C#中Foreach循环遍历的本质与枚举器详解
作者:SpringSun
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
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。
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。
到此这篇关于C#中Foreach循环遍历本质与枚举器的文章就介绍到这了,更多相关C# Foreach循环与枚举器内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!