Question

I’m receiving this error: “items collection must be empty before using itemssource” in a treeview.
My XAML code just contains:

<TreeView ItemsSource="{Binding Groups}">
</TreeView>

Groups is a class where contains only two properties: GroupID and GroupName. I’m sure that my collection has items, but I’m not sure what’s the reason for thix exception.

Answer 1

This error occurs when you have items added to the Items collection and use the ItemsSource, e.g.

<ListBox ItemsSource="{Binding Data}">
    <ListBoxItem Content="A concrete item"/>
</ListBox>

You can of course create such a problem in code too. So make sure you did not add anything manually somewhere.