Question

I’m using a WPF TreeView control, which I’ve bound to a simple tree structure based on ObservableCollections. Here’s the XAML:

<TreeView Name="tree" Grid.Row="0"> 
    <TreeView.ItemTemplate> 
        <HierarchicalDataTemplate ItemsSource="{Binding Path=Children}"> 
            <TextBlock Text="{Binding Path=Text}"/> 
        </HierarchicalDataTemplate> 
    </TreeView.ItemTemplate> 
</TreeView>  

And the tree structure:

public class Node : IEnumerable { 
    private string text; 
    private ObservableCollection<Node> children; 
    public string Text { get { return text; } } 
    public ObservableCollection<Node> Children { get { return children; } } 
    public Node(string text, params string[] items){ 
        this.text = text; 
        children = new ObservableCollection<Node>(); 
        foreach (string item in items) 
            children.Add(new Node(item)); 
    } 
    public IEnumerator GetEnumerator() { 
        for (int i = 0; i < children.Count; i++) 
            yield return children[i]; 
    } 
} 

I set the ItemsSource of this tree to be the root of my tree structure, and the children of that become root-level items in the tree (just as I want):

private Node root; 

root = new Node("Animals"); 
for(int i=0;i<3;i++) 
    root.Children.Add(new Node("Mammals", "Dogs", "Bears")); 
tree.ItemsSource = root; 

I can add new children to the various non-root nodes of my tree structure, and they appear in the TreeView right where they should.

root.Children[0].Children.Add(new Node("Cats", "Lions", "Tigers"));  

But, if I add a child to the root node:

root.Children.Add(new Node("Lizards", "Skinks", "Geckos")); 

The item does not appear, and nothing I’ve tried (such as setting the ItemsSource to null and then back again) has caused it to appear.

If I add the lizards before setting the ItemsSource, they show up, but not if I add them afterwards.

Any ideas?

Answer 1

You are setting ItemsSource = root which happens to implement IEnumerable but is not in and of itself observable. Even though you have a Children property which is observable, that’s not what you’re binding the TreeView to so the TreeView doesn’t have any way of listening to changes that occur through the Children property.

I would drop IEnumerable from the Node class altogether. Then set treeView.ItemsSource = root.Children;