XAML UserControl - Set Background based on Trigger

Question

I have a UserControl that contains an Expander :

<UserControl x:Class="Client.DevicesExpander"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300"
             x:Name="devicesExpander" >

    <Expander Margin="1" Name="expander" FontSize="11" BorderThickness="1" BorderBrush="DarkRed" Foreground="Black" Header="{Binding ElementName=devicesExpander, Path=Header}" FontWeight="Bold" MouseDoubleClick="Expander_MouseDoubleClick" Expanded="Expander_Expanded" IsExpanded="{Binding ElementName=devicesExpander, Path=IsExpanded}" Background="{Binding ElementName=devicesExpander, Path=Background}">
        <StackPanel Name="_devicesPanel">
            <ListBox BorderThickness="0,1,0,0"  Name="_devicesList" FontWeight="Normal" MouseDoubleClick="DevicesList_MouseDoubleClick" Background="{Binding ElementName=devicesExpander, Path=Background}" />
        </StackPanel>
        <Expander.Style>
            <Style TargetType="{x:Type Expander}">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding ElementName=devicesExpander, Path=IsExpanded}" Value="True">
                        <Setter Property="Background" Value="White" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
            </Expander.Style>
    </Expander>
</UserControl>

Ans basically all I'd like to do is change the Expander and StackPanel's Background color based on the IsExpanded of the UserControl (or Expander).

I've added three Dependancy Properties to the control :

    public static readonly DependencyProperty HeaderProperty = DependencyProperty.Register("Header", typeof(string), typeof(DevicesExpander));
    public static readonly DependencyProperty IsExpandedProperty = DependencyProperty.Register("IsExpanded", typeof(bool), typeof(DevicesExpander));
    public static readonly DependencyProperty BackgroundProperty = DependencyProperty.Register("Background", typeof(System.Windows.Media.Brush), typeof(DevicesExpander));

But my code does not work. The IsExpanded property from the usercontrol does work as the property changes accordingly (when the expander expands) when checked from within the window where the usercontrol is placed.

How can I change the background color of the Expander based on the UserControl.IsExpanded property ?

Thanks !

EDIT: I have in the meanwhile done the following:

<UserControl.Resources>
        <Style TargetType="{x:Type UserControl}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding ElementName=devicesExpander, Path=IsExpanded}" Value="True">
                    <Setter Property="Background" Value="White" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </UserControl.Resources>

    <Expander Margin="1" Name="expander" FontSize="11" BorderThickness="1" BorderBrush="DarkRed" Foreground="Black" Header="{Binding ElementName=devicesExpander, Path=Header}" FontWeight="Bold" MouseDoubleClick="Expander_MouseDoubleClick" Expanded="Expander_Expanded" IsExpanded="{Binding ElementName=devicesExpander, Path=IsExpanded}" Background="Transparent">
        <StackPanel Name="_devicesPanel">
            <ListBox BorderThickness="0,1,0,0"  Name="_devicesList" FontWeight="Normal" MouseDoubleClick="DevicesList_MouseDoubleClick" Background="Transparent" />
        </StackPanel>
    </Expander>

and removed the BackgroundProperty dependancy property. I actually thought this could work, but alas ...

Answer 1

I have managed to solve my problem. Herewith the solution (showing only the essential code) ...

My Dependancy Property is created as follows:

public static readonly DependencyProperty IsExpandedProperty = DependencyProperty.Register("IsExpanded", typeof(bool), typeof(DevicesExpander));

public bool IsExpanded
{
    get { return (bool)GetValue(IsExpandedProperty); }
    set { SetValue(IsExpandedProperty, value); }
}

and my XAML is :

<UserControl x:Class="TestApp.DevicesExpander"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             Name="devicesExpander">

    <Expander>
        <Expander.Style>
            <Style TargetType="Expander">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding ElementName=devicesExpander, Path=IsExpanded}" Value="True">
                        <Setter Property="Background" Value="Black" />
                    </DataTrigger>
                    <DataTrigger Binding="{Binding ElementName=devicesExpander, Path=IsExpanded}" Value="False">
                        <Setter Property="Background" Value="Red" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Expander.Style>
    </Expander>
</UserControl>

The solution in the end was to remove the Backgroud property from the element and specify a DataTrigger for IsExpanded=True and IsExpanded=False. It thus seems like the Background property when specified in the element properties overrides anything that the triggers tried to set.

Hope this helps someone !