[RadGridView] Add Combo Box in Cell (GridViewTextBoxColumn)

//DataGridView
            dataGridView1.Columns.Add("Item", "Item");
            dataGridView1.Columns.Add("Value", "Value");
            dataGridView1.Columns.Add("Type", "Type");
            dataGridView1.Columns.Add("Options", "Options");

            list = new List<string[]>();
            list.Add(new string[] { "Result", "DropDown", "Pass,Failed" });
            list.Add(new string[] { "UserId", "Text", "" });
            index = 0;
            foreach (string[] s in list)
            {
                dataGridView1.Rows.Add(s[0], "", s[1], s[2]);
                switch (s[1])
                {
                    case "DropDown":
                        DataGridViewComboBoxCell cmb = new DataGridViewComboBoxCell();
                        var selection = s[2].Split(new string[] { "," }, StringSplitOptions.RemoveEmptyEntries).ToList();
                        cmb.DataSource = selection;
                        this.dataGridView1.Rows[index].Cells[1] = cmb;
                        break;

                    case "Text":
                        this.dataGridView1.Rows[index].Cells[1].Value = "Text Field";
                        break;
                }
                index++;
            }