yield return Il代码讲解

反编译后，迭代器用的是状态机，栈本身就是状态机，由于协程本身也有栈， 我怀疑C#中的 迭代器和基于任务的异步编程是协程（未经过验证）

.class nested private auto ansi sealed beforefieldinit '<GetEnumerator>d__12'
    extends [System.Runtime]System.Object
    implements class [System.Runtime]System.Collections.Generic.IEnumerator`1<object>,
               [System.Runtime]System.Collections.IEnumerator,
               [System.Runtime]System.IDisposable

//这个位置相当于以上等于 private class  sealed <GetEnumerator>d__12：object，IEnumerator，IEnumerator，IDisposable
// nested ：嵌套    
//implements  ：实现接口
//extends：是继承父类，只要那个类不是声明为final或者那个类定义为abstract的就能继承，

beforefieldinit：beforefieldinit指示CLI在调用静态方法之前不必初始化类型。

{
    .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
        01 00 00 00
    )
自定义实例：.custom 声明，后跟类型的方法声明 构造函数
将 CompilerGeneratedAttribute 特性应用到任何应用程序元素，以指示该元素由编译器生成。

// Fields 字段 .field private int32 '<>1__state' .field private object '<>2__current' .field public class School.ClassOfStudent '<>4__this' .field private int32 '<index>5__1' // Methods 
//

    .method public hidebysig specialname rtspecialname
        instance void .ctor (
            int32 '<>1__state'
        ) cil managed 
//cil managed:托管代码
    {
        .custom instance void [System.Runtime]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
            01 00 00 00
        )
        // Method begins at RVA 0x2369
        // Code size 15 (0xf)
        .maxstack 8

        // {
        IL_0000: ldarg.0
        // (no C# code)
        IL_0001: call instance void [System.Runtime]System.Object::.ctor()
        // this.<>1__state = <>1__state;
        IL_0006: nop
        IL_0007: ldarg.0
        IL_0008: ldarg.1
        IL_0009: stfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<>1__state'
        // }
        IL_000e: ret
    } // end of method '<GetEnumerator>d__12'::.ctor

    .method private final hidebysig newslot virtual
        instance void System.IDisposable.Dispose () cil managed
    {
        .custom instance void [System.Runtime]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
            01 00 00 00
        )
        .override method instance void [System.Runtime]System.IDisposable::Dispose()
        // Method begins at RVA 0x2379
        // Code size 1 (0x1)
        .maxstack 8

        // }
        IL_0000: ret
    } // end of method '<GetEnumerator>d__12'::System.IDisposable.Dispose

    .method private final hidebysig newslot virtual
        instance bool MoveNext () cil managed
    {
        .override method instance bool [System.Runtime]System.Collections.IEnumerator::MoveNext()
        // Method begins at RVA 0x237c
        // Code size 133 (0x85)
        .maxstack 3
        .locals init (
            [0] int32,
            [1] int32,
            [2] bool
        )

        // switch (<>1__state)
        IL_0000: ldarg.0
        IL_0001: ldfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<>1__state'
        IL_0006: stloc.0
        // (no C# code)
        IL_0007: ldloc.0
        IL_0008: brfalse.s IL_0012

        IL_000a: br.s IL_000c

        IL_000c: ldloc.0
        IL_000d: ldc.i4.1
        IL_000e: beq.s IL_0014

        IL_0010: br.s IL_0016

        IL_0012: br.s IL_0018

        // return false;
        IL_0014: br.s IL_004f

        IL_0016: ldc.i4.0
        IL_0017: ret

        // <>1__state = -1;
        IL_0018: ldarg.0
        IL_0019: ldc.i4.m1
        IL_001a: stfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<>1__state'
        // <index>5__1 = 0;
        IL_001f: nop
        IL_0020: ldarg.0
        IL_0021: ldc.i4.0
        IL_0022: stfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<index>5__1'
        // break;
        IL_0027: br.s IL_0067

        // <>2__current = <>4__this.students[<index>5__1];
        IL_0029: nop
        IL_002a: ldarg.0
        IL_002b: ldarg.0
        IL_002c: ldfld class School.ClassOfStudent School.ClassOfStudent/'<GetEnumerator>d__12'::'<>4__this'
        IL_0031: ldfld class [System.Collections]System.Collections.Generic.List`1<class School.Student> School.ClassOfStudent::students
        IL_0036: ldarg.0
        IL_0037: ldfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<index>5__1'
        IL_003c: callvirt instance !0 class [System.Collections]System.Collections.Generic.List`1<class School.Student>::get_Item(int32)
        IL_0041: stfld object School.ClassOfStudent/'<GetEnumerator>d__12'::'<>2__current'
        // <>1__state = 1;
        IL_0046: ldarg.0
        IL_0047: ldc.i4.1
        IL_0048: stfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<>1__state'
        // return true;
        IL_004d: ldc.i4.1
        IL_004e: ret

        // <>1__state = -1;
        IL_004f: ldarg.0
        IL_0050: ldc.i4.m1
        IL_0051: stfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<>1__state'
        // <index>5__1++;
        IL_0056: nop
        IL_0057: ldarg.0
        IL_0058: ldfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<index>5__1'
        IL_005d: stloc.1
        // if (<index>5__1 < <>4__this.students.Count)
        IL_005e: ldarg.0
        IL_005f: ldloc.1
        IL_0060: ldc.i4.1
        IL_0061: add
        IL_0062: stfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<index>5__1'

        IL_0067: ldarg.0
        IL_0068: ldfld int32 School.ClassOfStudent/'<GetEnumerator>d__12'::'<index>5__1'
        IL_006d: ldarg.0
        IL_006e: ldfld class School.ClassOfStudent School.ClassOfStudent/'<GetEnumerator>d__12'::'<>4__this'
        IL_0073: ldfld class [System.Collections]System.Collections.Generic.List`1<class School.Student> School.ClassOfStudent::students
        IL_0078: callvirt instance int32 class [System.Collections]System.Collections.Generic.List`1<class School.Student>::get_Count()
        IL_007d: clt
        IL_007f: stloc.2
        IL_0080: ldloc.2
        IL_0081: brtrue.s IL_0029

        // return false;
        IL_0083: ldc.i4.0
        IL_0084: ret
    } // end of method '<GetEnumerator>d__12'::MoveNext

    .method private final hidebysig specialname newslot virtual
        instance object 'System.Collections.Generic.IEnumerator<System.Object>.get_Current' () cil managed
    {
        .custom instance void [System.Runtime]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
            01 00 00 00
        )
        .override method instance !0 class [System.Runtime]System.Collections.Generic.IEnumerator`1<object>::get_Current()
        // Method begins at RVA 0x240d
        // Code size 7 (0x7)
        .maxstack 8

        // return <>2__current;
        IL_0000: ldarg.0
        IL_0001: ldfld object School.ClassOfStudent/'<GetEnumerator>d__12'::'<>2__current'
        IL_0006: ret
    } // end of method '<GetEnumerator>d__12'::'System.Collections.Generic.IEnumerator<System.Object>.get_Current'

    .method private final hidebysig newslot virtual
        instance void System.Collections.IEnumerator.Reset () cil managed
    {
        .custom instance void [System.Runtime]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
            01 00 00 00
        )
        .override method instance void [System.Runtime]System.Collections.IEnumerator::Reset()
        // Method begins at RVA 0x2415
        // Code size 6 (0x6)
        .maxstack 8

        // throw new NotSupportedException();
        IL_0000: newobj instance void [System.Runtime]System.NotSupportedException::.ctor()
        IL_0005: throw
    } // end of method '<GetEnumerator>d__12'::System.Collections.IEnumerator.Reset

    .method private final hidebysig specialname newslot virtual
        instance object System.Collections.IEnumerator.get_Current () cil managed
    {
        .custom instance void [System.Runtime]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
            01 00 00 00
        )
        .override method instance object [System.Runtime]System.Collections.IEnumerator::get_Current()
        // Method begins at RVA 0x241c
        // Code size 7 (0x7)
        .maxstack 8

        // return <>2__current;
        IL_0000: ldarg.0
        IL_0001: ldfld object School.ClassOfStudent/'<GetEnumerator>d__12'::'<>2__current'
        IL_0006: ret
    } // end of method '<GetEnumerator>d__12'::System.Collections.IEnumerator.get_Current

    // Properties
    .property instance object 'System.Collections.Generic.IEnumerator<System.Object>.Current'()
    {
        .get instance object School.ClassOfStudent/'<GetEnumerator>d__12'::'System.Collections.Generic.IEnumerator<System.Object>.get_Current'()
    }
    .property instance object System.Collections.IEnumerator.Current()
    {
        .get instance object School.ClassOfStudent/'<GetEnumerator>d__12'::System.Collections.IEnumerator.get_Current()
    }

} // end of class <GetEnumerator>d__12