为什么我不能转换Foo< Bar>到IFoo< IBar> [英] Why can I not convert a Foo<Bar> to IFoo<IBar>

问题描述

为什么不能将 Foo< Bar> 转换为 IFoo< IBar> .

如果我尝试我会得到:

没有从 Foo< Bar> 到 IFoo< IBar>

推荐答案

如果您使用的是C#4，并且 IFoo 被声明为:

It would work if you were using C# 4 and IFoo were declared as:

public interface IFoo<out T>

假设 Bar 实现 IBar ，而 Foo< T> 实现 IFoo< T> .

但是，只有在安全的情况下才可以这样声明.如果 T 值进入" API并出现，那是不安全的.例如:

However, it could only be declared that way if it were safe. It's not safe if T values "go into" the API as well as coming out. For example:

public interface IFoo<T>
{
    T Value { get; set; }
}

此不能在T中是协变的，否则您可以这样写:

This can't be covariant in T, as otherwise you could write:

public class StringFoo : IFoo<string>
{
    public T Value { get; set; }
}

IFoo<string> fooString = new StringFoo(); // That's fine
IFoo<object> fooObject = fooString;       // This isn't, because...
fooObject.Value = new Object();           // ... this would violate type safety

阅读Eric Lippert的关于泛型方差的长博客系列有关更多信息.

Read Eric Lippert's long blog series on generic variance for much more information.

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