Workaround for inheriting from multiple classes?

Question

So I'm working on a GUI library and I have 3 classes: UIElement, the base for every UI object, UIContainer, which implements the possibility to hold other child-elements and UIRect which implements position and size for elements. Now I would like to create a class that uses both UIRect and UIContainer. Obviously this is impossible this way, but is there any elegant solution for this problem?

Answer 1

Here is one possibility: inherit from one of the classes (say, UIRect), and embed the other (say, UIContainer). Implement the interface of IUIContainer bu forwarding all calls to the embedded object.

class UIRect {
    ...
}
interface IUIContainer {
    IEnumerable<IUIElement> AllElements {get;}
    void AddElement(IUIElement toAdd);
}
class UIContainer : IUIContainer {
    public IEnumerable<IUIElement> AllElements {
        get {
            ...
        }
    }
    public void AddElement(IUIElement toAdd) {
        ...
    }
}
class Multiple : UIRect, IUIContainer {
    private readonly IUIContainer _cont = new UIContainer();
    ...
    public IEnumerable<IUIElement> AllElements {
        get {
            return _cont.AllElements;
        }
    }
    public void AddElement(IUIElement toAdd) {
        _cont.AddElement(toAdd);
    }
}

Another possibility is to use two interfaces, and share implementation through extension methods.