IL约束通话 [英] IL constrained call

问题描述

有关此代码：

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

    struct MyStruct { }
}

C＃编译器生成约束callvirt IL代码。

the C# compiler generates constrained callvirt IL code.

的这文章说：

例如，如果一个值V型覆盖Object.ToString（）方法，调用V.ToString（）指令发出的;如果不是这样，一个盒子指令和callvirt Object.ToString（）指令被发射。 。可能会出现一个版本问题< ...>如果一个覆盖后来加入

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：为什么会是一个问题，在这种情况下，如果编译器将生成一个箱代码，而不是一个约束的电话吗？

So, my question is: why would it be a problem in this case if the compiler will generate a box code, not a constrained call?

推荐答案

的箱指令可创建有问题的实例的副本。允许值类型的实例方法来修改他们是所谓的实例，如果他们这样做，静静地调用拷贝方法是错误的做法。

The box instruction creates a copy of the instance in question. Instance methods of value types are permitted to modify the instance they're called on, and if they do, silently calling the method on a copy is the wrong thing to do.

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();
        }
    }
}

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