DragDropFriendlyListBoxItem.cs

public class DragDropFriendlyListBoxItem : ListBoxItem
{
    private List<object> previousSelection = new List<object>();
    private List<object> currentSelection = new List<object>();

    private bool capturedSelectionOnMouseDown;

    /// <summary>
    /// Finds the parent DragDropFriendly list box.
    /// </summary>
    /// <returns>The parent <c>DragDropFriendlyListBox</c>.</returns>
    private DragDropFriendlyListBox FindParentDragDropFriendlyListBox()
    {
        Visual visual = this;
        while (visual != null && !(visual is DragDropFriendlyListBox))
        {
            visual = (Visual)VisualTreeHelper.GetParent(visual);
        }

        return visual as DragDropFriendlyListBox;
    }

    /// <summary>
    /// Called when the user presses the left mouse button over the <see cref="T:System.Windows.Controls.ListBoxItem"/>.
    /// </summary>
    /// <param name="e">The event data.</param>
    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
    {
        capturedSelectionOnMouseDown = false;

        if (this.IsSelected)
        {
            previousSelection.Clear();
            currentSelection.Clear();

            DragDropFriendlyListBox lb = FindParentDragDropFriendlyListBox();
            previousSelection.AddRange(lb.SelectedItems.Cast<object>());
            base.OnMouseLeftButtonDown(e);
            currentSelection.AddRange(lb.SelectedItems.Cast<object>());

            lb.UnselectAll();
            previousSelection.ForEach(item => lb.SelectedItems.Add(item));
            capturedSelectionOnMouseDown = true;
        }
        else
        {
            base.OnMouseLeftButtonDown(e);
        }
    }        

    /// <summary>
    /// Raises the <see cref="E:PreviewMouseLeftButtonUp"/> event.
    /// </summary>
    /// <param name="e">The <see cref="MouseButtonEventArgs"/> instance containing the event data.</param>
    protected override void OnPreviewMouseLeftButtonUp(MouseButtonEventArgs e)
    {
        // Apply the new selection if required.
        if (capturedSelectionOnMouseDown)
        {
            DragDropFriendlyListBox lb = FindParentDragDropFriendlyListBox();
            lb.UnselectAll();
            currentSelection.ForEach(item => lb.SelectedItems.Add(item));
        }

        // Clean up to store no references.
        currentSelection.Clear();
        previousSelection.Clear();            
    }         
}