Multi Select Combobox in Silverlight With C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Banthia_Mgt_System.Assets.Wrapper_Class;
using System.Collections.ObjectModel;
using Banthia_Mgt_System.Classes;
using Banthia_Mgt_System.Assets.Classes;

namespace Banthia_Mgt_System.Assets.Multiselect_Combo
{
    public partial class UC_Multi_Dept_Combo : UserControl
    {
        List<CLS_Multi_Dept_Checked_WrapperBE> _Coll_Wrapper_BE;
        private List<CLS_Multi_Dept_Checked_WrapperBE> Coll_Wrapper_BE
        {
            get
            {
                return _Coll_Wrapper_BE;
            }
            set
            {
                _Coll_Wrapper_BE = value;
                foreach (var item in Coll_Wrapper_BE)
                item.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(item_PropertyChanged);
                cmbDepartment.ItemsSource = value;
                if (cmbDepartment.ItemsSource != null)
                    SetString();
            }
        }

        public UC_Multi_Dept_Combo()
        {
            InitializeComponent();
            cmbDepartment.SelectionChanged += new SelectionChangedEventHandler(cmbDepartment_SelectionChanged);
            cmbDepartment.DropDownOpened += new EventHandler(cmbDepartment_DropDownOpened);
        }

        void cmbDepartment_DropDownOpened(object sender, EventArgs e)
        {
            if (Coll_Wrapper_BE == null)
                Coll_Wrapper_BE = CLS_Multi_Dept_Checked_Wrapper.Get_Wrapper_Coll(new List<int>());
        }

        public static readonly DependencyProperty Str_Department_IdsProperty = DependencyProperty.Register("Str_Department_Ids", typeof(string), typeof(UC_Multi_Dept_Combo), new PropertyMetadata(null, OnPropertyChangedCallback));
        public string Str_Department_Ids
        {
            get
            {
                return (string)GetValue(Str_Department_IdsProperty);
            }
            set
            {
                SetValue(Str_Department_IdsProperty, value);
            }
        }

        private static void OnPropertyChangedCallback(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            var obj = (UC_Multi_Dept_Combo)o;

            obj.Coll_Wrapper_BE = CLS_Multi_Dept_Checked_Wrapper.Get_Wrapper_Coll(Globle_Function.Get_Coll_From_Str_CSV(obj.Str_Department_Ids));
        }

        void item_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            if (e.PropertyName == "IS_CHECKED")
            {
                SetString();
                //Department_id_String();
            }
        }

        public void SetString()
        {
            string str_Department_Name = "";
            string str_Department_Ids = "";
            foreach (var item in Coll_Wrapper_BE)
            {
                if (item.IS_CHECKED)
                {
                    if (str_Department_Name != "") str_Department_Name += ", ";
                    if (str_Department_Ids != "") str_Department_Ids += ", ";
                    str_Department_Name += item.DEPT_MST.DEPARTMENT_NAME;
                    str_Department_Ids += item.DEPT_MST.DEPARTMENT_ID.ToString();
                }
            }

            SetValue(Str_Department_IdsProperty, str_Department_Ids);
            tbDepartment.Text = str_Department_Name;
            var Tool = new ToolTip() { Content = str_Department_Name };
            ToolTipService.SetToolTip(tbDepartment, Tool);
            ToolTipService.SetToolTip(cmbDepartment, Tool);
        }

        void cmbDepartment_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            cmbDepartment.SelectedItem = null;
        }

        private void tbDepartment_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            cmbDepartment.IsDropDownOpen = true;
        }
    }
}

XAML Code

<UserControl x:Class="Banthia_Mgt_System.Assets.Multiselect_Combo.UC_Multi_Dept_Combo"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 mc:Ignorable="d" d:DesignWidth="300" >

 <Grid x:Name="LayoutRoot" Background="White">
         <ComboBox Name="cmbDepartment" Height="23" HorizontalAlignment="Stretch">
             <ComboBox.ItemTemplate>
                 <DataTemplate>
                     <CheckBox Content="{Binding DEPT_MST.DEPARTMENT_NAME}" IsChecked="{Binding IS_CHECKED,Mode=TwoWay}" />
                 </DataTemplate>
             </ComboBox.ItemTemplate>
         </ComboBox>
         <TextBlock x:Name="tbDepartment" TextTrimming="WordEllipsis" MouseLeftButtonDown="tbDepartment_MouseLeftButtonDown" HorizontalAlignment="Stretch" VerticalAlignment="Center" MinHeight="15" Text="" Margin="5,0,20,0" />

     </Grid>
 </UserControl>

Wrapper Class

using System;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Banthia_Mgt_System.ServiceReference1;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using Banthia_Mgt_System.Classes;
using System.ComponentModel;

namespace Banthia_Mgt_System.Assets.Wrapper_Class
{
    public class CLS_Multi_Dept_Checked_WrapperBE : INotifyPropertyChanged
    {
        private bool _IS_CHECKED;
        public bool IS_CHECKED
        {
            get { return _IS_CHECKED; }
            set
            {
                _IS_CHECKED = value;
                OnPropertyChanged("IS_CHECKED");
            }
        }

        public DEPARTMENT_MASTER DEPT_MST { get; set; }
        #region INotifyPropertyChanged Members
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            if (this.PropertyChanged != null)
            {
                this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    #endregion
    }

    public static class CLS_Multi_Dept_Checked_Wrapper
    {
        public static List<CLS_Multi_Dept_Checked_WrapperBE> Get_Wrapper_Coll(List<int> Coll_Dept_ID)
        {
            var CollBE = new List<CLS_Multi_Dept_Checked_WrapperBE>();
            foreach (var item in GlobalClass.GColl_DeptMaster)
            {
                var obj = new CLS_Multi_Dept_Checked_WrapperBE()
                {
                    IS_CHECKED = (Coll_Dept_ID.Where(ar => ar == item.DEPARTMENT_ID).Count() > 0),
                    DEPT_MST = item
                };
                CollBE.Add(obj);
            }
            return CollBE;
        }
        public static List<int> Get_Object_BE_Coll(List<CLS_Multi_Dept_Checked_WrapperBE> Coll)
        {
            List<int> NewColl = new List<int>();
            if (Coll != null)
                NewColl = new List<int>(Coll.Where(ar => ar.IS_CHECKED == true).Select(ar => ar.DEPT_MST.DEPARTMENT_ID));
            return NewColl;
        }
    }
}