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C#中Foreach循环遍历的本质与枚举器详解

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

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 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);

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<T>泛型类

[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。