WPF ComboBox, whenever bound data changed, set SelectedIndex to 0?

Question

I couldn't find the right event to achieve the functionality.

TargetUpdated event didn't work.

setting SelectedIndex to 0 on xaml would only affect the first load of data.

Answer 1

Have you tried the SourceUpdated event?

Answer 2

You can:

• Set NotifyOnTargetUpdated on the binding
• Add an event handler for Binding.TargetUpdated
• In that event handler register for ItemsSource.CollectionChanged
• In that event handler set the selected index to zero

The issue is most likely that you didn't set NotifyonTargetUpdated in the binding so the first event wasn't fired or that the collection was being updated but it was the same collection so the second event is necessary.

Here's a working example using a ListBox as the ItemsControl and a MessageBox as a proxy for doing whatever you want to do when the event fires.

Here is the markup:

<Grid>
    <DockPanel>
        <Button DockPanel.Dock="Top" Content="Update Target" Click="ButtonUpdateTarget_Click"/>
        <Button DockPanel.Dock="Top" Content="Update Item" Click="ButtonUpdateItem_Click"/>
        <ListBox Name="listBox" Binding.TargetUpdated="ListBox_TargetUpdated" ItemsSource="{Binding Items, NotifyOnTargetUpdated=True}"/>
    </DockPanel>
</Grid>

and here is the code-behind:

public class ViewModel : INotifyPropertyChanged
{
    ObservableCollection<string> items;
    public ObservableCollection<string> Items
    {
        get { return items; }
        set { items = value; OnPropertyChanged("Items"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

void SetDataContext()
{
    DataContext = viewModel;
    viewModel.Items = new ObservableCollection<string> { "abc", "def", "ghi" };
}

ViewModel viewModel = new ViewModel();

private void ButtonUpdateTarget_Click(object sender, RoutedEventArgs e)
{
    viewModel.Items = new ObservableCollection<string> { "xyz", "pdq" };
}

private void ButtonUpdateItem_Click(object sender, RoutedEventArgs e)
{
    viewModel.Items[0] = "xxx";
}

private void ListBox_TargetUpdated(object sender, DataTransferEventArgs e)
{
    MessageBox.Show("Target Updated!");
    (listBox.ItemsSource as INotifyCollectionChanged).CollectionChanged += new NotifyCollectionChangedEventHandler(listBox_CollectionChanged);
}

void listBox_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    MessageBox.Show("Item Updated!");
}

Answer 3

I faced same problem. To overcome this problem, I used the following steps:

• create a TextBox
• Set visibility of TextBox to Collapsed

• Bind Text to ListBox.Items.Count

  <TextBox x:Name="txtCount" TextChanged="TextBox_TextChanged" Text="{Binding ElementName=ListBox1, Path=Items.Count, Mode=OneWay}" Visibility="Collapsed" />
  

• In the TextBox_TextChanged event, set SelectedIndex to 0

  private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
  {
      int count = 0;
  
      if(int.TryParse(txtCount.Text,out count) && count>0)
          ListBox1.SelectedIndex = 0;
  
  }