在WPF的MVVM设计中有仅有一个控件ComboBox，其IsEditable属性设置为true，该控件绑定后台ViewModel中的数据类型是public ObservableCollection<string> Source，Source的来源是sqlite数据库中的MSJ.db数据库的表Source的数据，该表只有一列数据，列名为Source，表里有两行数据分别为“上级配发”和“本级自购”，要求实现第一点：ComboBoxItem选择发生改变时，弹窗提示用户选择前后的数据；第二点：ComboBox的输入框在接受自定义输入后，判断用户输入不属于ComboBoxItem后，自动将其加入ComboBoxItem中，同时更新后台sqlite数据库的MSJ.db数据库的表Source的数据。请给出实现上述功能的完整实例代码

以下是实现上述功能的完整实例代码：

XAML代码：

<Window x:Class="ComboBoxDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="ComboBox Demo" Height="150" Width="300">
    <Grid>
        <ComboBox x:Name="comboBox" Margin="10" IsEditable="True"
                  ItemsSource="{Binding Source}" SelectedItem="{Binding SelectedItem}"
                  Text="{Binding Text}">
            <ComboBox.ItemContainerStyle>
                <Style TargetType="{x:Type ComboBoxItem}">
                    <EventSetter Event="Selected" Handler="ComboBoxItemSelected"/>
                </Style>
            </ComboBox.ItemContainerStyle>
        </ComboBox>
    </Grid>
</Window>

C#代码：

using System.Collections.ObjectModel;
using System.Data.SQLite;
using System.Windows;

namespace ComboBoxDemo
{
    public partial class MainWindow : Window
    {
        private ViewModel viewModel;

        public MainWindow()
        {
            InitializeComponent();
            viewModel = new ViewModel();
            DataContext = viewModel;
        }

        private void ComboBoxItemSelected(object sender, RoutedEventArgs e)
        {
            MessageBox.Show($"Selected: {viewModel.SelectedItem}");
        }
    }

    public class ViewModel
    {
        private ObservableCollection<string> source;

        public ViewModel()
        {
            source = new ObservableCollection<string>();
            LoadData();
        }

        public ObservableCollection<string> Source
        {
            get { return source; }
            set { source = value; }
        }

        private string selectedItem;

        public string SelectedItem
        {
            get { return selectedItem; }
            set { selectedItem = value; }
        }

        private string text;

        public string Text
        {
            get { return text; }
            set
            {
                text = value;
                if (source.IndexOf(text) == -1)
                {
                    source.Add(text);
                    UpdateData();
                }
            }
        }

        private void LoadData()
        {
            using (SQLiteConnection connection = new SQLiteConnection("Data Source=MSJ.db"))
            {
                connection.Open();
                using (SQLiteCommand command = new SQLiteCommand("SELECT Source FROM Source", connection))
                {
                    using (SQLiteDataReader reader = command.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            source.Add(reader.GetString(0));
                        }
                    }
                }
            }
        }

        private void UpdateData()
        {
            using (SQLiteConnection connection = new SQLiteConnection("Data Source=MSJ.db"))
            {
                connection.Open();
                using (SQLiteCommand command = new SQLiteCommand("INSERT INTO Source (Source) VALUES (@Source)", connection))
                {
                    command.Parameters.AddWithValue("@Source", text);
                    command.ExecuteNonQuery();
                }
            }
        }
    }
}

注意：为了运行上述代码，需要在项目中添加以下文件：MSJ.db、SQLite.Interop.dll、System.Data.SQLite.dll。其中，MSJ.db是SQLite数据库文件，SQLite.Interop.dll和System.Data.SQLite.dll是SQLite的.NET Framework驱动程序。