Question

Why TextBlock remains black?

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <Style TargetType="TextBlock" x:Key="style">
            <Style.Triggers>
                <DataTrigger Binding="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Tag}" Value="True">
                    <Setter Property="Foreground" Value="Red" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <StackPanel>
        <TextBlock Text="Test" Tag="True" Style="{StaticResource style}" />
    </StackPanel>
</Window>

Update: Ok now I have another problem. The style does not react on property change:

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <Style TargetType="TextBlock" x:Key="style">
            <Style.Triggers>
                <Trigger Property="Tag" Value="True">
                    <Setter Property="Foreground" Value="Red" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <StackPanel>
        <TextBlock Text="{Binding Prop}" Tag="{Binding Prop}" Style="{StaticResource style}" x:Name="text" />
        <Button Content="Test" Click="Button_Click" />
    </StackPanel>
</Window>

Backing Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
using System.ComponentModel;

namespace WpfApplication4
{
    public partial class MainWindow : Window
    {
        private MyClass a = new MyClass();

        public MainWindow()
        {
            InitializeComponent();
            DataContext = a;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            a.Prop = true;
            a.OnPropertyChanged("Prop");
        }
    }

    public class MyClass : INotifyPropertyChanged
    {
        public bool Prop { get; set; }

        public event PropertyChangedEventHandler PropertyChanged;
        public virtual void OnPropertyChanged(string property)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }
}

TextBlock’s text changes but color does not

Answer 1

Change it to Property Trigger

    <Style TargetType="TextBlock" x:Key="style">
        <Style.Triggers>
            <Trigger Property="Tag" Value="True">
                <Setter Property="Foreground" Value="Red" />
            </Trigger>
        </Style.Triggers>
    </Style>

TemplatedParent works inside a ControlTemplate. Your Binding is incorrect. Thats why it doesn’t work.

If you want to use DataTrigger for some reason then the correct Binding would be

<DataTrigger Binding="{Binding Tag,RelativeSource={RelativeSource Self}}" Value="True">