Question

Can anyone explain why isn’t this possible (at least in .Net 2.0):

public class A<T>
{
    public void Method<U>() where U : T
    {
        ...
    }
}

...

A<K> obj = new A<K>();
obj.Method<J>();

with K being the superclass of J

EDIT

I’ve tried to simplify the problem in order to make the question more legible, but I’ve clearly overdo that. Sorry!

My problem is a little more specific I guess. This is my code (based on this):

public class Container<T>
{
    private static class PerType<U> where U : T
    {
        public static U item;
    }

    public U Get<U>() where U : T
    {
        return PerType<U>.item;
    }

    public void Set<U>(U newItem) where U : T
    {
        PerType<U>.item = newItem;
    }
}

and I’m getting this error:

Container.cs(13,24): error CS0305: Using the generic type
Container<T>.PerType<U>' requires2′ type argument(s)

Answer 1

Actually it is possible. This code compiles and runs just fine:

public class A<T>
{
    public void Act<U>() where U : T
    {
        Console.Write("a");
    }
}

static void Main(string[] args)
{
  A<IEnumerable> a = new A<IEnumerable>();
  a.Act<List<int>>();
}

What is not possible is using covariance / contravariance in generics, as explained here:

IEnumerable<Derived> d = new List<Derived>();
IEnumerable<Base> b = d;