为什么在通用上下文中不能将继承的接口转换为其基本接口？ [英] Why an inherited interface can't be converted to its base interface in generic context?

问题描述

我正在尝试在我的C＃项目中实现接口继承系统，但是我无法使其正常工作。

I'm trying to implement an interface inheritance system in my C# project, but I can't get it to work.

这是一个简化的版本：

public interface BaseInterface {}

public abstract class AbstractClass<T> where T : BaseInterface {}

public interface ChildInterface : BaseInterface {}

public class ConcreteClass : AbstractClass<ChildInterface> {}

我想按以下方式使用它：

I want to use it as follow:

AbstractClass<BaseInterface> c = new ConcreteClass();

最后一行代码给我以下错误：

The last line of code gives me the following error:

不能将类型'ConcreteClass'隐式转换为'AbstractClass< BaseInterface>'

Cannot implicitly convert type 'ConcreteClass' to 'AbstractClass<BaseInterface>'

为什么

推荐答案

您无法进行分配，因为基类 AbstractClass< T> 是不变的。您希望能够进行这种分配的是协变类型。定义协方差和相反方差仅限于接口，因此这意味着我们需要另一个接口。

You aren't able to make the assignment because the base class, AbstractClass<T>, is invariant. What you want to be able to make that kind of assignment is a covariant type. Defining Covariance and Contravariance is limited to interfaces, so that means we need another interface.

public interface IAbstractClass<out T> where T : BaseInterface { }  
public abstract class AbstractClass<T> : IAbstractClass<T> where T : BaseInterface { }

输出关键字将通用类型参数标记为协变。然后，我们在 AbstractClass< T> 中实现该接口，并且其他类型可以通过该接口正常工作。这些也是我们唯一需要做的更改，其他类型定义保持不变：

The out keyword marks the generic type parameter as covariant. We then implement that interface in AbstractClass<T>, and our other types can work expected through the interface. These are also the only alterations we need to make, we leave the other type definitions the same:

public interface BaseInterface { }
public interface ChildInterface : BaseInterface { }

public class ConcreteClass : AbstractClass<ChildInterface> { }

我们现在有了 AbstractClass< T> 实现，您可以执行所需的分配，但是必须定位 IAbstractClass 接口。

We now have a covariant interface that AbstractClass<T> implements, and you can do the kind of assignment you desire, but you'll have to target the IAbstractClass interface.

public void Main() {
    IAbstractClass<BaseInterface> c = new ConcreteClass();
}

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