Binding a TextBox's Width to its parent container's ActualWidth - windows-phone-7

Question

I'm loading a Textbox and a Button into a horizontal StackPanel programmatically. The size of the button (which only contains an Image) is fixed, but I can't get the textbox to fill the available width of its parent. This is what the code looks like:
StackPanel parent = new StackPanel() {
Orientation = Orientation.Horizontal,
HorizontalAlignment = HorizontalAlignment.Stretch,
VerticalAlignment = VerticalAlignment.Top,
};
TextBox textbox = new TextBox() {
HorizontalAlignment = HorizontalAlignment.Stretch,
VerticalAlignment = VerticalAlignment.Top,
//MinWidth = 375,
};
Button btn = new Button() {
Content = new Image() {
MaxHeight = 40,
MaxWidth = 40,
MinHeight = 40,
MinWidth = 40,
Margin = new Thickness( 0 ),
Source = new BitmapImage(
new Uri( "btnimage.png", UriKind.Relative ) ),
},
HorizontalAlignment = HorizontalAlignment.Right,
BorderBrush = new SolidColorBrush( Colors.Transparent ),
Margin = new Thickness( 0 ),
};
btn.Click += ( ( s, e ) => OnBtnClicked( s, e, textbox ) );
parent.Children.Add( textbox );
parent.Children.Add( btn );
If I uncomment the MinWidth setting for the textbox it is displayed as I want it to, but I'd like to not have to specify a width explicitly. I tried adding a binding as follows but that doesn't work at all (the textbox just disappears!)
Binding widthBinding = new Binding() {
Source = parent.ActualWidth,
};
passwdBox.SetBinding( TextBox.WidthProperty, widthBinding );
Thanks for your help in advance!

Answer 1

Instead of using StackPanel (which always tries to keep elements as small as it can such that they all fit), use a Grid. Then you could do something like this:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/> <!-- Note "*" means to use the rest of the space -->
<ColumnDefinition Width="40"/>
</Grid.ColumnDefinitions>
<TextBox Grid.Column="0"/>
<Button Grid.Column="1">
<Button.Content>
<Image .../>
</Button.Content>
</Button>
</Grid>
You can convert this to code instead of XAML if you prefer, XAML's just easier to type here on-the-fly.

Answer 2

The right answer is don't do it. See my answer at this question and the same idea applies to Silverlight on Windows Phone.
In your example, you should be using a DockPanel.
<toolkit:DockPanel>
<Button toolkit:DockPanel.Dock="Right" />
<TextBox /> <!-- fill is implied -->
</toolkit:DockPanel>