[] args) cil managed{.entrypoint.maxstack 2.locals init ([0] class [mscorlib]System.Collections.Generic.List`1 list,[1] string str,[2] class [mscorlib]System.Collections.Generic.List`1 list2,[3] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator`0 enumerator,[4] bool flag)L_0000: nopL_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1::.ctor()L_0006: stloc.2L_0007: ldloc.2L_0008: ldstr "\u5f20\u4e09"L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)L_0012: nopL_0013: ldloc.2L_0014: ldstr "\u674e\u56db"L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)L_001e: nopL_001f: ldloc.2L_0020: ldstr "\u738b\u4e94"L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)L_002a: nopL_002b: ldloc.2L_002c: stloc.0L_002d: nopL_002e: ldloc.0L_002f: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator`0 [mscorlib]System.Collections.Generic.List`1::GetEnumerator()L_0034: stloc.3L_0035: br.s L_0048L_0037: ldloca.s enumeratorL_0039: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator`0::get_Current()L_003e: stloc.1L_003f: nopL_0040: ldloc.1L_0041: call void [mscorlib]System.Console::WriteLine(string)L_0046: nopL_0047: nopL_0048: ldloca.s enumeratorL_004a: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator`0::MoveNext()L_004f: stloc.s flagL_0051: ldloc.s flagL_0053: brtrue.s L_0037L_0055: leave.s L_0066L_0057: ldloca.s enumeratorL_0059: constrained. [mscorlib]System.Collections.Generic.List`1/Enumerator`0L_005f: callvirt instance void [mscorlib]System.IDisposable::Dispose()L_0064: nopL_0065: endfinallyL_0066: nopL_0067: call valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()L_006c: popL_006d: ret.try L_0035 to L_0057 finally handler L_0057 to L_0066}}

在反编译的IL代码中我们看到除了构建List和其他输出，然后多了三个方法：GetEnumerator()，get_Current() ，MoveNext() ，于是通过反编译reflector查看List泛型类，在List里面找到GetEnumerator方法是继承自接口IEnumerable 的方法，List实现的GetEnumerator方法代码

public Enumerator GetEnumerator() => new Enumerator((List) this);

【C#中Foreach循环遍历的本质与枚举器详解】即返回一个Enumerator泛型类，然后传入的参数是List泛型自己 this。接下来查看 Enumerator泛型类

[Serializable, StructLayout(LayoutKind.Sequential)]public struct Enumerator : IEnumerator, IDisposable, IEnumerator{private List list; private int index; private int version; private T current; internal Enumerator(List list){this.list = list; this.index = 0; this.version = list._version; this.current = default(T); } public void Dispose(){} public bool MoveNext(){List list = this.list; if ((this.version == list._version) && (this.index < list._size)){this.current = list._items[this.index]; this.index++; return true; }return this.MoveNextRare(); } private bool MoveNextRare(){if (this.version != this.list._version){ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion); }this.index = this.list._size + 1; this.current = default(T); return false; } public T Current =>this.current; object IEnumerator.Current{get{if ((this.index == 0) || (this.index == (this.list._size + 1))){ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen); }return this.Current; }}void IEnumerator.Reset(){if (this.version != this.list._version){ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion); }this.index = 0; this.current = default(T); }}

我们看到这个Enumerator泛型类实现了接口IEnumerator的方法，也就是我们测试的ForeachTest程序集反编译后IL代码中出现的get_Current() ，MoveNext() 方法。所以foreach实际上是编译器编译后先调用GetEnumerator方法返回Enumerator的实例，这个实例即是一个枚举器实例。通过MoveNext方法移动下标来查找下一个list元素，get_Current方法获取当前查找到的元素，Reset方法是重置list。

3、总结
因此要使用Foreach遍历的对象是继承了IEnumerable接口然后实现GetEnumerator方法。返回的实体对象需要继承IEnumerator接口并实现相应的方法遍历对象。因此Foreach的另一种写法如下。
C#中Foreach循环遍历的本质与枚举器详解