datagridview的combobox的问题

两个combobox，选择第一个后，第二个联动加载数据源，这里已经实现了
问题也随之而来，选择了第一行数据后，再去点击选择第二行数据则报错
报错信息：无法将类型为“System.Data.DataRowView”的对象强制转换为类型“System.IConvertible”。代码：private static readonly WareHouse.BLL.T_IventoryCheck bll = new BLL.T_IventoryCheck();        public frmIventoryCheck()
        {
            InitializeComponent();
            proclass.DataSource = bll.GetProclass().Tables[0].DefaultView;
            proclass.DisplayMember = "ClassCode";
            proclass.ValueMember = "ClassID";
        }        private void dgv_ADDCheckDetail_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
        {
            DataGridView dgv = (DataGridView)sender;
            if (dgv.CurrentCell.GetType().Name == "DataGridViewComboBoxCell")
            {                
                if (dgv.CurrentCell.ColumnIndex == 0)
                {
                    ComboBox cmb1 = (ComboBox)e.Control;
                    cmb1.SelectedIndexChanged += new EventHandler(comboBox_SelectedIndexChanged);
                }
                else
                {
                    ComboBox cmb2 = (ComboBox)e.Control;
                    cmb2.SelectedIndexChanged += new EventHandler(comboBox_SelectedIndexChanged2);
                }
            }
        }        public void comboBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            ComboBox combox = (ComboBox)sender;
            int classID = Convert.ToInt32(combox.SelectedValue);
            productcode.DataSource = bll.GetProCode(classID).Tables[0].DefaultView;
            productcode.DisplayMember = "ProductCode";
            productcode.ValueMember = "ProductID";
        }