C#将泛型子类型转换为父级 [英] C# Casting Generic Child Type to Parent

问题描述

假设我们有以下几种类型:

Let's say we have these types:

class A {}
class B : A {}

class X<T> {}

为什么我们不能这样做?

Why we can't do this?

X<A> var = new X<B>();

有没有可用的解决方法?

Is there any workaround available?

我尝试使用协方差，但是失败了，因为我想访问X内部的T类型的属性，而C#不允许在接口中使用T类型:

I tried to use covariance but it failed because I want to access a property inside X that is of type T and C# does not allow using type T in the interface:

interface IX<out T> {
    T sth {set; get;}
}

class X<T>: IX<T> {
    T sth { set; get; }
}

我也尝试过，但是失败了:

I also tried this but it failed:

class X<T> where T : A
{
    public T sth { set; get; }

    public static implicit operator X<T>(X<B> v)
    {
        return new X<T>
        {
            sth = v.sth,
        };
    }
}

奇怪的是，C#不允许强制转换'sth'.

It's strange that C# does not allow to the casting for 'sth'.

推荐答案

问题是类不支持协方差和协方差，仅支持接口:

The problem is that classes does not support Covariance and Contravariance, only interfaces:

class A { }
class B : A { }

class X<T> : P<T> { }

interface P<out T>
{
}

...

P<A> var = new X<B>();

协方差和逆差常见问题解答

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