c# - WPF: ComboBox with reset item

Question

I want to make a ComboBox in WPF that has one null item on the top, when this gets selected, the SelectedItem should be set to null (reset to default state). I've searched like forever, but didn't find a solution that was satisfying.

If possible I would want it to do it only with XAML code or an attached behaviour, because I don't really like changing stuff in the ViewModel for the View, or overriding standard controls.

Here is what I've come up with so far (shortened code):

[...]
<Popup x:Name="PART_Popup" [...]>
    <Border x:Name="PopupBorder" [...]>
        <ScrollViewer x:Name="DropDownScrollViewer" [...]>
            <StackPanel [...]>
                <ComboBoxItem>(None)</ComboBoxItem>
                <ItemsPresenter x:Name="ItemsPresenter"/>
            </StackPanel>
        </ScrollViewer>
    </Border>
</Popup>
[...]

I think the best way would be to somehow add an event trigger that sets the SelectedIndex to -1 when the item gets selected, but here is where I've got stuck.

Any ideas how to do this? Or an better way, like an attached behaviour?

Answer 1

Think about implementing a Null Object Pattern for the "None" combobox item and add this item to your items list. Then implement custom logic for saving null object in that class, or just check if selected item is of NullItem type.