Applying a (non-multi) ValueConverter the output of a MultiBinding + StringFormat

Question

Is there a way to apply a (single, not multi) ValueConverter to the output of a MultiBinding which uses StringFormat (i.e. after the string has been formatted).

It would be the equivalent of that code, in which I used an intermediary collapsed TextBlock to do the trick :

   <StackPanel>
        <TextBox x:Name="textBox1">TB1</TextBox>
        <TextBox x:Name="textBox2">TB2</TextBox>

        <TextBlock x:Name="textBlock" Visibility="Collapsed">
            <TextBlock.Text>
                <MultiBinding StringFormat="{}{0}{1}">
                    <Binding ElementName="textBox1" Path="Text"/>
                    <Binding ElementName="textBox2" Path="Text"/>
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>

        <TextBlock Text="{Binding ElementName=textBlock,
                   Path=Text, Converter={StaticResource SingleValueConverter}}" />

    </StackPanel>

Answer 1

Here is a hack that does what you want:

public static class Proxy
{
    public static readonly DependencyProperty TextProperty = DependencyProperty.RegisterAttached(
        "Text",
        typeof(string),
        typeof(Proxy),
        new PropertyMetadata(string.Empty));

    public static void SetText(this TextBlock element, string value)
    {
        element.SetValue(TextProperty, value);
    }

    [AttachedPropertyBrowsableForChildren(IncludeDescendants = false)]
    [AttachedPropertyBrowsableForType(typeof(TextBlock))]
    public static string GetText(this TextBlock element)
    {
        return (string) element.GetValue(TextProperty);
    }
}

---

<StackPanel>
    <TextBox x:Name="textBox1">TB1</TextBox>
    <TextBox x:Name="textBox2">TB2</TextBox>

    <TextBlock Text="{Binding Path=(local:Proxy.Text), 
                              RelativeSource={RelativeSource Self}, 
                              Converter={StaticResource SingleValueConverter}}">
        <local:Proxy.Text>
            <MultiBinding StringFormat="{}{0}{1}">
                <Binding ElementName="textBox1" Path="Text" />
                <Binding ElementName="textBox2" Path="Text" />
            </MultiBinding>
        </local:Proxy.Text>
    </TextBlock>
</StackPanel>