C＃泛型 - 为什么需要一个明确的类型从一个具体类型回T？ [英] C# generics - why is an explicit cast needed to from a concrete type back to T?

问题描述

这已经困扰了我一段时间。我有一个小麻烦了解为什么需要在下面的代码显式转换：

This has been bothering me for a while. I'm having a little trouble understanding why explicit casts are needed in the following code:

public static class CastStrangeness
{
    public class A
    {
    }

    public class B
        : A
    {
    }

    public static void Foo<T>(T item)
        where T : A
    {
        // Works.
        A fromTypeParameter = item;

        // Does not compile without an explicit cast.
        T fromConcreteType = (T)fromTypeParameter;

        // Does not compile without an explicit cast.
        Foo<T>((T)fromTypeParameter);
    }

    public static void Bar(A item)
    {
        // Compiles.
        Foo<A>(item);
    }
}

在我看来，T是保证是一个A，所以肯定是编译器可以推断，A的任何实例是保证分配到T？否则，我将无法到A或A B传递到美孚（）。所以，我想什么？

It seems to me that T is guaranteed to be an A, so surely the compiler could infer that any instance of A is guaranteed to be assignable to T? Otherwise, I wouldn't be able to pass an A or a B into Foo(). So what am I missing?

PS。我试图寻找关于这个关键字的无尽排列，但每次结果似乎风指的协变和逆变WRT通用接口：）

PS. I've tried searching for endless permutations of keywords on this, but every result seems to wind up referring to covariance and contravariance WRT generic interfaces :)

推荐答案

取的情况：

Foo(new B());

您的第一个任务是确定的：

Your first assignment is ok:

A fromtypeParameter = item;

由于B：A

Since B : A.

不过，此分配是不正常：

But this assignment is not ok:

T fromConcreteType = fromTypeParameter;

由于你很可能已经分配fromTypeParameter为：

Because you could very well have assigned fromTypeParameter as:

fromTypeParameter = new A();

，你显然不能转换为T（即b在这种情况下）。 T比一个更具体的，它可以从A派生所以，你可以走一条路而不是其他，没有一个明确的CAST（可能失败）。

Which you obviously cannot cast to T (which is B in this instance). T is more specific than A, it could be derived from A. So you can go one way but not the other, without an explicit cast (which may fail).

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