WPF override IsEnabled from Parent

Question

I just searched for a way to enable a child control while the parent control has IsEnabled = false. All answers that I have found up to now say that it is not possible - one has to enable the parent and disable the child controls except the ones that should still be enabled.

However, by overriding the Metadata for the IsEnabledProperty in the App.xaml.cs file, I was able to change this default behavior:

protected override void OnStartup(StartupEventArgs e)
{
    UIElement.IsEnabledProperty.OverrideMetadata(typeof(FrameworkElement),
             new UIPropertyMetadata(true,IsEnabledChanged, CoerceIsEnabled));
}

private void IsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var childrenCount = VisualTreeHelper.GetChildrenCount(d);
    for (int i = 0; i < childrenCount; ++i)
    {
        var child = VisualTreeHelper.GetChild(d, i);
        child.CoerceValue(UIElement.IsEnabledProperty);
    }
}
private object CoerceIsEnabled(DependencyObject d, object basevalue)
{
    var parent = VisualTreeHelper.GetParent(d) as FrameworkElement;
    if (parent != null && parent.IsEnabled == false)
    {
        if (d.ReadLocalValue(UIElement.IsEnabledProperty) == DependencyProperty.UnsetValue)
        {
            return false;
        }
    }
    return basevalue;
}

Now you can manually set the IsEnabled property on a child, which overrides the parent value.

Are there any drawbacks of this approach?

Answer 1

This worked for my situation on a control used several times with some slight modifications.

Placing here to help any future web searchers in a similar situation:

• placed it in a static constructor instead of an event, otherwise it tried to set it multiple times and threw a "PropertyMetadata is already registered for type '{type}'." exception.
• Changed the type to match the control

Code:

Make sure to find and replace [CustomControl] with the type name of your control.

static [CustomControl]()
{
    UIElement.IsEnabledProperty.OverrideMetadata(typeof([CustomControl]), new UIPropertyMetadata(true, [CustomControl]_IsEnabledChanged, CoerceIsEnabled));
}

private static void [CustomControl]_IsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var childrenCount = VisualTreeHelper.GetChildrenCount(d);
    for (int i = 0; i < childrenCount; ++i)
    {
        var child = VisualTreeHelper.GetChild(d, i);
        child.CoerceValue(UIElement.IsEnabledProperty);
    }
}

private static object CoerceIsEnabled(DependencyObject d, object basevalue)
{
    var parent = VisualTreeHelper.GetParent(d) as FrameworkElement;
    if (parent != null && parent.IsEnabled == false)
    {
        if (d.ReadLocalValue(UIElement.IsEnabledProperty) == DependencyProperty.UnsetValue)
        {
            return false;
        }
    }
    return basevalue;
}

Answer 2

The drawback is at least, that you break the basic concept, and the IsEnabled is not used for the intended scope. This workaround also makes maintenance a bit more complex (the developer has to understand first, why it works differently).

As it is suggested in comments, I would say, that a redesign of this window would help. Especially, if I would like to forbid only the editing (data modification) in the form, I would use other properties like IsReadOnly.

Answer 3

Another option is to override the FrameworkPropertyMetadataOptions to remove the Inherits property. I had a similar problem with the FontSize and this worked well:

FontSizeProperty.OverrideMetadata(
   typeof(YourControl), 
   new FrameworkPropertyMetadata(8.0, 
      FrameworkPropertyMetadataOptions.None, changed));