T必须是contravariantly有效 [英] T must be contravariantly valid

问题描述

什么是不对的。

interface IRepository<out T> where T : IBusinessEntity
{
    IQueryable<T> GetAll();
    void Save(T t);
    void Delete(T t);
}

它说：

It says:

无效差异：类型参数'T'必须是'MyNamespace.IRepository.Delete（T）contravariantly有效。 'T'是协变

Invalid variance: The type parameter 'T' must be contravariantly valid on 'MyNamespace.IRepository.Delete(T)'. 'T' is covariant.

推荐答案

考虑会发生什么，如果编译器允许的：

Consider what would happen if the compiler allowed that:

interface IR<out T>
{
    void D(T t);
}

class C : IR<Mammal>
{
    public void D(Mammal m)
    {
        m.GrowHair();
    }
}
...
IR<Animal> x = new C(); 
// legal because T is covariant and Mammal is convertible to Animal
x.D(new Fish()); // legal because IR<Animal>.D takes an Animal

和你只是想上长出头发一条鱼。

And you just tried to grow hair on a fish.

停止是指T的输出位置仅用于。您正在使用它输入位置。

The "out" means "T is only used in output positions". You are using it in an input position.

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