Question

I have a ListBox that can have multiple items selected at once. I have a UserControl that needs needs to be visible if exactly one item in the ListBox is selected.

Here is the pane that needs to be hidden:

<views:WebMethodsPane x:Name="WebMethodsPane"  Grid.Column="1" Grid.Row="0"   Margin="5,5,5,0" 
Visibility="{Binding SelectedList, Converter={StaticResource SelectionToVisibilityConverter}}" />

The SelectedList object is an ObservableCollection that is filled with items that are selected by the user in the ListBox. (I used a behavior to do this.)

The SelectionToVisibilityConverter goes as follows:

public class SelectionToVisibilityConverter : IValueConverter
{

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var selectedServices = value as ObservableCollection<WebService>;
        return (selectedServices.Count == 1 ? Visibility.Visible : Visibility.Collapsed);
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

When I run the app, the pane is hidden and stays hidden. The visibility is not being updated when I select different numbers of items from the ListBox. How can I make sure the Visibility updates? Maybe I need to use INotifyPropertyChanged, but I don’t know exactly how to.

Answer 1

The approach I would take would be to add a new property to the bound object, a SingleItemSelected boolean property.

Something along the lines of:-

 public class YourClass : INotifyPropertyChanged
 {

     public ObservableCollection<WebService> SelectedList {get; private set; }

     // ctor
     public YourClass()
     {
         SelectedList = new ObservableCollection<WebService>();
         SelectedList.CollectionChanged += SelectedList_CollectionChanged;
     }

     private void SelectedList_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
     {
         SingleItemSelected = SelectedList.Count == 1;
     }

     private bool mySingleItemSelected
     public bool SingleItemSelected
     {
         get { return mySingleItemSelected; }
         private set
         {
              if (mySingleItemSelected != value)
              {
                   mySingleItemSelected = value;
                   PropertyChanged(this, new PropertyChangedEventArgs("SingleItemSelected"));
              }
         }
     }

     public event PropertyChangedEventHandler PropertyChanged = delegate {};

}

So now what you need is a simple BoolToVisibilityConverter, there are plenty of examples of such a thing, I prefer my own here.

Then you xaml (assuming you placed an instance of the converter in the resources with the key “BtoV”):

<views:WebMethodsPane x:Name="WebMethodsPane"  Grid.Column="1" Grid.Row="0" Margin="5,5,5,0"
    Visibility="{Binding SingleItemSelected, Converter={StaticResource BtoV}}" />