如何编辑和替换包含逗号的文本文件中的特定代码行来分隔它们？

Question

我正在创建一个像商店一样的程序。店主创建一个新产品，并写下名称，价格，GTIN-8代码和可用产品的数量。当客户使用GTIN-8代码购买物品并指定数量时，程序应更新文本文件并更改剩余库存量。例如。如果有50个表并且用户购买了10个，则文本文件应该更新，然后显示40个。

我的代码

    Dim aLine As String = ""
    Dim theField As String = ""
    Dim aChar As Char

    findstring = IO.File.OpenText("Stock.txt")

    Do Until findstring.EndOfStream

        aLine = findstring.ReadLine()

        theField = ""
        GtinField = ""
        NameField = ""
        PriceField = ""

        For i = 0 To aLine.Length() - 1
            aChar = aLine(i)
            If aChar <> "," Then
                theField = theField + aChar
            Else
                If GtinField = "" Then
                    GtinField = theField
                ElseIf NameField = "" Then
                    NameField = theField

                End If
                theField = ""
            End If
        Next
        PriceField = theField

        If GtinField = txtGTIN.Text Then

            receipt.lstreceipt.Items.Add(GtinField & Space(3) & NameField & Space(2) & txtquantity.Text & Space(2) & PriceField & Space(2) & "£" & PriceField * txtquantity.Text)
            finalPrice = finalPrice + (PriceField * txtQuantity.Text)
        End If

        theField = ""

    Loop

    findstring.Close()

End Sub