What could be preventing my combobox dropdown from showing after app focus is lost?

Question

I have a ComboBox in a WPF app that has recently been refactored to use the MVVM pattern. An apparent side effect to this change is that changing focus to another application while the combobox dropdown is visible completely prevents the dropdown from being visible again, until the app has been restarted.

The ComboBox DataContext is set to my ViewModel, with its ItemsSource bound to an ObservableCollection<String> SearchSuggestions, and IsDropdownOpen bound to a property SuggestionsVisible in the ViewModel.

The desired effect is a search box with autocomplete suggestions. It should close if there are no suggestions in the ObservableCollection, if the user cancels the search, if the user runs the search, or if the user clicks away from the text field - either inside the app or outside it.

The ViewModel explicitly sets the SuggestionsVisible property to true or false based on whether SearchSuggesions contains any items after user input. This process continues to take place after this bug manifests itself, just with no visible change to the UI. Any idea why losing focus while the dropdown is open renders the dropdown un-openable for the rest of the app's session?

Here's how I have things wired together:

<ComboBox DataContext="{Binding SearchBoxVm}" Name="cmboSearchField" Height="0.667" 
          VerticalAlignment="Top" IsEditable="True" StaysOpenOnEdit="True" 
          PreviewKeyUp="cmboSearchField_OnKeyUp" 
          PreviewMouseLeftButtonUp="cmboSearchField_OnPreviewMouseLeftButtonUp" 
          Background="White" ItemsSource="{Binding SearchTopics}" 
          IsDropDownOpen="{Binding SuggestionsVisible, 
          UpdateSourceTrigger=PropertyChanged}" 
          Margin="50.997,15.333,120.44,0" 
          RenderTransformOrigin="0.5,0.5" Grid.Row="1" >

<!-- SNIP STYLING -->
</ComboBox>

ViewModel:

public class SearchBoxViewModel : INotifyPropertyChanged
{
    public void ResetSearchField(bool preserveContents = false)
    {
        if (!preserveContents || string.IsNullOrEmpty(Query))
        {
            Foreground = Brushes.Gray;
            QueryFont = FontStyles.Italic;
            Query = DEFAULT_TEXT;
        }
    }

    public bool OnKeyUp(Key key)
    {
        bool showDropdown = SuggestionsVisible;
        bool changeFocusToCombobox = false;

        if (keyInValidRange(key))
        {
            SearchSuggestions = GetSearchSuggestions(Query);
            if (SearchSuggestions.Count > 0)
            {
                SuggestionsVisible = true;
            }
        }

        return changeFocusToCombobox;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(String propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    bool _suggestionsVisible = false;
    public bool SuggestionsVisible 
    {
        get { return _suggestionsVisible; }
        set
        { 
                    // this section is still called after this issue manifests,
                    // but no visible change to the UI state is made
            _suggestionsVisible = value;
            NotifyPropertyChanged("SuggestionsVisible");
        }
    }

    public ObservableCollection<String> SearchTopics = new ObservableCollection<String>();
 }

The OnKeyUp() method is called by the MainWindow class ( haven't gotten as far as binding events to handlers specified in the ViewModel ), while but there's also a call to ResetSearechField from the MainWindow:

// Note: removing references to this event handler does not have any effect
// on the issue at hand... only including here for completeness
void window_Deactivated(object sender, EventArgs e)
{
    SearchBoxVm.SuggestionsVisible = false; 
    SearchBoxVm.ResetSearchField(true);            
}

I've spent quite a bit of time trying to debug this, and haven't seen any internal state changes that might account for this. The NotifyPropertyChanged event is otherwise behaving as it did before, and the stack trace window isn't showing any exceptions having been encountered.

Setting the binding mode on the IsDropdownOpen property to 'TwoWay' in the XAML hasn't had any effect either. Lastly, wrapping the assignment to SuggestionsVisible in a Dispatcher call on the main thread has had no effect on the issue either.

Any assistance would be appreciated.

Answer 1

@BrMcMullin, since you have stated that:

The desired effect is a search box with autocomplete suggestions.

may I ask, why do you choose to use standard ComboBox instead of specialized AutoCompleteBox that is available in the WPF Toolkit - February 2010 Release and seems like was especially designed for your case?

You may have noticed that first link points to documentation for its Silverlight predecessor, but don't worry - WPF Toolkit library include fully functional official WPF port of AutoCompleteBox from Silverlight. There is more info about this "event": AutoCompleteBox: Now with 100% more WPF.

With that control your auto complete popup could looks as simple as:

or as complex as:

So, if you will not manage to solve your issue with ComboBox's popup visibility, feel free to give a try to AutoCompleteBox. With it you could even leverage dynamic sorting of your suggestions if needed (just use answer from @adabyron).