WPF binding IValueConverter and the width of another control - wpf

Question

The following code worked fine. There was an error elsewhere in the code. Still, the advice given is good.
I am trying to bind the Width of a TextBox to a percentage of the Width of a parent control. I know I can accomplish something similar by simply setting a Margin, but I was wondering why this doesn't work.
First, I set a reference to an IValueConverter in the resources collection of my user control:
<UserControl.Resources>
<local:TextBoxWidthConverter x:Key="txtWidthConv" />
</UserControl.Resources>
In the main xaml, I have the following:
<StackPanel Name="parentPanel" Width="300">
<ScrollViewer HorizontalScrollBarVisibility="Auto"
VerticalScrollBarVisibility="Hidden" Name="scroller" Width="{Binding Width,
ElementName=parentPanel, Converter={StaticResource txtWidthConv}}">
<StackPanel Orientation="Horizontal">
<TextBox></TextBox>
</StackPanel>
</ScrollViewer>
</StackPanel>
The ivalueconverter looks like this:
public class TextBoxWidthConverter : IValueConverter
{
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
double result = (double)value;
if (!Double.IsNaN(result))
{
result = result * .25;
}
else
{
result = 100D;
}
return result;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new System.NotImplementedException("Not implemented.");
}
#endregion
}
Setting the width property does nothing here, let alone setting the IValueConverter. I would expect the ScrollViewer to be 1/4 the width of the parent StackPanel.

Answer 1

Set the ScrollViewer's HorizontalAlignment to something other than Stretch.
Also, you should bind to the ActualWidth property.

Answer 2

Let the layout system work for you instead of fighting it. Grid will automatically handle relative sizing:
<StackPanel Name="parentPanel" Width="300">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="3*" />
</Grid.ColumnDefinitions>
<ScrollViewer HorizontalScrollBarVisibility="Auto"
VerticalScrollBarVisibility="Hidden" Name="scroller">
<StackPanel Orientation="Horizontal">
<TextBox></TextBox>
</StackPanel>
</ScrollViewer>
</Grid>
</StackPanel>