WPF: Setting the Width (and Height) as a Percentage Value

Question

Say I want a TextBlock to have its Width equal to it\'s Parent container\'s Width (ie, stretch from side to side) or a percentage of

Answer 1

I know it's not Xaml but I did the same thing with SizeChanged event of the textbox:

private void TextBlock_SizeChanged(object sender, SizeChangedEventArgs e)
{
   TextBlock textBlock = sender as TextBlock;
   FrameworkElement element = textBlock.Parent as FrameworkElement;
   textBlock.Margin = new Thickness(0, 0, (element.ActualWidth / 100) * 20, 0);
}

The textbox appears to be 80% size of it's parent (well right side margin is 20%) and stretches when needed.