无法将类型“X"隐式转换为“字符串"——它何时以及如何确定它“不能"?

Cannot implicitly convert type #39;X#39; to #39;string#39; - when and how it decides that it quot;cannotquot;?(无法将类型“X隐式转换为“字符串——它何时以及如何确定它“不能?)

问题描述

现在我正在使用 Guids.

我当然记得，在整个代码中，这种隐式转换在某些地方有效，而在其他地方则无效.直到现在我都看不到这种模式.

I certainly remember that throughout the code in some places this implicit conversion works, in others it does not. Until now I fail to see the pattern.

编译器如何决定什么时候不能?我的意思是，类型方法 Guid.ToString() 是存在的，不是在需要这种转换时调用吗?

How the compiler decides when it cannot? I mean, the type method Guid.ToString() is present, isn't it called whenever this transformation is needed?

谁能告诉我这种转换是在什么情况下自动完成的，我什么时候必须显式调用 myInstance.ToString()?

Can someone please tell me under what circumstances this transformation is done automatically and when I have to call myInstance.ToString() explicitly?

推荐答案

简而言之，当定义了隐式或显式转换运算符时:

In short, when there is an implicit or explicit conversion operator defined:

class WithImplicit {
    public static implicit operator string(WithImplicit x) {
        return x.ToString();}
}
class WithExplicit {
    public static explicit operator string(WithExplicit x) {
        return x.ToString(); }
}
class WithNone { }

class Program {
    static void Main() {
        var imp = new WithImplicit();
        var exp = new WithExplicit();
        var none = new WithNone();
        string s1 = imp;
        string s2 = (string)exp;
        string s3 = none.ToString();
    } 
}

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