Question

There is a generic class XXValue like this, where the Type T can be value type or reference type, e.g int, string, struct object

public class XXValue<T>
{
   public T DefaultValue;
}

And there is another generic class XXAttribute

public class XXAttribute<T>
{
   public T Value;
}

But the type T for XXAttribute should be the class or sub-class of type XXValue, so how to write the where statement for XXAttribute? Which one is correct?

public class XXAttribute<T> where T : XXValue<T>

public class XXAttribute<VT, AT> where AT : XXValue<VT>

Answer 1

But the type T for XXAttribute should be the class or sub-class of type XXValue

Given this statement, it should be the second one.

public class XXAttribute<TBase, T> where T : XXValue<TBase>
{
    public T Value;
}

This specifies that the type parameter for XXAttribute should inherit from XXValue. But since XXValue is also a generic type, you need to specify its type parameter in XXAtribute also, as TBase, and pass it along.

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Alternatively (it’s hard to tell given the scope of the question, but you could possibly) change how Value is defined:

public class XXAttribute<TBase>
{
    public XXValue<TBase> Value;
}