Return a dynamic resource from a converter

Question

I want to change the color of a WPF control depending on the state of a bool, in this case the state of a checkbox. This works fine as long as I'm working with StaticResources:

My control

<TextBox Name="WarnStatusBox" TextWrapping="Wrap" Style="{DynamicResource StatusTextBox}" Width="72" Height="50" Background="{Binding ElementName=WarnStatusSource, Path=IsChecked, Converter={StaticResource BoolToWarningConverter}, ConverterParameter={RelativeSource self}}">Status</TextBox>

My converter:

[ValueConversion(typeof(bool), typeof(Brush))]

public class BoolToWarningConverter : IValueConverter
{
    public FrameworkElement FrameElem = new FrameworkElement();

    public object Convert(object value, Type targetType,
        object parameter, CultureInfo culture)
    {                      
        bool state = (bool)value;
        try
        {              
            if (state == true)
                return (FrameElem.TryFindResource("WarningColor") as Brush);
            else
                return (Brushes.Transparent);
        }

        catch (ResourceReferenceKeyNotFoundException)
        {
            return new SolidColorBrush(Colors.LightGray);
        }
    }

    public object ConvertBack(object value, Type targetType,
        object parameter, CultureInfo culture)
    {
        return null;
    }
}

The problem is that I have several definitions of the Resource "WarningColor" dependant on setting day mode or night mode. These events does not trig the WarningColor to change. Is there a way to make the return value dynamic or do I need to rethink my design?

Answer 1

The way to return a dynamic resource reference is pretty simple using a DynamicResourceExtension constructor and supplying it a resource key.

Usage:

return new DynamicResourceExtension(Provider.ForegroundBrush);

Definition of the Provider class should contains the key:

public static ResourceKey ForegroundBrush 
{ 
    get 
    { 
        return new ComponentResourceKey(typeof(Provider), "ForegroundBrush"); 
    } 
}

And the value for the key would be declared in the resource dictionary:

<ResourceDictionary
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
     xmlns:theme="clr-namespace:Settings.Appearance;assembly=AppearanceSettingsProvider">

<Color x:Key="{ComponentResourceKey TypeInTargetAssembly={x:Type theme:Provider}, ResourceId=ForegroundColor}">#FF0000FF</Color>

<SolidColorBrush x:Key="{ComponentResourceKey {x:Type theme:Provider}, ForegroundBrush}" Color="{DynamicResource {ComponentResourceKey {x:Type theme:Provider}, ForegroundColor}}" />
</ResourceDictionary>

This way, the converter would dynamically assign a DynamicResource to the bound property depending on the resource key supplied.

Answer 2

You cannot return something dynamic from a converter, but if your only condition is a bool you can easily replace the whole converter with a Style using Triggers:

e.g.

<Style TargetType="TextBox">
    <Setter Property="Background" Value="Transparent" />
    <Style.Triggers>
        <DataTrigger Binding="{Binding IsChecked, ElementName=WarnStatusSource}" Value="True">
            <Setter Property="Background" Value="{DynamicResource WarningColor}" />
        </DataTrigger>
    </Style.Triggers>
</Style>

If now the resource with that key is changed the background should change as well.