Friday, 27 March 2009

Binding Combo Boxes in WPF with MVVM

I was recently creating a simple WPF application and was trying to use the MVVM pattern. In this pattern, all the controls on your form are data bound to properties on your “View Model” class. While there are lots of examples of how to do this with Text boxes, List boxes, and even master-detail views, it seems that examples for binding Combo boxes are a little thin on the ground.

What I wanted to do was bind the items in the ComboBox to a list in my ViewModel and to track the currently selected item. I found that there does not seem to be one “official” way of doing this. One approach bound both the ItemsSource and the SelectedValue properties of the Combo box to corresponding properties on ViewModel. The approach I went with uses a CollectionView which is a class included with .NET that encapsulates a list and the concept of a current item as well as supporting the INotifyPropertyChanged interface.

So here is the XAML first. I set the IsSychronizedWithCurrentItem to true to allow us to track the current item on the ItemsSource.

<ComboBox ItemsSource="{Binding Path=Queries}"                 
          DisplayMemberPath="Name" />

The code in the ViewModel first creates the List, and then creates a CollectionView based on that list. This allows us to set the CurrentItem from the ViewModel as well as get notified whenever the CurrentItem changes.

public MainWindowViewModel()
    IList<Query> availableQueries = new List<Query>();
    // fill the list...

    Queries = new CollectionView(availableQueries);
    Queries.CurrentChanged += new EventHandler(queries_CurrentChanged);

public CollectionView Queries { get; private set; }

void queries_CurrentChanged(object sender, EventArgs e)
    Query currentQuery = (Query)Queries.CurrentItem;

I’m not sure yet whether using CollectionView is a better approach than the alternatives I have seen which bind the SelectedValue or SelectedItem property. I would be interested to hear in the comments if you think either approach has benefits over the other. One consideration is that Silverlight doesn’t seem to support CollectionView at the moment.