public override void OnApplyTemplate() { if (_comboBox != null) { _comboBox.ItemSelected -= ComboBox_SelectionChanged; _comboBox.PreviewProvider = null; } _comboBox = GetTemplateChild(@"PART_ComboBox") as HierarchicalComboBox; if (_comboBox != null) { _comboBox.ItemSelected += ComboBox_SelectionChanged; _comboBox.PreviewProvider = this; } base.OnApplyTemplate(); }