不能从基类继承

Question

My code is as follows

class BaseClass<T> where T : class
{
    class DerivedClass<U, V>
        where U : class
        where V : U
    {
        BaseClass<V> _base;
    }

}

error: The type 'V' must be reference type.

Isn't 'V' here of type class ??

Answer 1

You can resolve this issue by adding a class constraint to the V type parameter:

class BaseClass<T> where T : class
{
    class DerivedClass<U, V>
        where U : class
        where V : class, U
    {
        BaseClass<V> _base;
    }
}

For an explanation, see from Eric Lippert's article (as commented above by Willem van Rumpt ).

Answer 2

Isn't 'V' here of type class ??

No it is not. V could be of System.ValueType or any enum, or any ValueType .

Your constraint just says that V should be derived from U where as U is class. It doesn't says that V should be a class.

For example the following is perfectly valid, which contradicts with the constraint where T : class .

DerivedClass<object, DateTimeKind> derived;

So you need to add where V : class also.

Eric Lippert has blogged the very same question .