StringFormat is ignored

Question

This is my binding (shortened, Command-Property is also bound)

<MenuItem Header="Key" CommandParameter="{Binding StringFormat='Key: {0}', Path=PlacementTarget.Tag, RelativeSource={RelativeSource AncestorType=ContextMenu}}"/>

The Tag-Property of ContectMenu's PlacementTarget is a String like

"Short.Plural"

What i expect to receive in the Command-Handler is:

Key: Short.Plural

But what i acutally receive is:

Short.Plural

Answer 1

Label does not use StringFormat but ContentStringFormat. Use it this way:

<TextBlock x:Name="textBlock" Text="Base Text"/>
<Label Content="{Binding Path=Text, ElementName=textBlock}" ContentStringFormat="FORMATTED {0}"/>

Answer 2

I'm astounded, but my tests just show that StringFormat only applies if the target d-prop is of type String. I've never noticed this before, nor heard it mentioned. I don't have more time to look into it right now, but this seems ridiculous.

Seriously, this works:

<TextBlock x:Name="textBlock" Text="Base Text"/>
<TextBlock Text="{Binding StringFormat=FORMATTED {0}, Path=Text, ElementName=textBlock}"/>

This does not:

<TextBlock x:Name="textBlock" Text="Base Text"/>
<Label Content="{Binding StringFormat=FORMATTED {0}, Path=Text, ElementName=textBlock}"/>

Since Label.Content is not a String.

Answer 3

use Binding Converter:

public class CommandParamConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is string)
        {
            return string.Format("Key {0}", value);
        }
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Add it to Windows\UserControl resource:

<Window.Resources>
    <local:CommandParamConverter x:Key="commandParamConverter" />
</Window.Resources>

Refer it in Menu CommandParameter binding:

<MenuItem Header="Key" CommandParameter="{Binding Converter={StaticResource commandParamConverter}, Path=PlacementTarget.Tag, RelativeSource={RelativeSource AncestorType=ContextMenu}}"/>