WPF Drag & drop from ListBox with SelectionMode Multiple

Question

I've almost got this working apart from one little annoying thing...

Because the ListBox selection happens on mouse down, if you start the drag with the mouse down when selecting the last item to drag it works fine, but if you select all the items to drag first and then click on the selection to start dragging it, the one you click on gets unselected and left behind after the drag.

Any thoughts on the best way to get around this?

<DockPanel LastChildFill="True">
    <ListBox ItemsSource="{Binding SourceItems}"
             SelectionMode="Multiple"
             PreviewMouseLeftButtonDown="HandleLeftButtonDown"
             PreviewMouseLeftButtonUp="HandleLeftButtonUp"
             PreviewMouseMove="HandleMouseMove"
             MultiSelectListboxDragDrop:ListBoxExtension.SelectedItemsSource="{Binding SelectedItems}"/>
    <ListBox ItemsSource="{Binding DestinationItems}"
             AllowDrop="True"
             Drop="DropOnToDestination"/>
<DockPanel>

...

public partial class Window1
{
    private bool clickedOnSourceItem;

    public Window1()
    {
        InitializeComponent();
        DataContext = new WindowViewModel();
    }

    private void DropOnToDestination(object sender, DragEventArgs e)
    {
        var viewModel = (WindowViewModel)
                            e.Data.GetData(typeof(WindowViewModel));
        viewModel.CopySelectedItems();
    }

    private void HandleLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        var sourceElement = (FrameworkElement)sender;
        var hitItem = sourceElement.InputHitTest(e.GetPosition(sourceElement))
                      as FrameworkElement;

        if(hitItem != null)
        {
            clickedOnSourceItem = true;
        }
    }

    private void HandleLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        clickedOnSourceItem = false;
    }

    private void HandleMouseMove(object sender, MouseEventArgs e)
    {
        if(clickedOnSourceItem)
        {
            var sourceItems = (FrameworkElement)sender;
            var viewModel = (WindowViewModel)DataContext;

            DragDrop.DoDragDrop(sourceItems, viewModel, DragDropEffects.Move);
            clickedOnSourceItem = false;
        }
    }
}

Answer 1

I've found a very simple way to enable Windows Explorer like drag/drop behaviour when having multiple items selected. The solution replaces the common ListBox with a little derived shim that replaces the ListBoxItem with a more intelligent version. This way, we can encapsulate the click state at the right level and call into the protected selection machinery of the ListBox. Here is the relevant class. For a complete example, see my repo on github.

public class ListBoxEx : ListBox
{
    protected override DependencyObject GetContainerForItemOverride()
    {
        return new ListBoxItemEx();
    }

    class ListBoxItemEx : ListBoxItem
    {
        private bool _deferSelection = false;

        protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
        {
            if (e.ClickCount == 1 && IsSelected)
            {
                // the user may start a drag by clicking into selected items
                // delay destroying the selection to the Up event
                _deferSelection = true;
            }
            else
            {
                base.OnMouseLeftButtonDown(e);
            }
        }

        protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
        {
            if (_deferSelection)
            {
                try
                {
                    base.OnMouseLeftButtonDown(e);
                }
                finally
                {
                    _deferSelection = false;
                }
            }
            base.OnMouseLeftButtonUp(e);
        }

        protected override void OnMouseLeave(MouseEventArgs e)
        {
            // abort deferred Down
            _deferSelection = false;
            base.OnMouseLeave(e);
        }
    }
}