附加属性作用于多选控件，用于获取所有选中项

    using System.Collections;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Controls.Primitives;
    /// <summary>
    /// 附加属性作用于多选控件，用于获取所有选中项
    /// </summary>
    public static class SelectorExt
    {

        public static readonly DependencyProperty SelectedItemsProperty =
            DependencyProperty.RegisterAttached("SelectedItems", typeof(IList), typeof(SelectorExt), new PropertyMetadata(OnSelectedItemsChanged));
        public static IList GetSelectedItems(DependencyObject obj)
        {
            return obj.GetValue(SelectedItemsProperty) as IList;
        }

        public static void SetSelectedItems(DependencyObject obj, IList value)
        {
            obj.SetValue(SelectedItemsProperty, value);
        }

        private static void OnSelectedItemsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d == null) return;
            //MultiSelector
            MultiSelector multiSelector = d as MultiSelector;
            if (multiSelector != null)
            {
                multiSelector.SelectionChanged -= MultiSelector_SelectionChanged;
                multiSelector.SelectedItems.Clear();
                if ((e.NewValue as IList) != null)
                {
                    foreach (object item in e.NewValue as IList)
                    {
                        multiSelector.SelectedItems.Add(item);
                    }
                }
                multiSelector.SelectionChanged += MultiSelector_SelectionChanged;
                return;
            }

            ListBox listbox = d as ListBox;//增加Listbox多选附加,ListView继承ListBox，也可以使用
            if (listbox != null)
            {
                listbox.SelectionChanged -= MultiSelector_SelectionChanged;
                listbox.SelectedItems.Clear();
                if ((e.NewValue as IList) != null)
                {
                    foreach (object item in e.NewValue as IList)
                    {
                        listbox.SelectedItems.Add(item);
                    }
                }
                listbox.SelectionChanged += MultiSelector_SelectionChanged;
                return;
            }
        }
        private static void MultiSelector_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            IList source = GetSelectedItems(sender as DependencyObject);
            if (source == null) return;
            foreach (var item in e.AddedItems) source.Add(item);        //添加选中的项
            foreach (var item in e.RemovedItems) source.Remove(item);   //移除选中项
        }
    }