Binding SelectedItems

MultiSelectorHelper.cs

using System;
using System.Collections;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;

namespace DataGridView
{
    public class MultiSelectorHelper
    {
        #region Bindable SelectedItems

        public static IList GetBindableSelectedItems(DependencyObject obj)
        {
            return (IList)obj.GetValue(BindableSelectedItemsProperty);
        }

        public static void SetBindableSelectedItems(DependencyObject obj, IList value)
        {
            obj.SetValue(BindableSelectedItemsProperty, value);
        }

        public static readonly DependencyProperty BindableSelectedItemsProperty =
            DependencyProperty.RegisterAttached(
                "BindableSelectedItems",
                typeof(IList),
                typeof(MultiSelectorHelper),
                new PropertyMetadata(null)
            );

        #endregion

        #region Monitor SelectionChanged

        public static bool GetMonitorSelectionChanged(DependencyObject obj)
        {
            return (bool)obj.GetValue(MonitorSelectionChangedProperty);
        }

        public static void SetMonitorSelectionChanged(DependencyObject obj, bool value)
        {
            obj.SetValue(MonitorSelectionChangedProperty, value);
        }

        public static readonly DependencyProperty MonitorSelectionChangedProperty =
            DependencyProperty.RegisterAttached(
                "MonitorSelectionChanged",
                typeof(bool),
                typeof(MultiSelectorHelper),
                new PropertyMetadata(false, MonitorSelectionChangedPropertyChangedCallback)
            );

        private static void MonitorSelectionChangedPropertyChangedCallback(
            DependencyObject d,
            DependencyPropertyChangedEventArgs e
        )
        {
            if (d is not MultiSelector selector)
                throw new InvalidOperationException();

            if ((bool)e.NewValue)
                selector.SelectionChanged += OnSelectionChanged;
            else
                selector.SelectionChanged -= OnSelectionChanged;
        }

        private static void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var selector = (MultiSelector)sender;
            SetBindableSelectedItems(selector, selector.SelectedItems);
            selector.GetBindingExpression(BindableSelectedItemsProperty).UpdateSource();
        }

        #endregion
    }
}

SelectedItemsBehavior.cs

using Microsoft.Xaml.Behaviors;
using System.Collections;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;

namespace DataGridView
{
    public class SelectedItemsBehavior : Behavior<MultiSelector>
    {
        protected override void OnAttached()
        {
            base.OnAttached();

            AssociatedObject.SelectionChanged += OnSelectionChanged;
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();

            AssociatedObject.SelectionChanged -= OnSelectionChanged;
        }

        private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var name = nameof(OnSelectionChanged);
            var func = (int x) => x.ToString();
            var selector = (MultiSelector)sender;
            BindableSelectedItems = selector.SelectedItems;
            selector.GetBindingExpression(BindableSelectedItemsProperty)?.UpdateSource();
        }

        #region Bindable SelectedItems

        public IList BindableSelectedItems
        {
            get { return (IList)GetValue(BindableSelectedItemsProperty); }
            set { SetValue(BindableSelectedItemsProperty, value); }
        }

        public static readonly DependencyProperty BindableSelectedItemsProperty =
            DependencyProperty.Register(
                nameof(BindableSelectedItems),
                typeof(IList),
                typeof(SelectedItemsBehavior),
                new PropertyMetadata(null)
            );

        public static int GetMyProperty(DependencyObject obj)
        {
            return (int)obj.GetValue(MyPropertyProperty);
        }

        public static void SetMyProperty(DependencyObject obj, int value)
        {
            obj.SetValue(MyPropertyProperty, value);
        }

        public static readonly DependencyProperty MyPropertyProperty =
            DependencyProperty.RegisterAttached(
                "MyProperty",
                typeof(int),
                typeof(SelectedItemsBehavior),
                new PropertyMetadata(0)
            );

        #endregion
    }
}