C# Foreach循环本质与枚举器

首先创建一个控制台应用程序用来探究foreach的本质

在Program类中写一个foreach循环。

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace ForeachTest{    class Program    {        static void Main(string[] args)        {            List<string> peopleList = new List<string>() { "张三", "李四", "王五" };            foreach (string people in peopleList)            {                Console.WriteLine(people);            }            Console.ReadKey();        }    }}

然后生成项目将项目debug目录下的ForeachTest.exe程序集用Reflection反编译后查看Program类IL代码

.class private auto ansi beforefieldinit Program    extends [mscorlib]System.Object{    .method public hidebysig specialname rtspecialname instance void .ctor() cil managed    {        .maxstack 8        L_0000: ldarg.0         L_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<string> list,            [1] string str,            [2] class [mscorlib]System.Collections.Generic.List`1<string> list2,            [3] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator`0<string> enumerator,            [4] bool flag)        L_0000: nop         L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor()        L_0006: stloc.2         L_0007: ldloc.2         L_0008: ldstr "\u5f20\u4e09"        L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)        L_0012: nop         L_0013: ldloc.2         L_0014: ldstr "\u674e\u56db"        L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)        L_001e: nop         L_001f: ldloc.2         L_0020: ldstr "\u738b\u4e94"        L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)        L_002a: nop         L_002b: ldloc.2         L_002c: stloc.0         L_002d: nop         L_002e: ldloc.0         L_002f: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator`0<!0> [mscorlib]System.Collections.Generic.List`1<string>::GetEnumerator()        L_0034: stloc.3         L_0035: br.s L_0048        L_0037: ldloca.s enumerator        L_0039: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator`0<string>::get_Current()        L_003e: stloc.1         L_003f: nop         L_0040: ldloc.1         L_0041: call void [mscorlib]System.Console::WriteLine(string)        L_0046: nop         L_0047: nop         L_0048: ldloca.s enumerator        L_004a: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator`0<string>::MoveNext()        L_004f: stloc.s flag        L_0051: ldloc.s flag        L_0053: brtrue.s L_0037        L_0055: leave.s L_0066        L_0057: ldloca.s enumerator        L_0059: constrained. [mscorlib]System.Collections.Generic.List`1/Enumerator`0<string>        L_005f: callvirt instance void [mscorlib]System.IDisposable::Dispose()        L_0064: nop         L_0065: endfinally         L_0066: nop         L_0067: call valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()        L_006c: pop         L_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<T>泛型类，发现GetEnumerator是接口IEnumerable 的方法，方法里面代码如下即使返回一个Enumerator泛型类，然后传入的参数是List泛型自己 this。

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

接下来查看 Enumerator<T>类

[Serializable, StructLayout(LayoutKind.Sequential)]public struct Enumerator : IEnumerator<T>, IDisposable, IEnumerator{    private List<T> list;    private int index;    private int version;    private T current;    internal Enumerator(List<T> list)    {        this.list = list;        this.index = 0;        this.version = list._version;        this.current = default(T);    }    public void Dispose()    {    }    public bool MoveNext()    {        List<T> 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<T>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，这个实例即使枚举器，枚举list里面的所有元素。通过MoveNext方法移动下标来查找下一个，get_Current方法获取当前查找到的元素。Reset方法重置下标。

于是我们可以写下面的代码来遍历list里面的值。

总结：foreach只是语法糖，封装了循环遍历简化代码，实际是使用一个枚举器类实现的。所以如果我们想自定义的集合类能够使用foreach遍历则：类实现接口IEnumerable，自定义一个枚举器类实现接口IEnumerator。