Question

I am trying to create a Blend behavior related to ComboBoxes. In order to get the effect I want, the ItemsPanel of the ComboBox has to have a certain element added to it. I don’t want to do this in every ComboBox that uses the behavior, so I want the Behavior to be able to inject the ItemsPanelTemplate programmatically. However, I can’t seem to find a way to do this. ItemsPanelTemplate does not seem to have a property/method that lets me set the visual tree. WPF ItemsPanelTemplate has the VisualTree but Silverlight does not.

Basically, what is the programmatic equivalent of this XAML?

    <ComboBox>
        <ComboBox.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel/>
            </ItemsPanelTemplate>
        </ComboBox.ItemsPanel>
    </ComboBox>

Edit:
Okay apparently that is not an easy question, so I started a bounty and I’m going to give some more background in case there is another way to go about this. I want to provide keyboard support for the Silverlight ComoboBox. Out of the box it only supports the up and down arrows but I also want it to work so that when the user hits a letter, the ComboBox jumps to the first item of that letter, similar to how ComboBoxes work in a browser or Windows app.

I found this blog post, which got me half way. Adapting that behavior code, the ComboBox will change selection based on letter input. However, it does not work when the ComboBox is opened. The reason for this, according to this blog post is that when the ComboBox is opened, you are now interacting with its ItemsPanel and not the ComboBox itself. So according to that post I actually need to add a StackPanel to the ItemsPanelTemplate and subscribe to the StackPanel’s KeyDown event, in order to take action when the ComboBox is opened.

So that is what prompted my question of how to get a StackPanel into the ItemsPanelTemplate of a ComboBox, from a behavior. If that is not possible, are there alternative ways of getting this to work? Yes, I know I could go to each ComboBox in the application and add a StackPanel and the event. But I want to do this through a behavior so that I don’t have to modify every ComboBox in the app, and so I can reuse this logic across applications.

AnthonyWJones’ answer below using XamlReader gets me part way, in that I can create the StackPanel and get it into the template. However, I need to be able to get at that SP programmatically in order to subscribe to the event.

Answer 1

I think, best way for you – extend combobox functionality not via behavior but using inheritance.
So, you can create own control MyComboBox:ComboBox. Create style for it – get default ComboBox Style
here

And write instead (look for ScrollViewer by name):

< ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="1" >

   < ItemsPresenter />

< /ScrollViewer >

this

< ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="1" >

< StackPanel x:Name="StackPanel" >

   < ItemsPresenter />

< /StackPanel >

< /ScrollViewer >

This StackPanel you can get in code:

public class MyComboBox: ComboBox{

    public CM()
    {
        DefaultStyleKey = typeof (MyComboBox);
    }
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        StackPanel stackPanel = (StackPanel)GetTemplateChild("StackPanel");
        stackPanel.KeyUp += (s, e) => { /*do something*/ };
    }

}

Inheritance is more powerful. It’s allow work with template elements.
If you decided to inject ItemsPanel, you must understand that:

1)it’s impossible from code with keeping reference on injected panel.
2)to get reference to injected panel, this panel must registered itself in some storage, e.g.

< ComboBox>

   < ComboBox.ItemsPanel>

       < ItemsPanelTemplate>

           < StackPanel>

             < i:Interaction.EventTriggers>

               < i:EventTrigger EventName="Loaded">

                  < RegisterMyInstanceInAccessibleFromCodePlaceAction/>

               < /i:EventTrigger>

             < /i:Interaction.EventTriggers>

          < /StackPanel>

       < /ItemsPanelTemplate>

   < /ComboBox.ItemsPanel>

< /ComboBox>

Good luck!