数据集在visual studio 2010中没有更新

我是visual studio-2010的新手。我试图创建一个从sql数据库读取数据的应用程序，并显示在数据网格上。我实现了我在网上找到的代码。

下面是sql_button_Click函数

private void sql_button_Click(object sender, RoutedEventArgs e)
    {
         string ConnectionString = "Data Source=.''SQLEXPRESS;AttachDbFilename='C:''Users''jakkulv''Downloads''VS projects''employee''employee''SampleDatabase.mdf';Integrated Security=True;Connect Timeout=30;User Instance=True";
        string SQL = "select emp_id,first_name from employee";
        SqlConnection conn = new SqlConnection(ConnectionString);
        // open the connection
        conn.Open();
        //Create a SqlDataAdapter object
        SqlDataAdapter adapter = new SqlDataAdapter(SQL, conn);
        // Call DataAdapter's Fill method to fill data from the
        // Data Adapter to the DataSet
        DataSet ds = new DataSet("employee"); //database table name is "employee"
        ds.Clear();
        adapter.Fill(ds);
        // Bind data set to a DataGrid control
        dataGrid1.ItemsSource = ds.DefaultViewManager;

        if (ds.HasChanges(DataRowState.Added))
        {
            // New rows have been added to the dataset, add appropriate code.
            MessageBox.Show("data set has changes");
        }
        else
        {
            MessageBox.Show("data set has no changes");
            // No new rows have been added to the dataset, add appropriate code.
        }

但是当我执行相同的操作时，它显示消息框说-"数据集没有变化"。

为什么数据集没有更新??如何更新??