Developing a MultiSelector

using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Input;
using System.Windows.Media;

namespace SelectionAreaSample
{
    public sealed class SelectionAreaItem : ContentControl
    {
        #region Public Events

        public static readonly RoutedEvent SelectedEvent = Selector.SelectedEvent.AddOwner(typeof(SelectionAreaItem));

        public event RoutedEventHandler Selected
        {
            add
            {
                base.AddHandler(SelectedEvent, value);
            }
            remove
            {
                base.RemoveHandler(SelectedEvent, value);
            }
        }

        public static readonly RoutedEvent UnselectedEvent = Selector.UnselectedEvent.AddOwner(typeof(SelectionAreaItem));

        public event RoutedEventHandler Unselected
        {
            add
            {
                base.AddHandler(UnselectedEvent, value);
            }
            remove
            {
                base.RemoveHandler(UnselectedEvent, value);
            }
        }

        #endregion Public Events

        #region Public Properties

        public static readonly DependencyProperty IsSelectedProperty = Selector.IsSelectedProperty.AddOwner(typeof(SelectionAreaItem), new FrameworkPropertyMetadata(false, OnIsSelectedChanged));

        public bool IsSelected
        {
            get
            {
                return (bool)GetValue(IsSelectedProperty);
            }
            set
            {
                SetValue(IsSelectedProperty, value);
            }
        }

        private static void OnIsSelectedChanged(DependencyObject target, DependencyPropertyChangedEventArgs e)
        {
            var item = target as SelectionAreaItem;

            if ((bool)e.NewValue)
            {
                item.RaiseEvent(new RoutedEventArgs(Selector.SelectedEvent, item));
            }
            else
            {
                item.RaiseEvent(new RoutedEventArgs(Selector.UnselectedEvent, item));
            }
        }

        #endregion Public Properties

        #region Internal Properties

        internal SelectionArea ParentSelectionArea
        {
            get
            {
                return ItemsControl.ItemsControlFromItemContainer(this) as SelectionArea;
            }
        }

        #endregion Internal Properties

        #region Protected Methods

        protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
        {
            base.OnMouseLeftButtonDown(e);

            var parent = ParentSelectionArea;

            if (parent != null)
            {
                parent.NotifyItemClicked(this);
            }

            e.Handled = true;
        }

        protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
        {
            if (VisualTreeHelper.GetDescendantBounds(this).Contains(hitTestParameters.HitPoint))
            {
                return new PointHitTestResult(this, hitTestParameters.HitPoint);
            }

            return null;
        }

        protected override GeometryHitTestResult HitTestCore(GeometryHitTestParameters hitTestParameters)
        {
            var geometry = new RectangleGeometry(VisualTreeHelper.GetDescendantBounds(this));
            return new GeometryHitTestResult(this, geometry.FillContainsWithDetail(hitTestParameters.HitGeometry));
        }

        #endregion Protected Methods
    }
}