访问struct字段复制整个struct？

Question

我知道，将structs作为参数传递给方法，返回它们，或尝试将struct值赋值给变量，都会创建整个struct的副本。

现在，看看下面假设的代码：

//Regions is an array of a struct type. Color and r/g/b are structs too.

if(Regions[0].Color.r == Regions[0].Color.g && Regions[0].Color.b != 0){
  .
  ..      
}

要获得r/g/b值，遮罩下面会发生什么？只将r/g/b值复制到内存位置或整个区域三次？

Answer 1

区域是一个结构数组。我做了一个快速的例子来检查生成的IL：

public class Program
{

    public static void Main()
    {
        Foo[] Regions = new Foo[10];

        Regions[0] = new Foo();


        if(Regions[0].Color.R == Regions[0].Color.G && Regions[0].Color.B != 0)
        {

        }


    }
}

public struct Foo
{
    public Color Color { set; get; }
}

正如您在生成的IL下面看到的那样，get_Color被调用了3次，因此颜色的值被复制了3次并被检索。

  .method public hidebysig static void  Main() cil managed
  {
    // 
    .maxstack  3
    .locals init (valuetype Foo[] V_0,
             valuetype [System.Drawing]System.Drawing.Color V_1,
             bool V_2)
    IL_0000:  nop
    IL_0001:  ldc.i4.s   10
    IL_0003:  newarr     Foo
    IL_0008:  stloc.0
    IL_0009:  ldloc.0
    IL_000a:  ldc.i4.0
    IL_000b:  ldelema    Foo
    IL_0010:  initobj    Foo
    IL_0016:  ldloc.0
    IL_0017:  ldc.i4.0
    IL_0018:  ldelema    Foo
    IL_001d:  call       instance valuetype [System.Drawing]System.Drawing.Color Foo::get_Color()
    IL_0022:  stloc.1
    IL_0023:  ldloca.s   V_1
    IL_0025:  call       instance uint8 [System.Drawing]System.Drawing.Color::get_R()
    IL_002a:  ldloc.0
    IL_002b:  ldc.i4.0
    IL_002c:  ldelema    Foo
    IL_0031:  call       instance valuetype [System.Drawing]System.Drawing.Color Foo::get_Color()
    IL_0036:  stloc.1
    IL_0037:  ldloca.s   V_1
    IL_0039:  call       instance uint8 [System.Drawing]System.Drawing.Color::get_G()
    IL_003e:  bne.un.s   IL_0059

    IL_0040:  ldloc.0
    IL_0041:  ldc.i4.0
    IL_0042:  ldelema    Foo
    IL_0047:  call       instance valuetype [System.Drawing]System.Drawing.Color Foo::get_Color()
    IL_004c:  stloc.1
    IL_004d:  ldloca.s   V_1
    IL_004f:  call       instance uint8 [System.Drawing]System.Drawing.Color::get_B()
    IL_0054:  ldc.i4.0
    IL_0055:  ceq
    IL_0057:  br.s       IL_005a

    IL_0059:  ldc.i4.1
    IL_005a:  nop
    IL_005b:  stloc.2
    IL_005c:  ldloc.2
    IL_005d:  brtrue.s   IL_0061

    IL_005f:  nop
    IL_0060:  nop
    IL_0061:  ret
  } // end of method Program::Main