How to convert datagridview cell type at edit time

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WinForms

C#5.0

Hi friends,

I have a small requirement. I need to change the text column of a datagridview to combobox column at edit time.How to achieve this?. What i have done so far is given below. the problem i am facing is that. when the edit mode begins a new column is generated and all the rows of the specified column has changed to combobox type.
My datagridview is bounded to a datasource.
Any idea or help is appreciated

 private void dataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
        {
else if (colIndex == 6)
            {
                DataTable comboSource = new DataTable("comboSource");
                comboSource.Columns.AddRange(new DataColumn[] {
                new DataColumn("id"),
                new DataColumn("stat") });
                comboSource.Rows.Add(1, "");
                comboSource.Rows.Add(2, "Left");
                comboSource.Rows.Add(3, "Expire");             


                DataGridViewComboBoxColumn col = new DataGridViewComboBoxColumn();
                col.DataPropertyName = "stat";
                col.DataSource = comboSource;
                col.DisplayMember = "stat";
                col.ValueMember = "id";
                dataGridView1.Columns.Add(col);
            }
}