.net关于TextBox和DropDownList相结合?

我用DropDownList 需要用txtName
string txtName = "txtW" + (dayOfWeek - i).ToString();
((Label)gv.Rows[gv.EditIndex].FindControl(txtName)).Text = ((DropDownList)gv.Rows[gv.EditIndex].FindControl(txtName)).SelectedValue;
可老报TextBox不能强转为DropDownList?

补充：.NET技术 ,  C#