Extending a control (ComboBox) in silverlight - silverlight

Question

I have a simple problem. I am trying to add a dependency property to a combo box. I want to be able to display a value on the face of the combo box when it is initially displayed. There is a ContentPresenter with a TextBlock inside the ComboBox. That TextBlock gets set when a user selects an item in the ComboBox. How can I set that with a default value (not one of the items)? Show something like 'choose one'.
I can extend the ComboBox with a 'DefaultDisplay' dependency property but how do I 'link' that property to the TextBlock that is part of the control template?
Thanks for any help on this.
Pat

Answer 1

Instead of a dependency property I'd suggest using a behavior to do this. I actually already had one of these written for this problem. Give it a try by adding it to your combobox and setting the PromptText property on the behavior:
public class ComboBoxPromptBehavior : Behavior<ComboBox>
{
[Category("Display")]
public string PromptText
{
get { return (string)GetValue(PromptTextProperty); }
set { SetValue(PromptTextProperty, value); }
}
public static readonly DependencyProperty PromptTextProperty = DependencyProperty.Register("PromptText", typeof(string), typeof(ComboBoxPromptBehavior), new PropertyMetadata(" "));
protected override void OnAttached()
{
base.OnAttached();
AssociatedObject.Loaded += new RoutedEventHandler(AssociatedObject_Loaded);
AssociatedObject.LayoutUpdated += new EventHandler(AssociatedObject_LayoutUpdated);
}
void AssociatedObject_LayoutUpdated(object sender, EventArgs e)
{
SetPromptText();
}
protected override void OnDetaching()
{
base.OnDetaching();
AssociatedObject.Loaded -= new RoutedEventHandler(AssociatedObject_Loaded);
AssociatedObject.LayoutUpdated -= new EventHandler(AssociatedObject_LayoutUpdated);
}
void AssociatedObject_Loaded(object sender, RoutedEventArgs e)
{
SetPromptText();
}
private void SetPromptText()
{
var textbox = AssociatedObject.FindChild<TextBlock>();
if (textbox != null && string.IsNullOrWhiteSpace(textbox.Text))
{
textbox.Text = PromptText;
}
}
}
And the extension method for FindChild is:
public static T FindChild<T>(this DependencyObject element) where T : DependencyObject
{
var childCount = VisualTreeHelper.GetChildrenCount(element);
for (int i = 0; i < childCount; i++)
{
var child = VisualTreeHelper.GetChild(element, i);
if (child is T)
{
return (T)child;
}
var match = child.FindChild<T>();
if (match != null) return match;
}
return null;
}