c# - 与泛型的反/协方差 - 不能分配给 T

Question

代码:

public interface IAssignable
{
    IAssignable AssignMe { get; set; }
}

public class GenericAssignExample<T> where T : IAssignable
{
    private T _assignable;

    public void Valid(T toAssign)
    {
        _assignable.AssignMe = toAssign;
    }

    public void AlsoValid(T toAssign)
    {
        _assignable.AssignMe = toAssign.AssignMe;
    }

    public void AlsoValidAsWell(T toAssign)
    {
        _assignable = toAssign;
    }

    public void Invalid(T toAssign)
    {
        _assignable = toAssign.AssignMe;
    }
}

问题:

在最后一个示例方法 _assignable(它是 T，其中 T 是 IAssignable)不能被分配值 toAssign.AssignMe(这是输入 IAssignable), 编译器抛出

"Cannot implicitly convert type 'IAssignable' to 'T'".

这是为什么？ T 是 IAssignable 那么确定 IAssignable 的实例可以分配给它吗？

Answer 1

Why is this? T is IAssignable so surely an instance of IAssignable can be assigned to it?

啊，不是吗？ T 不是 IAssignable，如果是，您就不需要泛型。 T 只是实现 IAssignable。

仅仅因为 Button 和 TextBox 继承自 Control(或者在虚构的示例中实现 IControl)不会意味着您可以将 TextBox 分配给 Button，反之亦然。

---

public interface IAssignable
{
    IAssignable AssignMe { get; set; }
}

public class A : IAssignable
{
    public IAssignable AssignMe { get; set; }
}

public class B : IAssignable
{
    public IAssignable AssignMe { get; set; }
}

// You will be able to instantiate this class with either A or B and both must be valid
public class GenericAssignExample<T> where T : IAssignable
{
    // here, T refers to either A or B.
    private T _assignable;

    public void Valid(T toAssign)
    {
        // assigning either an A or B to the interface... 
        // always valid, both implement it
        _assignable.AssignMe = toAssign;
    }

    public void AlsoValid(T toAssign)
    {
        // assigns interface to interface. Always valid.
        _assignable.AssignMe = toAssign.AssignMe;
    }

    public void AlsoValidAsWell(T toAssign)
    {
        // assigns either an A to an A
        // or a B to a B.
        // both always valid.
        _assignable = toAssign;
    }

    public void Invalid(T toAssign)
    {
        // this tries to assign an interface to either an A or a B
        // always invalid.
        _assignable = toAssign.AssignMe;
    }
}