How do I write a datatrigger to change the value of a dependency property based on a change of a second dependency property?

Question

I want to change the state of a dependency property (Mode) in a UserControl triggered off of a change change of a second dependency property (ViewType).

I tried defining a DataTrigger as follows, but for some reason it doesn't work:

<UserControl.Resources>
    <Style TargetType="{x:Type UserControls:MyUserControl}">
        <Style.Triggers>
            <DataTrigger Binding="{Binding ViewType}" Value="ViewType2">
                <Setter Property="Mode" Value="Logon"/>
            </DataTrigger>
      </Style.Triggers>
    </Style>
</UserControl.Resources>

To verify that my binding was working correctly I wrote the following test XAML in the same user control:

    <Style x:Key="butstyle">
        <Style.Triggers>
            <DataTrigger Binding="{Binding ViewType}" Value="ViewType2">
                <Setter Property="Control.Foreground" Value="White"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>

When I assigned this style to a button, I see the foreground color change to white at the right time, which proves my Binding to ViewType is working.

I seems I need to use a style under usercontrol.resources (like in the first block of code above), but the "Mode" property is never set.

Is this the right way to do this? I'd like to try to keep this in XAML if possible, but I'm not comfortable how triggers should work when they are not related directly to visual elements.

In case its relevant, the "ViewType" bound property is defined in a parent UserControl, and the "Mode" property is defined on MyUserControl.

Update:

I used a converter to find out that my trigger is being called, however my setter is not getting engaged for some reason. Here is the code for my DP which I am trying to set. Breakpoints on neitehr the set() nor the OnModeChanged() are called. Is there anything wrong with my Dependency Property?

    public enum ModeStates { Logon, NextState }

    public ModeStates Mode
    {
        get { return (ModeStates)GetValue(ModeProperty); }
        set { SetValue(ModeProperty, value); }
    }

    protected static readonly DependencyProperty ModeProperty = DependencyProperty.Register(
            "Mode", typeof(ModeStates), typeof(MyUserControl),
            new FrameworkPropertyMetadata(new PropertyChangedCallback(OnModeChanged))
            );


    private static void OnModeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {

            // .......

    }

Update:

I was able to use DependencyPropertyDescriptor.AddValueChanged() to solve my problem in a pretty clean fashion.

This is comment and not an aswer as I have not tested it. You might be able to do this with a converter. Is there a reason you cannot just do this in the object you are binding to? — paparazzo
how to use DependencyPropertyDescriptor.AddValueChanged() to solve your problem ? Could you provide some code please ? — Gintama
Sorry, this post was from many years ago and I have long stopped working on this language and framework, nor do I remember the details. — Locksleyu

Adam Mills Adam Mills · Accepted Answer · 2011-10-18T16:38:38

If the dependency properties in question are defined on the UserControl you need to set it's style property, not add a default style inside the resources.

to set the style inline in XAML

<UserControl.Style>
    <Style TargetType="{x:Type UserControls:MyUserControl}">
      <!-- Triggers -- >
    </Style>
</UserControl.Style>

How do I write a datatrigger to change the value of a dependency property based on a change of a second dependency property?

2 Answers