how to add a combobox in a datagridview which accepts input and also allow user to add new items if the items is not there in the combobox?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
namespace WindowsFormsApplication1
{
public partial class Form2 : Form
{
int count = 0;
DataTable tblA = new DataTable();
List<object> lstArticles = new List<object>();
public Form2()
{
InitializeComponent();
}
private void dataGridViewEx1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
if (e.Control is DataGridViewComboBoxEditingControl)
{
((ComboBox)e.Control).DropDownStyle = ComboBoxStyle.DropDown;
((ComboBox)e.Control).AutoCompleteSource = AutoCompleteSource.ListItems;
((ComboBox)e.Control).AutoCompleteMode = System.Windows.Forms.AutoCompleteMode.SuggestAppend;
((ComboBox)e.Control).Validating -= Form2_Validating;
((ComboBox)e.Control).Validating += Form2_Validating;
}
}

void Form2_Validating(object sender, CancelEventArgs e)
{
DataGridViewComboBoxEditingControl cbo = sender as DataGridViewComboBoxEditingControl;
DataGridView grid = cbo.EditingControlDataGridView;
object value = cbo.Text;
DataGridViewComboBoxCell cboCell = (DataGridViewComboBoxCell)grid.CurrentCell;
if (!value.Equals(""))
{
if (cbo.Items.IndexOf(value) == -1)
{
if (DialogResult.Yes == MessageBox.Show("New ITEM", "Are you sure?", MessageBoxButtons.YesNo))
{
cbo.Items.Add(value);
Column2.Items.Add(value);
insertNewArticle(value.ToString());
grid.CurrentCell.Value = value;
}
else
{
SendKeys.Send("{LEFT}");
}
}
else
{
cboCell.Value = value.ToString();
}
}
else
{
SendKeys.Send("{LEFT}");
count++;
if (count == 2)
{
dataGridView1.Focus();
dataGridView1.Select();
}
}
}

private void Form2_Load(object sender, EventArgs e)
{
getArticles();
foreach (String item in lstArticles)
Column2.Items.Add(item);

}
void insertNewArticle(String value)
{
string strError = "";
string query = "insert into tblArticles(Article) values(@newArticle)";
List<oledbparameter> parameters = new List<oledbparameter>();
parameters.Add(new OleDbParameter("newArticle", value));
string result = PawnManagement.SQLHelper.RunCommand(query, parameters, ref strError);
if (result == "Done")
{
MessageBox.Show("new item successffullly added");
}
else
{
MessageBox.Show(result);
}
}

void getArticles()
{
String strError = "";
String query = "select Article from tblArticles";
tblA = PawnManagement.SQLHelper.GetDataTable(query, ref strError);
if (strError != "")
{
MessageBox.Show("Errorr in retrieving articles");
}
foreach (DataRow row in tblA.Rows)
{
lstArticles.Add(row["Article"].ToString());
}
}
}
}