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:
1 .class private auto ansi beforefieldinit Program 2 extends
[mscorlib]System.Object 3 { 4 .method public hidebysig specialname
rtspecialname instancevoid .ctor() cil managed 5 { 6 .maxstack 8 7 L_0000:
ldarg.0 8 L_0001: call instance void [mscorlib]System.Object::.ctor() 9
L_0006: ret10 } 11 12 .method private hidebysig static void Main(string[]
args) cil managed13 { 14 .entrypoint 15 .maxstack 2 16 .locals init ( 17 [0]
class [mscorlib]System.Collections.Generic.List`1<string> list, 18 [1] string
str,19 [2] class [mscorlib]System.Collections.Generic.List`1<string> list2, 20 [
3] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator`0<string>
enumerator,21 [4] bool flag) 22 L_0000: nop 23 L_0001: newobj instance void
[mscorlib]System.Collections.Generic.List`1<string>::.ctor() 24 L_0006: stloc.2
25 L_0007: ldloc.2 26 L_0008: ldstr "\u5f20\u4e09" 27 L_000d: callvirt instance
void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0) 28 L_0012:
nop29 L_0013: ldloc.2 30 L_0014: ldstr "\u674e\u56db" 31 L_0019: callvirt
instancevoid [mscorlib]System.Collections.Generic.List`1<string>::Add(!0) 32
L_001e: nop33 L_001f: ldloc.2 34 L_0020: ldstr "\u738b\u4e94" 35 L_0025:
callvirt instancevoid [mscorlib]System.Collections.Generic.List`1<string>::Add(!
0) 36 L_002a: nop 37 L_002b: ldloc.2 38 L_002c: stloc.0 39 L_002d: nop 40
L_002e: ldloc.0 41 L_002f: callvirt instance valuetype
[mscorlib]System.Collections.Generic.List`1/Enumerator`0<!0>
[mscorlib]System.Collections.Generic.List`1<string>::GetEnumerator() 42 L_0034:
stloc.3 43 L_0035: br.s L_0048 44 L_0037: ldloca.s enumerator 45 L_0039: call
instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator`0<string>
::get_Current()46 L_003e: stloc.1 47 L_003f: nop 48 L_0040: ldloc.1 49 L_0041:
callvoid [mscorlib]System.Console::WriteLine(string) 50 L_0046: nop 51
L_0047: nop52 L_0048: ldloca.s enumerator 53 L_004a: call instance bool
[mscorlib]System.Collections.Generic.List`1/Enumerator`0<string>::MoveNext() 54
L_004f: stloc.s flag55 L_0051: ldloc.s flag 56 L_0053: brtrue.s L_0037 57
L_0055: leave.s L_006658 L_0057: ldloca.s enumerator 59 L_0059: constrained.
[mscorlib]System.Collections.Generic.List`1/Enumerator`0<string> 60 L_005f:
callvirt instancevoid [mscorlib]System.IDisposable::Dispose() 61 L_0064: nop 62
L_0065: endfinally63 L_0066: nop 64 L_0067: call valuetype
[mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()65 L_006c:
pop66 L_006d: ret 67 .try L_0035 to L_0057 finally handler L_0057 to L_0066 68
}69 } View Code
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。

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。