Question

How can I change the default value for an inherited dependency property? In our case, we’ve created a subclass of Control which by default has its Focusable set to ‘true’. We want our subclass to have the default of ‘false’.

What we’ve been doing is simply setting it to ‘false’ in the constructor, but if someone uses ClearValue, it goes back to the default, not the value set in the constructor.

Here’s what I’m currently doing to achieve this (This is a test control with a DP of ‘Foo’ for an example.) I’m not a fan of the ‘new’ to hide the property although thanks to AddOwner, it does point to the same shared instance so I guess it’s ok. It looks like it inherits all the other metadata values as well so that’s good. Just wondering if this is correct?

public class TestControlBase : Control
{

    public static readonly DependencyProperty FooProperty = DependencyProperty.Register(
        "Foo",
        typeof(int),
        typeof(TestControlBase),
        new FrameworkPropertyMetadata(4) // Original default value
    );

    public int Foo
    {
        get { return (int)GetValue(FooProperty); }
        set { SetValue(FooProperty, value); }
    }

}

public class TestControl : TestControlBase
{

    public static readonly new DependencyProperty FooProperty = TestControlBase.FooProperty.AddOwner(
        typeof(TestControl),
        new FrameworkPropertyMetadata(67) // New default for this subclass
    );

}

Mark

UPDATE…

I think this is even better as it eliminates the ‘new’ call. You still access it via the FooProperty on the base class since this uses AddOwner. As such, it’s technically the same one.

public class TestControl : TestControlBase
{
    // Note this is private
    private static readonly DependencyProperty AltFooProperty = TestControlBase.FooProperty.AddOwner(
        typeof(TestControl),
        new FrameworkPropertyMetadata(67) // New default for this subclass
    );

}

Answer 1

The correct way to override a base class’s property is:

static TestControl() {

    FooProperty.OverrideMetadata(
        typeof(TestControl),
        new FrameworkPropertyMetadata(67)
    );
}

EDIT:

AddOwner is meant to share the same DependencyProperty across types that are not related (i.e. the TextProperty of TextBox and TextBlock).