Question

I’ve run into a bit of a problem with my class hierarchy, in a WPF application. It’s one of those issues where you have two inheritance trees merging together, and you can’t find any logical way to make your inheritance work smoothly without multiple inheritance. I’m wondering if anyone has any bright ideas for getting this kind of system working, without making it impossible to follow or debug.

I’m a low-level engineer, so my first thought is always, “Oh! I’ll just write some of these classes in native C++, and reference them externally! Then I can have all my old-school OO fun!” Alas, this doesn’t help when you need to inherit from managed controls…

Allow me to show a snippet of my current projected class diagram:

 ____________________________________      _____________________________________
| CustomizableObject                 |    | System.Windows.Controls.UserControl |
|____________________________________|    |_____________________________________|
|   string XAMLHeader()              |                        ▲
|   string XAMLFooter()              |◄--┐                    |
|   CustomizableObject LoadObject()  |   \                    |
|   <Possible other implementations> |    \                   |
|____________________________________|     \                  |
         ▲                      ▲           \                 |
         |                      |            \                |
         |                      |             \               |
 _________________    ______________________   \    _____________________
| SpriteAnimation |  | SpriteAnimationFrame |  └---| CustomizableControl |
|_________________|  |______________________|      |_____________________|
                                                      ▲             ▲
                                                      |             |
                                                      |             |
                                                  ________    _____________
                                                 | Sprite |  | SpriteFrame |
                                                 |________|  |_____________|

The problem is pretty clear: the separation of the CustomizableObject and CustomizableControl object trees — and the insertion of UserControl into one, but not both, of the trees.

It makes no practical sense to move the implementation of CustomizableObject into its derived classes, since the implementation doesn’t vary by class. Furthermore, it would be really confusing to have it implemented multiple times. So I really don’t want to make CustomizableObject an interface. The interface solution doesn’t make any sense to me. (Interfaces have never really made much sense to me, to be honest…)

So I say again, anyone have any bright ideas? This one’s a real pickle. I’d like to learn more about making interfaces work WITH my object tree, rather than against it. I’m making this simple sprite engine using WPF and C# as a solid exercise, more than anything. This is so easy to solve in C++ – but I need to figure out how to solve these problems in a managed environment, rather than throwing my hands in the air and running back to Win32 whenever the going gets tough.

Answer 1

One approach is to use extension methods with an interface to provide your “derived class” implementation, much like System.Linq.Queryable:

interface ICustomizableObject
{
    string SomeProperty { get; }
}

public static class CustomizableObject
{
    public static string GetXamlHeader(this ICustomizableObject obj)
    {
        return DoSomethingWith(obj.SomeProperty);
    }

    // etc
}

public class CustomizableControl : System.Windows.Controls.UserControl, ICustomizableObject
{
    public string SomeProperty { get { return "Whatever"; } }
}

Usage: As long as you have a using directive for (or are in the same namespace as) the namespace where your extension methods are defined:

var cc = new CustomizableControl();
var header = cc.GetXamlHeader();