未设置UWP ComboBox SelectedValue

Question

I obviously don't grasp using SelectedValue to change which item a combo box is showing in UWP

The XAML is simple

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <ComboBox x:Name="comboBoxColor" />
</Grid>

The code-behind isn't very deep

    public MainPage()
    {
        this.InitializeComponent();
    }

    public void page_Loaded(object sender, RoutedEventArgs e)
    {
        populateDdlMultiColor();

        comboBoxColor.SelectedValue = Colors.Red;
        //comboBoxColor.SelectedIndex = 0 works tho
    }

    private void populateDdlMultiColor()
    {
        comboBoxColor.ItemsSource = ColorDict();
        comboBoxColor.DisplayMemberPath = "Key";
        comboBoxColor.SelectedValuePath = "Value";


    }

    private Dictionary<string,Color> ColorDict()
    {
        Dictionary<string, Color> L = new Dictionary<string, Color>();
        L.Add("reddish",Colors.Red);

        return L;
    }

This is obviously tinker-toy but it fails the same way my code fails: After setting the SelectedValue, the combo box is on index -1 and SelectedValue is null. If I set SelectedIndex to a proper value [see comment] the combo box works - it has been loaded.

Answer 1

Thanks for your feedback. This is a known issue that SelectedValue doesn't work with enumeration type.

For now, as a workaround, you can use SelectedIndex or SelectedItem like you've done. However, please note that Dictionary is implemented as a hash table. There's no such concept of an "index" within a Dictionary . The order is undefined and we can't rely on it. So for your scenario, using SelectedItem might be better.

For more info, you can see this answer .

Answer 2

Thanks to Jay Zuo for putting me on the right track! In my scenario, it's then easy enough to convert the colors to integers as a workaround to the known issue. Won't work for every enumeration, but anyone building color dropdowns [in UWP] may find this handy. Note the Dictionary change.

    public MainPage()
    {
        this.InitializeComponent();
    }

    public void page_Loaded(object sender, RoutedEventArgs e)
    {
        populateDdlMultiColor();

        comboBoxColor.SelectedValue = ColorInt(Colors.Red);
    }

    private void populateDdlMultiColor()
    {
        comboBoxColor.ItemsSource = ColorDict();
        comboBoxColor.DisplayMemberPath = "Key";
        comboBoxColor.SelectedValuePath = "Value";


    }

    private Dictionary<string,int> ColorDict()
    {
        Dictionary<string, int> L = new Dictionary<string, int>();
        L.Add("reddish",ColorInt(Colors.Red));

        return L;
    }

    private int ColorInt(Color c)
    {

        return (c.A*16777216) + (c.R*65536) + (c.G*256) + c.B ;
    }