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 solution that is proposed in the question may be fine in some scenarios. However, changing the default framework behavior for all UIElements in the application, could introduce compatibility issues and it might be difficult in the future to understand where/why the behavior was changed.

An alternative approach would be to keep the default behavior of the framework and only override that behavior manually in specific places, when needed. One way to do this is be creating a simple wrapper element that breaks the IsEnabled inheritance chain from the parent.

The framework's default coerce callback checks the parent IsEnabled value and inherits it. This control sets a new coerce callback that just returns the value directly without checking inheritance.

public class ResetIsEnabled : ContentControl
{
    static ResetIsEnabled()
    {
        IsEnabledProperty.OverrideMetadata(
            typeof(ResetIsEnabled),
            new UIPropertyMetadata(
                defaultValue: true,
                propertyChangedCallback: (_, __) => { },
                coerceValueCallback: (_, x) => x));
    }
}

It can be used like this

<ParentControl IsEnabled="False">
  <!-- Any elements here will have IsEnabled set to false, inherited from the parent -->
  <ResetIsEnabled>
    <!-- Any child elements here will have IsEnabled set to true (the default value) -->
  </ResetIsEnabled>
</ParentControl>