1. MainWindow.xaml

<Window x:Class="MultiSelectDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ViewModel="clr-namespace:MultiSelectDemo"
        xmlns:control="clr-namespace:MultiSelectComboBox;assembly=MultiSelectComboBox"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <ViewModel:ViewModel x:Key="ViewModel"/>
    </Window.Resources>
    <Grid DataContext="{Binding Source={StaticResource ViewModel}}">
        <control:MultiSelectComboBox Width="100" Height="30" ItemsSource="{Binding Items}"
                                     DefaultText="input jjj"
                                     SelectedItems="{Binding SelectedItems,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                                     />
    </Grid>
</Window>

2. MainWindowViewModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;

namespace MultiSelectDemo
{
    public class MainWindowViewModel: NotificationObject
    {

        private Dictionary<string, object> _items;
        private Dictionary<string, object> _selectedItems;

        public Dictionary<string, object> Items
        {
            get
            {
                return _items;
            }
            set
            {
                _items = value;
                NotifyPropertyChanged("Items");
            }
        }

        public Dictionary<string, object> SelectedItems
        {
            get
            {
                return _selectedItems;
            }
            set
            {
                _selectedItems = value;
                NotifyPropertyChanged("SelectedItems");
            }
        }

        public MainWindowViewModel()
        {
            Items = new Dictionary<string, object>();
            Items.Add("Chennai", "MAS");
            Items.Add("Trichy", "TPJ");
            Items.Add("Bangalore", "SBC");
            Items.Add("Coimbatore", "CBE");

            SelectedItems = new Dictionary<string, object>();
            SelectedItems.Add("Chennai", "MAS");
            SelectedItems.Add("Trichy", "TPJ");
        }

        private void Submit()
        {
            foreach (KeyValuePair<string, object> s in SelectedItems)
                MessageBox.Show(s.Key);
        }

    }

}

MultiSelectComboBox(二)