WPF - Can't set combobox selectedIndex when itemssource changes

Question

I have a combobox that shows a list of items. The list of items displayed is determined by a set of radio buttons. I attach to the radio button clicked events and attempt to set a new itemssource on the combobox. I would like the selectedItem to default to 0, and not -1.

What am I doing wrong?

<Grid>
    <ComboBox Name="cb_Test" />
    <RadioButton Content="List 1" Name="radioButton1" Click="radioButton1_Click" />
    <RadioButton Content="List 2" Name="radioButton2" Click="radioButton2_Click" />
</Grid>

public partial class MainWindow : Window
{
    List<string> list1 = new List<string>() { "list 1", "list 1", "list 1" };
    List<string> list2 = new List<string>() { "list 2", "list 2", "list 2" };
    ComboBoxViewModel viewModel = new ComboBoxViewModel();

    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = viewModel;
        cb_Test.ItemsSource = list1;
    }

    private void radioButton1_Click(object sender, RoutedEventArgs e)
    {
        cb_Test.ItemsSource = list1;
        viewModel.SelectedIndex = 0;
    }

    private void radioButton2_Click(object sender, RoutedEventArgs e)
    {
        cb_Test.ItemsSource = list2;
        viewModel.SelectedIndex = 0;
    }
}

public class ComboBoxViewModel : INotifyPropertyChanged
{
    private int selectedIndex;
    public event PropertyChangedEventHandler PropertyChanged;

    public int SelectedIndex
    {
        get { return selectedIndex; }
        set
        {
            if (selectedIndex != value)
            {
                selectedIndex = value;
                NotifyPropertyChanged("SelectedIndex");
            }
        }
    }

    private void NotifyPropertyChanged(string controlName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(controlName));
        }
    }
}

Answer 1

You don't have any databinding to ComboBoxViewModel view model in your XAML, at least provided one. I think that is the problem.

Answer 2

You need to bind your combobox to your VM.

public partial class MainWindow : Window
{
    List<string> list1 = new List<string>() { "list 1", "list 1", "list 1" };
    List<string> list2 = new List<string>() { "list 2", "list 2", "list 2" };
    ComboBoxViewModel viewModel = new ComboBoxViewModel();

    public MainWindow()
    {
        InitializeComponent();
        cb_Test.DataContext = viewModel;
    }

    private void radioButton1_Click(object sender, RoutedEventArgs e)
    {
        viewModel.ItemsSource = list1;
        viewModel.SelectedIndex = 0;
    }

    private void radioButton2_Click(object sender, RoutedEventArgs e)
    {
        viewModel.ItemsSource = list2;
        viewModel.SelectedIndex = 0;
    }
}

and in XAML

<Grid>
    <ComboBox Name="cb_Test"
              ItemsSource="{Binding ItemsSourse}"
              SelectedIndex="{Binding SelectedIndex}"/>
    <RadioButton Content="List 1" Name="radioButton1" Click="radioButton1_Click" />
    <RadioButton Content="List 2" Name="radioButton2" Click="radioButton2_Click" />
</Grid>

Even better design would be to move the code viewModel.ItemsSource = list1; viewModel.SelectedIndex = 0; to the VM itself.

Answer 3

Agree with Tigran. You need to assign the ComboBoxViewModel as the DataContext of the Window/Page/Control whatever this is AND you would need to bind SelectedIndex in the declaration of the ComboBox in the xaml. Since you need to bind, you should also move the List collections to one ViewModel with the SelectedIndex, and bind the ComboBox ItemsSource to a Property in your ViewModel that you can set under certain conditions.