how to store data in dropdownlist( under gridview) from database

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C#

ASP.NET

i written below code in page load and all gridview events like rowdatabound,databound events

MySqlConnection mcon;
       MySqlCommand cmd;
       MySqlDataAdapter da ;

                cmd = new MySqlCommand(text, mcon);
                //cmd.CommandText = text;
                cmd.CommandType = cm;
                mcon.Open();
                da.SelectCommand = cmd;
                da.Fill(ds);
                mcon.Close();
               dropdownlist1.datasourcw=ds;//dropdownlist1 is from grid view
  dropdownlist1.datatextfield="deptname
  dropdownlist1.datavaluefield="deptno"
dropdownlist1.databind()

i am getting exception as object instance.....
how to solve that exception