WPF Default Value (using Style Trigger) with Binding - wpf

Question

I'm new to WPF so please bear with me. I have a ComboBox on my WPF window, the ItemSource property is bound to a List of strings property (Countries) and the SelectedItem is bound to a string property (SelectedCountry). Both of these properties are in the code behind - and I'm setting the DataContext to "this" (i.e. the Window).
The ComboBox xaml is:
<ComboBox Name="CountryComboBox"
VerticalAlignment="Center"
Width="200"
ItemsSource="{Binding Path=Countries, Mode=OneTime}"
SelectedItem="{Binding Path=SelectedCountry, Mode=TwoWay}">
</ComboBox>
I wanted to have a default "- Please Select -" option that is displayed when an item is not selected, therefore I placed the following xaml in App.xaml:
<Style TargetType="ComboBox">
<Style.Triggers>
<Trigger Property="SelectedItem" Value="{x:Null}">
<Setter Property="IsEditable" Value="true" />
<Setter Property="IsReadOnly" Value="true" />
<Setter Property="Text" Value="- Please Select -" />
</Trigger>
</Style.Triggers>
</Style>
When my window is first displayed, the combobox does have the "- Please Select -" text as expected. When I then select a value in the combobox, the SelectedCountry gets populated appropriately, but then when I assign "null" to the SelectedCountry property the combobox still has the same selected country when I'd expect it to go back to "- Please Select -". What am I doing wrong?
Thanks.

Answer 1

It may be a better option not to modify the ComboBox and simply overlay a TextBlock over the ComboBox when the SelectedItem is null.
Just wrap the ComboBox and a TextBlock in a Grid and set a DataTrigger on the TextBlock to check if the SelectedItem is null and toggle its Visibility
Example:
<Grid>
<ComboBox x:Name="combo" ItemsSource="{Binding Countries}" SelectedItem="{Binding SelectedItem}" />
<TextBlock x:Name="textblock" Text="- Please Select -" Margin="5,3,0,0" IsHitTestVisible="False">
<TextBlock.Style>
<Style TargetType="TextBlock">
<Setter Property="Visibility" Value="Hidden" />
<Style.Triggers>
<DataTrigger Binding="{Binding SelectedItem,ElementName=combo}" Value="{x:Null}">
<Setter Property="Visibility" Value="Visible" />
</DataTrigger>
</Style.Triggers>
</Style>
</TextBlock.Style>
</TextBlock>
</Grid>
Result:

Answer 2

You need to insert a record into the Countries list that has a value of null and the name " - Please Select - ".
Alternatively I suppose you could extend the ComboBox control and write your own so that you could specify the null vale in the list without having to put a record into the Countries selection.
But of the two, it is much easier to just add a record to Countries.