Combobox Not Updating Despite SelectedItem and ItemsSource Bindings

Question

I have the following ComboBox:

<ComboBox SelectedItem="{Binding SelectedTheme, Mode=TwoWay}"
          ItemsSource="{Binding Themes, Mode=OneTime}" />

It is bound to the following values in my VM:

private Theme _selectedTheme;
public Theme SelectedTheme
{
    get { return _selectedTheme; }
    set
    {
        if (_selectedTheme != value)
        {
            _selectedTheme = value;
            OnPropertyChanged();
        }
    }
}

public List<Theme> Themes =>
    Enum.GetValues(typeof(Theme)).Cast<Theme>().ToList();

I set SelectedThemes value in the VM's ctor, and the get member is being hit after I assign the VM instance to my Page's DataContext. My trouble is the UI does not reflect the binding value the first time I load the page; it updates works correctly all other times, but the combobox does not show any selection after the page is initially loaded.

score 6 · Accepted Answer · 2016-12-26 05:56:58Z

6

After struggling with this issue for about two hours, I realized that the UWP framework is connecting the bindings in the order they are set, so the SelectedItem is being set correctly, but is then cleared when the ItemsSource value is set. Changing my XAML to the following fixes the problem:

<ComboBox ItemsSource="{Binding Themes, Mode=OneTime}"
          SelectedItem="{Binding SelectedTheme, Mode=TwoWay}" />

edited Dec 26, 2016 at 5:56

answered Dec 26, 2016 at 1:50

user2509848

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8 Comments

M.kazem Akhgary Over a year ago

Besides that shouldnt you use observable collection instead of list? (interesting though never thought order of attributes would be important)

user2509848 Over a year ago

I don't need to because the list will never change.

M.kazem Akhgary Over a year ago

So use OneTime binding Mode instead of TwoWay.

user2509848 Over a year ago

I typically don't set the mode if the default one works; is it important to? Also, isn't the default one-way?

M.kazem Akhgary Over a year ago

Default mode varies on different properties. In your case its OneWay mode. But since your collection does not change and also can be made static, OneTime binding is lighter than other modes. So thats just optimizing hint.

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