protected DataSet dsUsers = new DataSet();

private void cmdDS_Click(object sender, System.EventArgs e)
{     
        ComboBox1.DataSource= dsUsers.Tables["USER"];
        ComboBox1.DisplayMember="UserName";
        ComboBox1.ValueMember="UserID";
        ComboBox1.SelectedIndex=0;
        lblAssociated.Text=ComboBox1.SelectedValue.ToString();
        GroupBox1.Visible=true;
        lblSource.Text="DataSet";
}