Iterating through DevExpress AspxGridVeiw

Protected Sub gridProducts_CustomCallback(ByVal sender As Object, ByVal e As ASPxGridViewCustomCallbackEventArgs)
        Dim colQty As GridViewDataColumn = DirectCast(gridProducts.Columns(3), GridViewDataColumn)
        Dim colDiscount As GridViewDataColumn = DirectCast(gridProducts.Columns(4), GridViewDataColumn)
        Dim cart As New ShoppingCart()
        For i As Integer = 0 To gridProducts.VisibleRowCount - 1
            Dim pID As Integer = CInt(gridProducts.GetRowValues(i, "ID"))
            Dim spinMainQuantity As ASPxSpinEdit = TryCast(gridProducts.FindRowCellTemplateControl(i, colQty, "spinMainQuantity"), ASPxSpinEdit)
            Dim spinMainDiscount As ASPxSpinEdit = TryCast(gridProducts.FindRowCellTemplateControl(i, colDiscount, "spinMainDiscount"), ASPxSpinEdit)

            If CInt(spinMainQuantity.Number) > 0 Then
                cart.AddItem(InventoryItem.GeteCommerceInventoryItem(pID), CInt(spinMainQuantity.Number), spinMainDiscount.Number)
            End If
        Next
        cart.Save()
    End Sub