Question

why do I get a compiler error in the following code stating: Cannot implicty convert type SpecialNode to T even though T must derive from NodeBase as I defined in the where clause and even though SpecialNode actually derived from NodeBase?

    public static T GetNode<T>() where T : NodeBase
    {
        if (typeof(T) == typeof(SpecialNode))
        {
            return ThisStaticClass.MySpecialNode; // <-- compiler error
        }
        if (typeof(T) == typeof(OtherSpecialNode))
        {
            return ThisStaticClass.MyOtherSpecialNode; // <-- compiler error
        }
        ...
        return default(T);
    }

Answer 1

The compiler doesn’t read your if check to realize that in this particular line, T must be SpecialNode.

You need to cast to NodeBase first, like this:

return (T)(NodeBase)MySpecialNode;

You need to casts because (as far as the compiler knows) T might be MyOtherSpecialNode, and you cannot cast a MyOtherSpecialNode to MySpecialNode.

EDIT: You can do it with a single cast like this:

NodeBase retVal;

if (typeof(T) == typeof(SpecialNode))
    retVal = MySpecialNode;
else if (typeof(T) == typeof(OtherSpecialNode))
    retVal = MyOtherSpecialNode;

return (T)retVal;