Using enum as ComboBox value in WinUI3 doesn't work

[suhailmahmood]

Consider this XAML:

<StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center">
    <TextBlock Text="Theme:" VerticalAlignment="Center" Margin="10 0" />
    <ComboBox Name="themeCB"
        ItemsSource="{x:Bind Themes}"
        SelectedValue="{x:Bind SelectedTheme}"
        DisplayMemberPath="DisplayText"
        SelectedValuePath="Value" />

    <TextBlock Text="Backdrop:" VerticalAlignment="Center" Margin="10 0" />
    <ComboBox Name="backdropCB"
                ItemsSource="{x:Bind Backdrops}"
                SelectedValue="{x:Bind SelectedBackdrop}"
                DisplayMemberPath="DisplayText"
                SelectedValuePath="Value" />
</StackPanel>

And the code behind:

public MainWindow()
{
    InitializeComponent();

    SelectedTheme = ElementTheme.Light;
    SelectedBackdrop = BackdropType.Mica;
}

public List<ComboBoxItem<ElementTheme>> Themes =>
[
    new ComboBoxItem<ElementTheme>("Light", ElementTheme.Light),
    new ComboBoxItem<ElementTheme>("Dark", ElementTheme.Dark),
    new ComboBoxItem<ElementTheme>("System", ElementTheme.Default),
];

public List<ComboBoxItem<BackdropType>> Backdrops =>
[
    new ComboBoxItem<BackdropType>("None", BackdropType.None),
    new ComboBoxItem<BackdropType>("Mica", BackdropType.Mica),
    new ComboBoxItem<BackdropType>("Mica Alt", BackdropType.MicaAlt),
    new ComboBoxItem<BackdropType>("Acrylic", BackdropType.Acrylic),
    new ComboBoxItem<BackdropType>("Acrylic Thin", BackdropType.AcrylicThin),
];

public ElementTheme SelectedTheme { get; set; }
public BackdropType SelectedBackdrop { get; set; }

The ComboBoxItem and BackdropType types are as follows:

public class ComboBoxItem<T>
{
    public string DisplayText { get; set; }
    public T Value { get; set; }

    public ComboBoxItem() { }
    public ComboBoxItem(string displayText, T value)
    {
        DisplayText = displayText;
        Value = value;
    }
}

public enum BackdropType
{
    None,
    Mica,
    MicaAlt,
    Acrylic,
    AcrylicThin
}

ElementTheme is from Microsoft.UI.Xaml.

The problem is, the Theme: ComboBox gets populated with the value set, which is "Light", but the Backdrop: ComboBox does NOT get populated with the value set, which is "Mica". Why is that? What am I missing?

P.S.: I am not looking for a solution to make it work - I can make it work in a number of ways. I am trying to understand why it works for one enum but doesn't for another.

P.P.S.: I posted this same question on SO but got no answer so far.

[DarranRowe]

Xaml works based upon what is projected out to the Windows Runtime. ElementTheme is in the WinUI 3 metadata, it is in Microsoft.UI.Xaml.winmd. But have you actually made your BackdropType enum available to the Windows Rumtime?

[suhailmahmood]

@DarranRowe Not sure how to do that. But I did try adding the same three attributes that ElementTheme has to my BackdropType enum, namely the following, but it didn't work still.

[WindowsRuntimeType("Microsoft.UI.Xaml")]
[WinRTExposedType(typeof(EnumTypeDetails<BackdropType>))]
[ContractVersion(typeof(WinUIContract), 65536u)]
public enum BackdropType
{
    None,
    Mica,
    MicaAlt,
    Acrylic,
    AcrylicThin
}

[suhailmahmood]

The ComboBox works just fine when changing the selected value, SelectionChanged event gets fired correctly, populating using the SelectedIndex works fine too, only using SelectedValue does not work. To me this seems like a bug or something.

[DarranRowe]

It is probably trying to query for something extra. But I'd have to actually get the current public source and see what it is trying to do. But it is a Sunday.

[DarranRowe]

Well, after a little look through the WinUI 3 code shows the following.

ElementTheme is getting special treatment because it is an enum that is known to Xaml.

Custom enumerations don't get this special treatment, and so the runtime is querying for something else, but I'm not sure what. The IIDs are not well known to Visual Studio.

Does the documentation state anything about the requirements of the object you are using in the ComboBox?