通用约束和接口实现/继承 [英] Generic constraints and interface implementation/inheritance

问题描述

不完全确定如何解释这个问题，因为它是为什么这不起作用？类型的查询。

Not entirely sure how to phrase the question, because it's a "why doesn't this work?" type of query.

我将特定问题简化为此代码：

I've reduced my particular issue down to this code:

public interface IFoo
{
}

public class Foo : IFoo
{
}

public class Bar<T> where T : IFoo
{
    public Bar(T t)
    {
    }

    public Bar()
        : this(new Foo()) // cannot convert from 'Foo' to 'T'
    {
    }
}

现在，在 Bar< T> 类 T >必须实现IFoo。那为什么编译器会给我评论中的错误？当然，Foo的一个实例是IFoo，因此可以作为泛型类型的代表 T ？

这是一个编译器限制还是缺少一些东西？

Is this a compiler limitation or am I missing something?

推荐答案

你也可以有一个实现IFoo的Fiz以任何其他方式与Foo相关：

You could also have a Fiz that implements IFoo that is not related to Foo in any other way:

public interface IFoo
{
}

public class Foo : IFoo
{
}

public class Fiz : IFoo
{
}

Foo foo = new Foo();
Fiz fiz = foo; // Not gonna compile.

你想要的可能更像是：

What you want is probably more like:

public class Bar<T> where T : IFoo, new()
{
    public Bar(T t)
    {
    }

    public Bar()
        : this(new T()) 
    {
    }
}

所以你可以有

So you can have

Bar<Foo> barFoo = new Bar<Foo>();
Bar<Fiz> barFiz = new Bar<Fiz>();

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