Question

Short version:
WPF seems to always set a local value when using an IValueConverter in a binding, even if that converter returns Binding.DoNothing.
My question is: What do I have to return or do to tell WPF to use the inherited value?

Please note: I don’t want to use DataTriggers as this would bloat up my code significantly because I would need one data trigger along with a converter for every color my current converter returns.

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Long version with reproduction:

Imagine the following scenario:
I have a Button in which a TextBlock is located. There exists a style for the Button that sets the Foreground property. This value is inherited by the TextBlock.
Now I want to create a value converter that converts the value of the TextBlock to a Brush to be used as the Foreground – but only in some cases. In the cases in which I don’t want to set a special color, I return Binding.DoNothing. My understanding was that this would make the TextBlock to continue to use the inherited value.

Unfortunatelly, my understanding was not correct. Even when returning Binding.DoNothing a local value is set. This has been verified with Snoop.

The problem can be easily reproduced with this simple example:

XAML:

<Window x:Class="WpfApplication1.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:WpfApplication1="clr-namespace:WpfApplication1"
        Title="MainWindow" Height="350" Width="525">
  <Window.Resources>
    <WpfApplication1:DummyConverter x:Key="DummyConverter" />
    <Style TargetType="{x:Type Button}">
      <Setter Property="Foreground" Value="Red" />
    </Style>
    <Style TargetType="{x:Type TextBlock}">
          <Setter Property="Foreground"
                  Value="{Binding Path=Text, RelativeSource={RelativeSource Self}, Converter={StaticResource DummyConverter}}" />
    </Style>
  </Window.Resources>
  <StackPanel>
    <Button><TextBlock>Text1</TextBlock></Button>
    <Button><TextBlock>Text2</TextBlock></Button>
  </StackPanel>
</Window>

Converter:

public class DummyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value.ToString() == "Text2")
            return Brushes.Cyan;
        return Binding.DoNothing;
    }
}

As you can see, the first button has a black text instead of red. If you remove the style for TextBlock both buttons will have the correct red text.

Question:
What do I have to do to prevent this? Is there some value to return that tells the engine to continue using the inherited value?

Answer 1

To answer your question: according to this thread, no. As soon as you give the TextBlock a style setter (#4), any value returned will override inherited properties (#7).

Instead, you could create a MultiBinding like so:

public class DummyConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (values[0].ToString() == "Text2")
            return Brushes.Cyan;

        return values[1];
    }
}

<Window x:Class="Spritefire.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:Spritefire"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <local:DummyConverter x:Key="DummyConverter" />
        <Style TargetType="{x:Type Button}">
            <Setter Property="Foreground" Value="Red" />
        </Style>
        <Style TargetType="{x:Type TextBlock}">
            <Setter Property="Foreground">
                <Setter.Value>
                    <MultiBinding Converter="{StaticResource DummyConverter}">
                        <Binding Path="Text" RelativeSource="{RelativeSource Self}" />
                        <Binding Path="Foreground" ElementName="ExampleButton" />
                    </MultiBinding>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <StackPanel>
        <Button x:Name="ExampleButton">
            <TextBlock>Text1</TextBlock>
        </Button>
        <Button>
            <TextBlock>Text2</TextBlock>
        </Button>
    </StackPanel>
</Window>