IL 约束调用

Question

对于此代码：

class Program
{
    static void Main()
    {
        Console.WriteLine(new MyStruct().ToString());
    }

    struct MyStruct { }
}

C# 编译器生成 constrained callvirt IL 代码。

This文章说：

For example, if a value type V overrides the Object.ToString() method, a call V.ToString() instruction is emitted; if it does not, a box instruction and a callvirt Object.ToString() instruction are emitted. A versioning problem can arise <...> if an override is later added.

所以，我的问题是：如果编译器将生成 box 代码而不是约束调用，为什么在这种情况下会出现问题？

Answer 1

box 指令创建了相关实例的副本。允许值类型的实例方法修改调用它们的实例，如果这样做，则在副本上静默调用方法是错误的做法。

static class Program
{
    static void Main()
    {
        var myStruct = new MyStruct();
        Console.WriteLine(myStruct.i); // prints 0
        Console.WriteLine(myStruct.ToString()); // modifies myStruct, not a copy of myStruct
        Console.WriteLine(myStruct.i); // prints 1
    }

    struct MyStruct {
        public int i;
        public override string ToString() {
            i = 1;
            return base.ToString();
        }
    }
}

IL 约束调用

IL constrained call

c#

cil

boxing