Generic Parent For Generic Class

Question

I am using a fictional example for this. Say, I have a Widget class like:

abstract class Widget
{
Widget parent;
}

Now, my other classes would be derived from this Widget class, but suppose I want to put some constraint in the class while defining the derived types such that only a particular "type" of widget can be parent to a particular type of Widget.

For example, I have derived two more widgets from the Widget class, WidgetParent and WidgetChild. While defining the child class, I want to define the type of parent as WidgetParent, so that I dont have to type cast the parent every time I use it.

Precisely, what I would have liked to do is this:

// This does not works!
class Widget<PType>: where PType: Widget
{
    PType parent;
}

class WidgetParent<Widget>
{
    public void Slap();
}

class WidgetChild<WidgetParent>
{
}

So that when I want to access the parent of WidgetChild, instead of using it this way:

WidgetParent wp = wc.parent as WidgetParent;
if(wp != null)
{
    wp.Slap();
}
else throw FakeParentException();

I want to use it this way(if I could use generics):

wc.parent.Slap();

Answer 1

You should be able to use the code you've got by still having the non-generic class Widget and making Widget<T> derive from it:

public abstract class Widget
{
}

public abstract class Widget<T> : Widget where T : Widget
{
}

You then need to work out what belongs in the generic class and what belongs in the non-generic... from experience, this can be a tricky balancing act. Expect to go back and forth a fair amount!