MVVM and Multiple Selection – Part IV - DataGrid

Download the source for this article.

Previous posts in this series:

• http://grokys.blogspot.com/2010/07/mvvm-and-multiple-selection-part-iii.html
• http://grokys.blogspot.com/2010/07/mvvm-and-multiple-selection-part-ii.html
• http://grokys.blogspot.com/2010/07/mvvm-and-multiple-selection-part-i.html

Please note, this is WPF only - I don't use Silverlight!

It's been a while since my previous post in this series, MVVM and Multiple Selection – Part III. In the comments to that post, I had a number of people asking about the DataGrid control, however by that point I was no longer in WPF-land and didn't have the time or inclination to investigate properly.

Recently though, I needed to track multiple selections on a DataGrid and as the commenters pointed out, it didn't work. The problem came down to the method I was using to track changes to the ItemsSource property on a control:

static MultiSelect()
{
    Selector.ItemsSourceProperty.OverrideMetadata(typeof(Selector), new FrameworkPropertyMetadata(ItemsSourceChanged));
}

This doesn't work for DataGrid as the ItemsSource metadata on that control is already overridden, so I needed to find a different approach.

Here's what I came up with:

static void IsEnabledChanged(object sender, DependencyPropertyChangedEventArgs e)
{
    Selector selector = sender as Selector;
    bool enabled = (bool)e.NewValue;

    if (selector != null)
    {
        DependencyPropertyDescriptor itemsSourceProperty =
            DependencyPropertyDescriptor.FromProperty(Selector.ItemsSourceProperty, typeof(Selector));
        IMultiSelectCollectionView collectionView = selector.ItemsSource as IMultiSelectCollectionView;

        if (enabled)
        {
            if (collectionView != null) collectionView.AddControl(selector);
            itemsSourceProperty.AddValueChanged(selector, ItemsSourceChanged);
        }
        else
        {
            if (collectionView != null) collectionView.RemoveControl(selector);
            itemsSourceProperty.RemoveValueChanged(selector, ItemsSourceChanged);
        }
    }
}

Instead of overriding the property metadata globally for all Selectors, I attach a listener to the PropertyDescriptor. Now, the only problem here is that the event handler for PropertyDescriptor changed events are plain old EventHandler delegates, so by the time the property has changed we can't get hold of the old IMultiSelectCollectionView in order to remove the control from it. I get round this by storing all attached controls in a static Dictionary.

This leaves us the problem that we're going to be keeping controls alive when after they're gone, but that's a post for another day. In the meantime, download the code!