Generic style with specific trigger values

Question

I have several TextBlocks in my UserControl that I want to change to become Bold and have a Red font when a Property is triggered. The thing is that each of them are changed by different properties. I saw a solution here with Tags, but couldn't quite get it to work for me. I apologize if this is a duplicate, but I couldn't find any solution that solved my problem.

My Style looks like this:

<Style x:Key="TextBlockTrigger" TargetType="{x:Type TextBlock}">
  <Style.Triggers>
    <Trigger Property="Tag" Value="true">
      <Setter Property="FontWeight" Value="Bold"/>
      <Setter Property="Foreground" Value="Red"/>
    </Trigger>
  </Style.Triggers>
</Style>

And this is the TextBlock I'm trying to use it on:

<TextBlock Name="TextBlock1" x:Uid="TextBlock1" Text="This text should become bold and Red"
Style="{StaticResource TextBlockTrigger}" Tag="{Binding Path=TriggerProperty, UpdateSourceTrigger=PropertyChanged}"/>

I added a button with a codebehind function that reads the Tag, and a breakpoint shows that Tag is set to true, but the text is still regular black.

TriggerProperty is set by a function call in the View Constructor, after InitializeComponent:

public MyWindow()
{
  InitializeComponent();
  UpdateServerProperties();
}

public void UpdateServerProperties()
{
    //other code
    if(ServerValue == true)
    {
        TriggerProperty = true;
        OnPropertyChanged("TriggerProperty");
    }
}

It's a bit simplified, but the actual code is overly complicated, but results in the same thing. ServerValue gets a value, and I have confirmed TriggerProperty does get updated to true.

Do you set the TriggerProperty property dynamically? Do you implement INotifyPropertyChanged? — mm8

Mike Strobel Mike Strobel · Accepted Answer · 2017-10-02T12:51:58

The Tag property has an object type. Xaml has no way of knowing that true represents a bool value, so it just assumes you meant it to be a string. Assuming you are setting Tag to a boolean value, your Trigger is evaluating Equals(true, "true"), so the condition fails.

Try using {x:Static} to point to some constant boolean value. I keep a KnownBoxes class around for this sort of thing:

public static class KnownBoxes
{
    public static readonly object True = true;
    public static readonly object False = false;

    // ... more common values ...
}

These values are easily referenced from Xaml, e.g., {x:Static ns:KnownBoxes.True}.

Alternatively, you could use the element syntax:

<Trigger Property="Tag">
  <Trigger.Value>
    <s:Boolean xmlns:s="clr-namespace:System;assembly=mscorlib">True</s:Boolean>
  </Trigger.Value>
  <Setter Property="FontWeight" Value="Bold"/>
  <Setter Property="Foreground" Value="Red"/>
</Trigger>

Or you could set Tag to the string "true", though that may cause some confusion when someone else works on your code :).

Generic style with specific trigger values

2 Answers