WPF Style DataTrigger with binding to DataContext not working

Question

I have a TextBox with a style that has a DataTrigger which changes the text, like this:

<Grid>
    <TextBlock Text="Foo">
        <TextBlock.Style>
            <Style BasedOn="{StaticResource TextStyle}" TargetType="TextBlock">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding MyBool}" Value="True">
                        <Setter Property="Text" Value="Bar"/>
                    </DataTrigger>
                 </Style.Triggers>
             </Style>
         </TextBlock.Style>
     </TextBlock>
</Grid>

But it's not working, the text never changes to "Bar". I have tested using another TextBlock with Text="{Binding MyBool}" and this text changes from "False" to "True". Snoop reveals no errors that I can see and there is nothing in the output.

This question may seem like a duplicate of WPF Trigger binding to MVVM property, but my code does not seem different from the accepted answer there (http://www.thejoyofcode.com/Help_Why_cant_I_use_DataTriggers_with_controls_in_WPF.aspx, section "Using a style") in any relevant way. And using a DataTemplate as suggested in the actual answer seems wrong since I only want this to apply to a single TextBlock, but if it is correct, I'm not sure how to write a DataTemplate for this...

EDIT:

This is what the property I'm binding to looks like:

public bool MyBool
{
    get { return _myBool; }
    set
    {
        if (_myBool== value)
            return;

        _myBool= value;
        NotifyPropertyChanged();
    }
}
private bool _myBool;

Rena

First thing you need to do is turn up debug messages for databinding: i.sstatic.net/MF8i5.png Next, re-run and check the output window and see what errors are there. You may have done this; the fact that you mentioned snoop indicates you're far ahead of the pack. Probably need the relevant parts of the VM in the question, as well. – Kinsley

I'd check your output window for errors. It may be something as simple as the binding being messed up. – Kloof

Yes, as I mentioned: "there is nothing in the output". Warnings for binding errors are enabled. I'll update the main post with the property... but as I also mentioned, I can bind to and display the value of the same property just fine, it just doesn't work in the trigger. – Rena

Answer 1

Dependency Properties can be set from many different places; inline, animations, coercion, triggers, etc. As such a Dependency Property Value Precedence list was created and this dictates which changes override which other changes. Because of this order of precedence, we can't use a Trigger to update a property that is explicitly set inline in your XAML. Try this instead:

<Grid>
    <TextBlock>
        <TextBlock.Style>
            <Style BasedOn="{StaticResource TextStyle}" TargetType="TextBlock">
                <!-- define your default value here -->
                <Setter Property="Text" Value="Foo" />
                <Style.Triggers>
                    <DataTrigger Binding="{Binding MyBool}" Value="True">
                        <!-- define your triggered value here -->
                        <Setter Property="Text" Value="Bar" />
                    </DataTrigger>
                 </Style.Triggers>
             </Style>
         </TextBlock.Style>
     </TextBlock>
</Grid>

Lakesha

Now THAT is just silly... is there actually a good reason for this? – Rena

I would disagree with you there... it's all about the Dependency Property Value Precedence as there are many ways to set a DependencyProperty. It just makes more sense if you are aware of it. – Lakesha

That's so weird.. I remember many times that I explicitly set a property in Xaml and then added a Trigger and it worked. Check this out: https://mcmap.net/q/661832/-concatenated-text-in-textblock-datatrigger For now I'm enjoying your solution - It works for me so +1 for solution (explanation need source!) – Eby

I updated my answer to make my point clearer. Setting a property inline will stop it from being changed by a Trigger. Setting it as above in my answer is the correct way to set an initial value for a property when using a Trigger. I hope that is clearer now. – Lakesha

In this code, when MyBool is set to false, Text will become Foo again right? In other words, there's no need to specify another DataTrigger for the False case? – Pinchpenny