How to set the SystemColors.HighlightBrushKey in WPF with a Converter

Question

I im trying to set the SystemColors.HighlightBrushKey always a bit darker than the Background of the selected Row. Therefore im using this Code:

App.xaml:

    <WPFTests2:SelectionBackgroundConverter x:Key="SelectionBackgroundConverter"/>

    <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="{Binding Background, Converter={StaticResource SelectionBackgroundConverter}}"/>

</Application.Resources>

Window1.xaml:

<Window x:Class="WPFTests2.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300" Loaded="Window_Loaded">
<Grid>
    <ListBox x:Name="LB" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
</Grid>

Window1.xaml.cs:

using System; 
using System.Globalization;
using System.Windows;
using System.Windows.Data;
using System.Windows.Media;

namespace WPFTests2
{
/// <summary>
/// Interaction logic for Window1.xaml
/// </summary>
public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        LB.Items.Add("Text1");
        LB.Items.Add("Text2");
        LB.Items.Add("Text3");
        LB.Items.Add("Text4");
        LB.Items.Add("Text5");
    }
}

public class SelectionBackgroundConverter : IValueConverter
{
    public object Convert(object value, Type targetType,
                          object parameter, CultureInfo culture)
    {
        if (value != null)
        {
            SolidColorBrush brush = (SolidColorBrush)value;
            Color newCol = brush.Color;
            newCol.R -= 10;
            newCol.G -= 10;
            newCol.B -= 10;
            BrushConverter conv = new BrushConverter();
            Brush newBrush = (Brush)conv.ConvertTo(newCol, typeof(Brush));
            return newBrush;
        }
        return Brushes.Transparent;
    }

    public object ConvertBack(object value, Type targetType,
                              object parameter, CultureInfo culture)
    {
        //never called
        return null;
    }
}
}

The Problem is that the Converter never gets called... Does anyone know how to set the Background of the selected Row a bit darker than it was before selecting it?

Any help is appreciated!

Update

It looks like its working but unfortunately not completely. I've corrected the Converter to look like this:

public class SelectionBackgroundConverter : IValueConverter
{
    public object Convert(object value, Type targetType,
                          object parameter, CultureInfo culture)
    {
        if (value != null)
        {
            SolidColorBrush brush = (SolidColorBrush)value;
            Color newCol = brush.Color;
            newCol.R -= 10;
            newCol.G -= 10;
            newCol.B -= 10;
            return new SolidColorBrush(newCol);
        }
        return Brushes.Transparent;
    }

    public object ConvertBack(object value, Type targetType,
                              object parameter, CultureInfo culture)
    {
        // we don't intend this to ever be called
        return null;
    }

Now the problem is that the Converter only gets called once. I mean if I start the Program and click on any Row the Converter gets called. If I then click into another Row, DataGrid or Control the Converter doesn't get called.

Any Idea how to fix this?

Answer 1

The problem is in this binding:

Color="{Binding Background, Converter={StaticResource SelectionBackgroundConverter}}"

There is no Source, and the Background property won't exist in the current context. Change it to this:

Color="{Binding Source={x:Static SystemColors.HighlightBrush}, Converter={StaticResource SelectionBackgroundConverter}}"

And your converter will be called. You have bugs in your converter though, but that should get you started. Also consider:

• freezing the Brush
• caching the Brush (if you do this a lot in your app)