Need a simple Example of cascading combo boxes using MVVM

Question

Need a simple Example of cascading combo boxes using MVVM

Wpf / Silverlight

Answer 1

If I understand your question you want to have the next combobox to fill with data based on the previous value.

I have a generic ViewModel that you can have to capture the list of items and the selected item

class ItemListViewModel<T> : INotifyPropertyChanged where T : class
{
    private T _item;
    private ObservableCollection<T> _items;

    public ItemListViewModel()
    {
        _items = new ObservableCollection<T>();
        _item = null;
    }

    public void SetItems(IEnumerable<T> items)
    {
        Items = new ObservableCollection<T>(items);
        SelectedItem = null; 
    }

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

    public T SelectedItem
    {
        get { return _item; }
        set
        {
            _item = value;
            RaisePropertyChanged("SelectedItem");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void RaisePropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Then have the main viewmodel that will be bound to the DataContext of the view. Have the Load methods do what you want

class MyViewModel : INotifyPropertyChanged
{
    public MyViewModel()
    {
        First = new ItemListViewModel<string>();
        Second = new ItemListViewModel<string>();
        Third = new ItemListViewModel<string>();

        First.PropertyChanged += (s, e) => Update(e.PropertyName, First, Second, LoadSecond);
        Second.PropertyChanged += (s, e) => Update(e.PropertyName, Second, Third, LoadThird);

        LoadFirst();
    }

    public ItemListViewModel<string> First { get; set; }
    public ItemListViewModel<string> Second { get; set; }
    public ItemListViewModel<string> Third { get; set; }

    private void LoadFirst()
    {
        First.SetItems(new List<string> { "One", "Two", "Three" });
    }
    private void LoadSecond()
    {
        Second.SetItems(new List<string> { "First", "Second", "Third" });
    }
    private void LoadThird()
    {
         Third.SetItems(new List<string> { "Firsty", "Secondly", "Thirdly" });
    }

    private void Update<T0, T1>(string propertyName, ItemListViewModel<T0> parent, ItemListViewModel<T1> child, Action loadAction)
        where T0 : class
        where T1 : class
    {
        if (propertyName == "SelectedItem")
        {
            if (parent.SelectedItem == null)
            {
                child.SetItems(Enumerable.Empty<T1>());
            }
            else
            {
                loadAction();
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

And in your view have this code somewhere.

    <ComboBox ItemsSource="{Binding First.Items}" SelectedItem="{Binding First.SelectedItem}" />
    <ComboBox ItemsSource="{Binding Second.Items}" SelectedItem="{Binding Second.SelectedItem}" />
    <ComboBox ItemsSource="{Binding Third.Items}" SelectedItem="{Binding Third.SelectedItem}" />

You can refactor to make it nicer, use MVVM frameworks or derive the ItemListViewModel specifically for the list of items and have the load in there for better encapsulation. Its up to you.

If any parent combobox value gets changed then all child lists will get cleared.

HTH