Datagridview in c#

Question

Here is the my code to add columns to datagridview runtime :

 DataGridViewComboBoxColumn c5 = new DataGridViewComboBoxColumn();

            c5.HeaderText = "Item_SrNo";

            con.connCheck();

            OleDbCommand cmd = new OleDbCommand("select Item_SrNo from Item_Master", con.cn);

            OleDbDataReader dr = cmd.ExecuteReader();

            while (dr.Read()==true)

            {

                c5.Items.Add(dr["Item_SrNo"].ToString());

            }

            dataGridView1.Columns.Add(c5);

            DataTable dt = new DataTable();

            dt.Columns.Add(new DataColumn("Item_Code", typeof(string)));

            dt.Columns.Add(new DataColumn("Sale_Unit", typeof(string)));

            dt.Columns.Add(new DataColumn("Inventory_Rate", typeof(int)));

            dataGridView1.DataSource = dt;


Here when i press 'enter' in datagridview then comboBox list dropdowns Please Help me...