How to select an item in the Silverlight Treeview?

Question

I am having problems to select the first TreeViewItem in a TreeView in Silverlight. The following code just gives a null value in the method SelectFistItemInCatTreeView. Why? Any ideas?

<controls:TreeView  x:Name="treeCategories" Grid.Column="1" Grid.Row="2" SelectedItemChanged="treeCategories_SelectedItemChanged">
  <controls:TreeView.ItemTemplate>
    <common:HierarchicalDataTemplate  ItemsSource="{Binding SubCats}">
      <StackPanel >
        <TextBlock Text="{Binding Name}"></TextBlock>
      </StackPanel>
    </common:HierarchicalDataTemplate>
  </controls:TreeView.ItemTemplate>
</controls:TreeView> 



1            public MainPage()
2            {
3                InitializeComponent();
4    
5                DBService.DocTrackingServeceClient webService = new DockTracking.DBService.DocTrackingServeceClient();
6                webService.GetDocCategoriesCompleted += new EventHandler(webService_GetDocCategoriesCompleted);
7                webService.GetDocCategoriesAsync();
8                treeCategories.Loaded += new RoutedEventHandler(treeCategories_Loaded);
9            }
10   
11           void webService_GetDocCategoriesCompleted(object sender, DockTracking.DBService.GetDocCategoriesCompletedEventArgs e)
12           {
13   
14               List cats = new List();
15               cats = GetCats(e.Result.ToList(), null);
16               treeCategories.ItemsSource = cats;
17           }
18   
19           void treeCategories_Loaded(object sender, RoutedEventArgs e)
20           {
21               SelectFistItemInCatTreeView();
22           }
23   
24           private void SelectFistItemInCatTreeView()
25           {
26               TreeViewItem item = treeCategories.ItemContainerGenerator.ContainerFromItem(treeCategories.Items[0]) as TreeViewItem;
27               if (item != null)
28               {
29                   item.IsSelected = true;
30               }
31           }

Answer 1

It looks like the ContainerFromItem method is supposed to be passed the actual item you're binding to, rather than just an tv.Items[0].

I'd look at the following:

• Post by Justin Josef-Angel about using the ContainerFromItem method
• Previous SO question about selecting a TreeViewItem in code

Answer 2

Calling UpdateLayout() prior to get the TreeViewItem does the trick:

private void SelectFistItemInCatTreeView()
{

  treeCategories.UpdateLayout();

  TreeViewItem item = treeCategories.ItemContainerGenerator.ContainerFromItem(treeCategories.Items[0]) as TreeViewItem;
  if (item != null)
  {
    item.IsSelected = true;
  }
}

Answer 3

If you have Hierarchical data, this will only work when selecting items at the top level. To select subitems underneath the top level, you have to expand and traverse the tree til the TreeViewItem for the desired item has been created.

The answer is here -> http://silverlightfeeds.com/post/543/Silverlight_TreeView_Advanced_Scenarios_TreeViewExtended.aspx

His TreeViewExtended class has a method called SetSelectedItem that actually works!