DataTrigger not firing

Question

I have the following xaml:

Answer 1

The issue here is Property Value Precedence.

You are currently setting the Background to blue directly on the DockPanel. This explicit property will override any value set by the trigger.

Instead, you must set the original "Background" as a setter in the style.

<DockPanel DockPanel.Dock="Left" Width="10">
    <DockPanel.Style>
        <Style>  
            <Setter Property="DockPanel.Background" Value="Blue" /> 
            <Style.Triggers>                    
                <DataTrigger Binding="{Binding Path=Test}" Value="True">                        
                    <Setter Property="DockPanel.Background" Value="Yellow" />                       
                </DataTrigger>
            </Style.Triggers>            
        </Style>        
    </DockPanel.Style>    
</DockPanel></DockPanel>