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.