Pass the value of one control to a Converter to set the width on another control

Question

I want to set the width of a TextBlock based on the width of its container, minus the margins set on the TextBlock.

Here is my code:

<TextBlock x:Name="txtStatusMessages" 
           Width="{Binding ElementName=LayoutRoot,Path=ActualWidth }"
                   TextWrapping="WrapWithOverflow" 
           Foreground="White" 
           Margin="5,5,5,5">This is a message
</TextBlock>

And that works great except for the fact that the TextBlock is 10 units too big due to the Left and Right Margins bbeing set to 5.

OK, so I thought... Let's use a Converter. But I don't know how to pass the ActualWidth of my container control (SEE ABOVE: LayoutRoot).

I know how to use converters, and even converters with parameters, just not a parameter like... Binding ElementName=LayoutRoot,Path=ActualWidth

For example, I can't make this work:

Width="{Binding Converter={StaticResource PositionConverter},  
       ConverterParameter={Binding ElementName=LayoutRoot,Path=ActualWidth }}"

I hope I made this clear enough and hope that you can help because Google is no help for me tonight.

Answer 1

you're supposed to use the other control as the source, not the parameter. The parameter has to be a constant and in your case can be -5.

I'm not near VS at the moment so the syntax maybe inaccurate, however, it is something like:

Width="{Binding ElementName=LayoutRoot, Path=ActualWidth,
Converter={StaticResource PositionConverter}, ConverterParameter=-5}"

(The converter will receive -5 as a string and will have to convert it into a number before using it.)

From my experience it is better to use the OnXXXChanged callback of DependecyProperty XXX, and not bind controls within the same window/root control one to another. One of the reasons for this is that you may want to bind them to an external element later on.

Or alternatively, use multibinding:

<TextBlock>
    <TextBlock.Width>
        <MultiBinding Converter="{StaticResource yourConverter}">
            <MultiBinding.Bindings>
                <Binding /> <!-- Bind to parameter 1 here -->
                <Binding /> <!-- Bind to parameter 2 here -->
          </MultiBinding.Bindings>
        </MultiBinding>
    </TextBlock.Width>
</TextBlock>

and and a converter which converts the two parameters to the value you want.