如果foreach是一个结构数组,它是否会复制每个元素？

Question

我有一系列结构.foreach在通过数组进行迭代时,运算符是否会复制每个元素？据我所知foreach,只是引擎盖下的语法糖转换为for.所以似乎答案是肯定的,但我很乐意得到一些确认.

PS:似乎有人应该已经问过但我不能轻易找到任何东西.所以请以提供的参考形式投票.

Answer 1

是的,将进行值类型实例的副本.迭代数组时,foreach确实会使用数组访问而不是使用枚举器,但仍会复制每个数组槽中的值.

这段代码:

struct AStruct
{
    public string a;
    public int b;

    static void Main()
    {
        var structs = new AStruct[10];

        foreach (var x in structs) {
            Console.WriteLine(x);
        }
    }
}

为该Main()方法生成以下IL :

.method private static hidebysig
       default void Main ()  cil managed
{
    .entrypoint
    .maxstack 4
    .locals init (
            valuetype AStruct[]     V_0,
            valuetype AStruct[]     V_1,
            int32   V_2,
            valuetype AStruct       V_3)
    IL_0000:  ldc.i4.s 0x0a
    IL_0002:  newarr AStruct
    IL_0007:  stloc.0
    IL_0008:  ldloc.0
    IL_0009:  stloc.1
    IL_000a:  ldc.i4.0
    IL_000b:  stloc.2
    IL_000c:  br IL_002d

    IL_0011:  ldloc.1
    IL_0012:  ldloc.2
    IL_0013:  ldelema AStruct
    IL_0018:  ldobj AStruct
    IL_001d:  stloc.3
    IL_001e:  ldloc.3
    IL_001f:  box AStruct
    IL_0024:  call void class [mscorlib]System.Console::WriteLine(object)
    IL_0029:  ldloc.2
    IL_002a:  ldc.i4.1
    IL_002b:  add
    IL_002c:  stloc.2
    IL_002d:  ldloc.2
    IL_002e:  ldloc.1
    IL_002f:  ldlen
    IL_0030:  conv.i4
    IL_0031:  blt IL_0011

    IL_0036:  ret
} // end of method AStruct::Main

请注意IL_0013到IL_001d的说明.每个数组槽的整个值被压入堆栈并存储在本地V_3(x迭代变量)中.