implict type cast in generic method

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

Answer 1

You might see that a condition has been logically met, but the compiler does not "remember" this.

public static T GetNode() where T : NodeBase
{
    if (typeof(T) == typeof(SpecialNode)) // OK, you now know T is SpecialNode
    {
        // the compiler still insists on returning a T,
        // and will not assume that MySpecialNode is a T
        return MySpecialNode;
    }

    // ...

    return default(T);
}

It's true what others have already said: you must cast MySpecialNode: (T)(NodeBase)MySpecialNode (which you can do safely, because you have already checked that T is SpecialNode).

It's easy to think of this as a shortcoming of the compiler; but this is just a mistake stemming from how obvious it seems that MySpecialNode is a T. Suppose I had a method like this:

public T Get() {
    if (typeof(T).FullName.Equals("System.Int32"))
        return 5;
    else
        return default(T);
}

Should this compile? I should hope not; the compiler needs to guarantee that it's returning an object of type T, and it can't be sure that 5 will meet that requirement just from some bizarre check that I the developer have performed. (Yes, I know that T is int, but I would not expect any reasonable compiler to determine that from a comparison of the System.Type.FullName property.)

Checking if (typeof(T) == typeof(SpecialNode)) is really not so different from that.