private void InsertText()
{
var filter = FilterText ?? string.Empty;
var insertText = filter + FilteredItems.ElementAt(AutoCompleteBox.SelectedIndex).Substring(filter.Length);
var currentCaret = AssociatedObject.CaretIndex;
string newText = AssociatedObject.Text;
newText = newText.Remove(currentCaret - filter.Length, filter.Length);
currentCaret -= filter.Length;
newText = newText.Insert(currentCaret, insertText);
AssociatedObject.Text = newText;
AssociatedObject.CaretIndex = currentCaret + insertText.Length;
}