Question

So I have a class; lets use the ScrollViewer class as an example. It has a dependency property called Content which accepts anything of type System.Object, great!

Let’s say I derive a class from ScrollViewer, lets call it ZoomScrollViewer, it adds some basic zooming and panning using the mouse with a keyboard press. It also adds a dependency property of it’s own, AutoscaleContent.

Now, I want to be able to put a ZoomScrollViewer into a UI window, but I only want it to accept a Canvas as it’s content. Naturally, I go about creating a ZoomScrollViewer<T> class.

However, how can I change the Content property so that only accepts elements of type <T>? Can I override the dependency property? I got a little confused and tried:

    public new T Content
    {
        get { return (T)base.Content; }
        set { base.Content = value; }
    }

But of course this makes it no longer a dependency property, so all the XAML code fails when I set up the bindings.

Edit: It should also be noted that I’ve taken a look at using:

ZoomScrollViewer.ContentProperty.OverrideMetadata(typeof(ZoomScrollControl2<T>), new PropertyMetadata(...?));

To see if I could do anything using that, but it seems you can only override the default value, unless I’m missing something?

Update: I’ve now tried using the following:

public class ZoomScrollControl2<T> : ZoomScrollViewer where T : FrameworkElement
{
    static ZoomScrollControl2()
    {
        ContentProperty.OverrideMetadata(typeof(ZoomScrollControl2<T>), new FrameworkPropertyMetadata(typeof(ZoomScrollControl2<T>)));
    }
}

public class CanvasZoomControl : ZoomScrollControl2<Canvas>
{
}

Which I thought would work, but it still seems to accept a Content of any type.

Update: In short I’m not sure if what I want to do is even possible, so I’ve marked the discussion as the answer, even though it isn’t an answer per-se.

Answer 1

I suggest to try that approach as it suggested by this MSDN article.

It should override the referal type, so you can refer to it using derived type.

Dependency Property visibility is not made esplicit in .NET Framework for derived types, as searching right property among the types tree has a cost in terms of performance, and considering that we use DP on UI binding, it can lead to non desirable performance issues.