为什么LINQ的演员LT;＆GT;助手不与隐式转换运营商合作？ [英] Why does the Linq Cast<> helper not work with the implicit cast operator?

问题描述

的请投票决定像以前读到结尾重复... 的

我有一个实现了 A型隐式转换运营商为另一种类型：

I have a type that implements an implicit cast operator to another type:

class A
{
    private B b;
    public static implicit operator B(A a) { return a.b; }
}
class B
{
}

现在，显性和隐性的铸造工作就好了：

Now, implicit and explicit casting work just fine:

B b = a;
B b2 = (B)a;

...所以怎么来的Linq的 .Cast＆LT;＆GT; 不？

A[] aa = new A[]{...};
var bb = aa.Cast<B>();  //throws InvalidCastException

在源$ C ​​$ C

寻找 .Cast＆LT;＆GT; ，没有太多的神奇事情：一些特殊情况下，如果该参数确实是一个的IEnumerable＆LT; B＆GT; ，然后：

Looking at the source code for .Cast<>, there's not much magic going on: a few special cases if the parameter really is a IEnumerable<B>, and then:

foreach (object obj in source) 
    yield return (T)obj; 
    //            ^^ this looks quite similar to the above B b2 = (B)a;

那么，为什么的我的的显式类型转换的工作，但不是一个内部 .Cast＆LT;＆GT; ？

So why does my explicit cast work, but not the one inside .Cast<>?

该编译器糖了我明确的转换？

Does the compiler sugar-up my explicit cast ?

PS。只见这个问题但我不认为它真正的答案解释发生了什么事情。

PS. I saw this question but I don't think its answers really explain what's going on.

推荐答案

简短的答案是简单的：在演员LT; T＆GT; 方法不支持自定义转换操作符

The short answer would be simply: the Cast<T> method doesn't support custom conversion operators.

在第一个例子：

B b = a;
B b2 = (B)a;

编译器可以看到这个 B静态分析中（A一）运营商;编译器间$ P $点这是一个静态呼叫你的运营商定制的方法。在第二个例子中：

the compiler can see this B(A a) operator during static analysis; the compiler interprets this as a static call to your custom operator method. In the second example:

foreach (object obj in source) 
    yield return (T)obj; 

这具有的没有知识的操作人员;这是通过 unbox.any 实施（这是一样的 castclass 如果 T 是一个引用类型）。

that has no knowledge of the operator; this is implemented via unbox.any (which is the same as castclass if T is a ref-type).

还有第三个选择：如果通过动态去的时候，运行时实现试图模仿编译器的规则，所以这个的将会的发现运营商......但并不像C＃的一部分-to-IL编译步骤：

There is also a third option: if you went via dynamic, the runtime implementation tries to mimic compiler rules, so this will find the operator ... but not as part of the C#-to-IL compile step:

dynamic b = a; // note that `dynamic` here is *almost* the same as `object`
B b2 = b;

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