Textblock Style dataTrigger not working inside ItemsControl

Question

I have an ObservableCollection<Object1> type(Messages in code below) which is bound to an ItemsControl. Object1 is having two properties namely ErrMsg and IsError. I want to display the ErrMsg in red colour if its an Error(i.e. if IsError is true) otherwise Black.

<ItemsControl
    Height="Auto"
    Background="White"
    ItemsSource="{Binding Messages}">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <StackPanel>
                <TextBlock
                    Margin="5,0,0,0"
                    Text="{Binding ErrMsg}"
                    Width="Auto"
                    Foreground="Black">
                    <TextBlock.Style>  
                        <Style TargetType="{x:Type TextBlock}">       
                            <Style.Triggers>         
                                <DataTrigger
                                    Binding="{Binding IsError}"
                                    Value="true">      
                                    <Setter
                                        Property="TextBlock.Foreground"
                                        Value="Red" />         
                                </DataTrigger>       
                            </Style.Triggers>     
                        </Style>   
                    </TextBlock.Style> 
                </TextBlock>
            </StackPanel>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

Problem is that all the messages are always displayed in Black colour irrespective of IsError value?

How can I achieve this?

Answer 1

That's because you specify Foreground="Black" in your text block declaration. Local values (set on the element itself) override style values (including triggers).

To fix this, just move setting of black foreground to the style:

<TextBlock Margin="5,0,0,0"
           Text="{Binding Value}"
           Width="Auto">
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Setter Property="Foreground"
                    Value="Black"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding IsError}"
                             Value="true">
                    <Setter Property="Foreground"
                            Value="Red" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>