how to retrieve the row and its value from the grideview

Revision 2
i want to retrieve the values inside the row in gridview.
but i enable the autogenerated edit button and autogenerated column because i am selecting the database and table in the dropdownlist and then i open the table through the gridview.For editing i will clicks the edit button it will transfer all its values to the textbox after editing, i want that all cell values please tell me how can i get ,,,,
i used this but it returns null

  int rowindex = Convert.ToInt32(e.RowIndex);
string column_name = GridView4.Rows[rowindex].Cells[2].Text;