Element is already the child of another element.

Answer 1

Your items may be visual elements, instead of data objects.

If you provide the XAML, I can help make sure that is the case.

Answer 2

Usually this error occurs when the said Element is already attached to an existing Parent and somewhere in your code you're attempting to re-parent it (ie via just straight "add" when you in turn must remove the child from the parent first, then Add it to the children etc).

Where specifically the control is failing the above isn't enough info to digest.

Answer 3

The error is a generic catch-all exception that has many many causes. I've written a debugger utility that can help to identify which part of the XAML is actually causing the error. You can download it from my blog: http://whydoidoit.com/2010/08/30/debug-xaml-element-is-already-the-child-of-another-element/

Answer 4

Simple and stupid solution:

public class AutoCompleteTextBox : AutoCompleteBox
    {
        public override void OnApplyTemplate()
        {
            try
            {
                base.OnApplyTemplate();
            }
            catch { }
        }
    }