When you assign a value to the EditorControl property, you are telling the UltraTree where to get its embeddable editor from. Using that embedded editor within the UltraTree will not cause the control's events to fire. The following code sample demonstrates how to handle the EditorWithCombo-specific events (in this example, SelectionChanged) that are raised by the embedded editor:

using Infragistics.Win;
using Infragistics.Win.UltraWinEditors;
using Infragistics.Win.UltraWinTree;

public partial class Form1 : Form
{
    private EditorWithCombo editor = null;

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        //  Add some nodes
        this.tree.Nodes.Add( "Node 1" );
        this.tree.Nodes.Add( "Node 2" );
        this.tree.Nodes.Add( "Node 3" );

        //  Add some items
        this.comboEditor.Items.Add( "Item 1" );
        this.comboEditor.Items.Add( "Item 2" );
        this.comboEditor.Items.Add( "Item 3" );

        //  Enabled editing, using an EditorWithCombo provided by the UltraComboEditor
        this.tree.Override.LabelEdit = DefaultableBoolean.True;
        this.tree.Override.UseEditor = Infragistics.Win.DefaultableBoolean.True;
        this.tree.Override.EditorControl = this.comboEditor;

        //  Hook the UltraTree's AfterActivate event
        this.tree.AfterActivate += new AfterNodeChangedEventHandler(this.tree_AfterActivate);

    }

    //  Handles the UltraTree's AfterActivate event, for the purpose
    //  of hooking the SelectionChanged event of the EditorWithCombo
    //  associated with the ActiveNode.
    private void tree_AfterActivate(object sender, NodeEventArgs e)
    {
        UltraTreeNode node = e.TreeNode;
        if ( node == null )
            return;

        //  If the editor hasn't changed since the last time we did this,
        //  return since we are already listening to the right editor's event.
        EditorWithCombo editor = node.EditorResolved as EditorWithCombo;
        if ( this.editor == editor )
            return;

        //  If we have a non-null editor reference, unhook its SelectionChanged event
        if ( this.editor != null )
        {
            this.HookSelectionChanged( this.editor, false );
            this.editor = null;
        }

        //  Set our reference
        this.editor = editor;

        //  Hook the event on the new editor
        if ( this.editor != null )
            this.HookSelectionChanged( this.editor, true );
    }

    //  Handles the EditorWithCombo's SelectionChanged event.
    private void editor_SelectionChanged(object sender, EventArgs e)
    {
        EditorWithCombo editor = sender as EditorWithCombo;
        object value = editor.Value;
    }

    //  Hooks/unhooks the EditorWithCombo's SelectionChanged event.
    private void HookSelectionChanged( EditorWithCombo editor, bool hook )
    {
        if ( hook )
            editor.SelectionChanged += new EventHandler(this.editor_SelectionChanged);
        else
            editor.SelectionChanged -= new EventHandler(this.editor_SelectionChanged);
    }

    protected override void Dispose(bool disposing)
    {
        if ( disposing )
        {
            if ( this.components != null )
                this.components.Dispose();

            //  If we have a non-null editor reference, unhook its SelectionChanged event
            if ( this.editor != null )
            {
                this.HookSelectionChanged( this.editor, false );
                this.editor = null;
            }
        }

        base.Dispose(disposing);
    }

}

Thank you. My mistake.

I'm not sure I followed this, but the only way to get checkboxes to appear next to the items in UltraComboEditor's dropdown is using a draw filter or creation filter. The first hack that comes to mind is to assign an image to each item, then use the IUIElementCreationFilter interface to pull the image out, and replace it with a CheckBoxUIElement. The problem is that it will not respond to user interaction, since clicking on it will cause the dropdown to close. If this is not a concern, i.e., the checkbox is read-only anyway, you might also be able to show an image of a checkbox (have two images, one for each check state, assigning them based on what the item's "CheckState" is). If none of this is helpful, please repost with clarification and we can try to help.