Autocomplete for column not working

Question

I use following code for autocomplete first column but its not working plz help?

     private void dgvInovce_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
        {
            if (this.dgvInovce.CurrentCell.ColumnIndex == 0)
            {
                TextBox tb = e.Control as TextBox;

                if (tb != null)
                {
                    AutoCompleteStringCollection str = new AutoCompleteStringCollection();
                    str.Add("A");
                    str.Add("B");
                    tb.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
                    tb.AutoCompleteCustomSource = str;
                    tb.AutoCompleteSource = AutoCompleteSource.CustomSource;
                }
            txtDgvQuntity = e.Control as TextBox;
            txtDgvSaleRate = e.Control as TextBox;
            txtDgvDiscount = e.Control as TextBox;
            txtDgvQuntity.KeyPress -= new KeyPressEventHandler(txtDgvQuntity_KeyDown);
            txtDgvSaleRate.KeyPress -= new KeyPressEventHandler(txtDgvSaleRate_KeyDown);
            txtDgvDiscount.KeyPress -= new KeyPressEventHandler(txtDgvDiscount_KeyDown);
            if (this.dgvInovce.CurrentCell.ColumnIndex == 1)
            {

                txtDgvQuntity.KeyPress += new KeyPressEventHandler(txtDgvQuntity_KeyDown);
            }

            if (this.dgvInovce.CurrentCell.ColumnIndex == 3)
            {

                txtDgvSaleRate = e.Control as TextBox;
                if (txtDgvSaleRate.Text.Trim().Length > 0)
                    txtDgvSaleRate.Clear();

                txtDgvSaleRate.KeyPress += new KeyPressEventHandler(txtDgvSaleRate_KeyDown);
            }
            if (this.dgvInovce.CurrentCell.ColumnIndex == 4)
            {

                txtDgvDiscount = e.Control as TextBox;
                txtDgvDiscount.KeyPress += new KeyPressEventHandler(txtDgvDiscount_KeyDown);
            }
      
            }
        }