通用明确的转换失败C＃ [英] Generic explicit cast failure C#

问题描述

我已经用下面这段代码的一些问题。我想明确的字符串到一个对象，这是工作完全正常，但是，如果这个对象是一个通用类的一部分，这是一个具有以下错误异常失败：无法投类型的对象'System.String'键入test.B'。 。即使我重载方法

I've some issues with the following piece of code. I would like to explicit a string to an object, this is working perfectly fine, however, if this object is part of a generic class, this is failing with the following error exception: "Unable to cast object of type 'System.String' to type 'test.B'". Even though I've overloaded the the method.

using System;
using System.Collections.Generic;

namespace test {
    class Program {
        static void Main(string [] args) {
            // These two cast perfectly fine.
            B x = (B) "abc";
            C y = (C) "def";

            A <B> a = new A<B>();
            a.b();
            A <C> b = new A<C>();
            b.b();
        }
    }

    class A<T> {
        public List <T> a = new List<T>();

        public void b() {
            // Unable to cast object of type 'System.String' to type 'test.B'
            this.a.Add ((T) (object) "abc"); 
            this.a.Add ((T) (object) "def");
            this.a.Add ((T) (object) "ghi");
        }
    }

    class B {
        public string b;

        public static explicit operator B(string a) {
            B x = new B();
            x.b = a;
            return x;
        }
    }

    class C {
        public string c;

        public static explicit operator C(string a) {
            C x = new C();
            x.c = a;
            return x;
        }
    }
}

这将是真棒，如果有人为可这是为什么不正确铸件向我解释。

It would be awesome if somone could explain to me why this isn't casting properly.

感谢

推荐答案

转换运营商，只有当类型是静态已知适用;毕竟，通用的方法，需要使用完全一样的IL 每 T - 所以它不能打电话给你的运营商在某些情况下，和类型检查他人。

Conversion operators only apply when the type is known statically; after all, generic methods need to use the exact same IL for every T - so it can't call your operator in some cases, and a type-check in others.

此外，因为你的明确投地对象，将绝不会中使用;从对象是总是的一个简单的拆箱和类型检查。

Additionally, because you've explicitly cast to object, it will never be used; a cast from object is always a simple unbox or type check.

这是邪恶的铸造修复将是（我不喜欢这个）：

An evil fix would be (and I don't like this):

        this.a.Add((T)(dynamic)"abc");
        this.a.Add((T)(dynamic)"def");
        this.a.Add((T)(dynamic)"ghi");

会推迟分辨率运行时。它的工作原理，但我需要后洗我的眼睛。更一般地，虽然：运营商和仿制药的没有发挥很好的 - 所以：尽量不要使用该组合你的API中。 我的真正的将不会使用上面，个人！

which defers the resolution to runtime. It works, but I would need to wash my eyes after that. More generally, though: operators and generics do not play nicely - so: try not to use that combination in your API. I really wouldn't use the above, personally!

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