打字中协变与逆变位置的区别

public interface ICovariant<out T> {
    T Get();
}

public class Base { }

public class Derived : Base { }

public class BaseProvider : ICovariant<Base> {
    public Base Get() => new Base();
}

ICovariant<Base> baseProvider = new BaseProvider(); // 正常赋值
ICovariant<Derived> derivedProvider = baseProvider; // 这是允许的，因为BaseProvider实现了ICovariant<Base>

public interface IContravariant<in T> {
    void Set(T value);
}

public class Base { }

public class Derived : Base { }

public class BaseConsumer : IContravariant<Base> {
    public void Set(Base value) { /* ... */ }
}

IContravariant<Base> baseConsumer = new BaseConsumer(); // 正常赋值
IContravariant<Derived> derivedConsumer = baseConsumer; // 这是允许的，因为BaseConsumer的Set方法接受Base类型，而Derived是Base的子类