Adding Combobox control to datagrideview TextboxColumn cell

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C#3.0

C#

Windows

hi i am using window application and datagridview in it DGV is populated to a database table (patient_name,Test_Name,Result,Reference_Ranges).I want when the Result column receive input focus a combo box appear and user select value from it but want the combo box not for all cell of the result column because in some situation the user inter the value from keyboard because the combo box is read only and data source of that combo box is bond to database and select defrent value for each test ;
in short i want like this

DGV.rows[e.rowindex].cells["Result"].controls.add(cmb);

i tried the best and search the net but fail
i add the cmb like this

DGV.controls.add(cmb);

but it appear at the top of DGV and i want it specific result column cell which receive input focus.
help me withe some sample of code
thanks