Question

I have a databound ListBox that is behaving strangely. The ListBox’s SelectionMode property is set to MultiExtended, and on a button click, I need to copy the items to another control, in this case, a TreeView. However, for some reason, every iterator I’ve tried only loops once. I’ve attempted both SelectedItems and SelectedIndices. Code excerpt:

var movedItems = new List<ListBoxUnderlyingObject>();
foreach (var selectedItem in listBox.SelectedItems)
{
    var castItem = selectedItem as ListBoxUnderlyingObject;
    var newNode = new TreeNode(castItem.SomeString);
    newNode.Name = castItem.AnotherString;
    newNode.Tag = castItem;
    newNode.ForeColor = Color.RoyalBlue;
    //parentNode was set earlier
    parentNode.Nodes.Add(newNode);
    movedItems.Add(selectedItem);
}
//use movedItems to remove items from listBox's underlying databound object and rebind

No matter how many items are selected, the loop only executes once. Same with SelectedIndices. If I attempt it with a numbered iterator, it fails with an “index out of bounds of array” error.

for(var i = 0;i < listBox.SelectedItems.Count;i++)
{
    var castItem = listBox.SelectedItems[i] as ListBoxUnderlyingObject;
    //etc., the previous line bombs on the second iteration
}

If I throw a Debug.WriteLine(listBox.SelectedItems.Count) either before or during the loop, it always reflects the correct count. I know this is probably something stupid, but I’m stumped. Help!

Follow Up

I’ve created a separate winforms project that emulates the behavior almost exactly, and SelectedItems works. I am completely baffled. Now, I’m going to try and add a new form in the original project and see if I can recreate the behavior there.

Answer 1

Well, it turns out I did withhold some critical information. The listbox has drag-drop behavior enabled, and part of this is a handler for the MouseDown event. The handler has this code in it:

private void listBox_MouseDown(object sender, MouseEventArgs e)
{
    if (listBox.Items.Count = 0) return;
    listBox.DoDragDrop(listBox.SelectedItem, DragDropEffects.Move);
}

If I comment this handler out, SelectedItems behaves correctly. Now I have to figure out how to do the drag-drop stuff correctly, but that’s a different question.