Question

I am using MVVM light and have a list box with multiple selection. In my Mainpage.xaml I have

<ListBox Name="ListBox1" ItemsSource="{Binding Items}" Grid.Column="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="Transparent"  Margin="15,15,18,0" SelectionMode="Multiple" Height="100" />

In MainPage.xaml.cs I have (I do not want to use dependency property for some reason).

MainPage()
{
    ListBox1.SelectionChanged = new SelectionChangedEventHandler(ListBox1_SelectionChanged);
}

void ListBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
 var listBox = sender as ListBox;
 var viewModel = listBox.DataContext as MainViewModel;
 viewModel.SelectedItems.Clear();
 foreach (string item in listBox.SelectedItems)
     viewModel.SelectedItems.Add(item);
}

and this works fine and binds to my MainViewModel. But When the page is loaded I want the first item of the the collection items to be selected by default. Please let me know how to implement this

Answer 1

I’d recommend using the ListBox’s Loaded event and then bind to the first item in the collection:

MainPage()
{
    ListBox1.Loaded += new RoutedEventHandler( OnListBox1Loaded );
    ListBox1.SelectionChanged += new SelectionChangedEventHandler(ListBox1_SelectionChanged);
}

private void OnListBox1Loaded( object sender, RoutedEventArgs e )
{
    // make sure the selection changed event doesn't fire
    // when the selection changes
    ListBox1.SelectionChanged -= MyList_SelectionChanged;

    ListBox1.SelectedIndex = 0;
    e.Handled = true;

    // re-hook up the selection changed event.
    ListBox1.SelectionChanged += MyList_SelectionChanged;
}

Edit

If you can not use the Loaded event, then you’ll need to create another property in your Model that will hold the item you want selected and then assign that property to the SelectedItem property of the ListBox.

public class MyModel : INotifyPropertyChanged
{

  private ObservableCollection<SomeObject> _items;
  public ObservableCollection<SomeObject> Items
  {
    get { return _items; }
    set
    {
        _items = value;
        NotifyPropertyChanged( "Items" );
    }
  }

  private SomeObject _selected;
  public SomeObject  Selected
  {
    get { return _selected; }
    set
    {
        _selected = value;
        NotifyPropertyChanged( "Selected" );
    }
  }

  public void SomeMethodThatPopulatesItems()
  {
    // create/populate the Items collection

    Selected = Items[0];
  }

  // Implementation of INotifyPropertyChanged excluded for brevity

}

XAML

<ListBox ItemsSource="{Binding Path=Items}" 
         SelectedItem="{Binding Path=Selected}"/>

By having another property that holds the currently selected item, you also have access in your Model to that item as well whenever the selected item is changed by the user.