我正在使用带有 vb.net 代码的 ISBN 号从亚马逊网站并行获取数据。
它只工作一些 ISBN 号码..
当我们使用其他一些 ISBN 号时，它会显示以下错误：
nullreferenceexception 未处理。你调用的对象是空的。
如何解决这个问题？任何人帮助..
这是代码：
这段代码有错误吗？谁给个解决办法。。
Private Sub ButtonProductAdvertisingAPIItemLookup_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) 处理 ButtonProductAdvertisingAPIItemLookup.Click sender.Enabled = False
'Product Advertising API, ItemLookup: http://docs.aws.amazon.com/AWSECommerceService/2011-08-01/DG/ItemLookup.html
Dim ItemId(9) As String
ItemId(0) = System.Uri.EscapeDataString(TextBoxisbn1.Text)
ItemId(1) = System.Uri.EscapeDataString(TextBoxisbn2.Text)
ItemId(2) = System.Uri.EscapeDataString(TextBoxisbn3.Text)
ItemId(3) = System.Uri.EscapeDataString(TextBoxisbn4.Text)
ItemId(4) = System.Uri.EscapeDataString(TextBoxisbn5.Text)
ItemId(5) = System.Uri.EscapeDataString(TextBoxisbn6.Text)
ItemId(6) = System.Uri.EscapeDataString(TextBoxisbn7.Text)
ItemId(7) = System.Uri.EscapeDataString(TextBoxisbn8.Text)
ItemId(8) = System.Uri.EscapeDataString(TextBoxisbn9.Text)
ItemId(9) = System.Uri.EscapeDataString(TextBoxisbn10.Text)
Dim i As Integer = 0
'For Each ItemId As String In ItemIds
'Parallel.ForEach(ItemIds, Sub(ItemId)
System.Threading.Tasks.Parallel.ForEach(ItemIds, Sub(ItemId)
'For Each ItemId As String In ItemIds()
'Next
' Dim ItemId As String = System.IO.Path.GetFileName(currentFile)
My.Settings.ItemId = ItemId
Dim MyREST As New SprightlySoftAWS.REST
Dim RequestURL(i) As String
RequestURL(i) = "https://webservices.amazon.com/onca/xml?Service=AWSECommerceService&Operation=ItemLookup&Version=2011-08-01"
RequestURL(i) += "&SignatureVersion=2&SignatureMethod=HmacSHA256&Timestamp=" & Uri.EscapeDataString(DateTime.UtcNow.ToString("yyyy-MM-dd\THH:mm:ss.fff\Z"))
RequestURL(i) += "&AssociateTag="
RequestURL(i) += "&ResponseGroup=" & System.Uri.EscapeDataString("ItemAttributes,Large,OfferSummary,Offers,VariationSummary,Reviews")
RequestURL(i) += "&ItemId=" & System.Uri.EscapeDataString(ItemId)
'RequestURL(i) += "&ItemId=" & System.Uri.EscapeDataString(ItemId)
RequestURL(i) += "&IdType=ISBN"
RequestURL(i) += "&SearchIndex=Books"
Dim RequestMethod As String
RequestMethod = "GET"
Dim SignatureValue As String
SignatureValue = MyREST.GetSignatureVersion2Value(RequestURL(i), RequestMethod, "", "")
RequestURL(i) += "&Signature=" & System.Uri.EscapeDataString(SignatureValue)
Dim RetBool As Nullable(Of Boolean)
RetBool = MyREST.MakeRequest(RequestURL(i), RequestMethod, Nothing)
Debug.Print("")
Debug.Print(MyREST.LogData)
Debug.Print("")
Dim Y As Integer
Dim ResponseMessage As String = ""
If RetBool = True Then
Dim MyXmlDocument As System.Xml.XmlDocument
Dim MyXmlNamespaceManager As System.Xml.XmlNamespaceManager
Dim MyXmlNode As System.Xml.XmlNode
Dim MyXmlNodeList As System.Xml.XmlNodeList
MyXmlDocument = New System.Xml.XmlDocument
MyXmlDocument.LoadXml(MyREST.ResponseString)
MyXmlNamespaceManager = New System.Xml.XmlNamespaceManager(MyXmlDocument.NameTable)
MyXmlNamespaceManager.AddNamespace("amz", "http://webservices.amazon.com/AWSECommerceService/2011-08-01")
MyXmlNodeList = MyXmlDocument.SelectNodes("amz:ItemLookupResponse/amz:Items/amz:Item", MyXmlNamespaceManager)
If MyXmlNodeList.Count = 0 Then
ResponseMessage = "Item not found."
Else
For Each ItemXmlNode As System.Xml.XmlNode In MyXmlNodeList
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:ASIN", MyXmlNamespaceManager)
ResponseMessage += "Details of " & MyXmlNode.InnerText
'Dim vbCrLf As New String(ValueType)
ResponseMessage += vbCrLf
' ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:ItemAttributes/amz:Title", MyXmlNamespaceManager)
ResponseMessage += "Title = " & MyXmlNode.InnerText
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:MediumImage/amz:URL", MyXmlNamespaceManager)
ResponseMessage += "Image = " & MyXmlNode.InnerText
ResponseMessage += vbCrLf
MyXmlNodeList = ItemXmlNode.SelectNodes("amz:ItemAttributes/amz:Author", MyXmlNamespaceManager)
If IsNothing(MyXmlNodeList) = False Then
ResponseMessage += "Author = "
For Each MyXmlNode In MyXmlNodeList
ResponseMessage += MyXmlNode.InnerText & ", "
Next
End If
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:ItemAttributes/amz:ListPrice/amz:FormattedPrice", MyXmlNamespaceManager)
ResponseMessage += "ListPrice = "
If IsNothing(MyXmlNode) = False Then
ResponseMessage += MyXmlNode.InnerText
End If
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:Offers/amz:Offer/amz:OfferAttributes/amz:Condition", MyXmlNamespaceManager)
ResponseMessage += "Condition = " & MyXmlNode.InnerText
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:Offers/amz:TotalOffers", MyXmlNamespaceManager)
ResponseMessage += "TotalOffers = " & MyXmlNode.InnerText
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:OfferSummary/amz:TotalNew", MyXmlNamespaceManager)
ResponseMessage += "TotalNew = " & MyXmlNode.InnerText
ResponseMessage += vbCrLf
MyXmlNodeList = ItemXmlNode.SelectNodes("amz:OfferSummary/amz:LowestNewPrice/amz:FormattedPrice", MyXmlNamespaceManager)
ResponseMessage += "LowestNewPrice = "
Y = 0
For Each MyXmlNode In MyXmlNodeList
If Y < 25 Then
ResponseMessage += "" & MyXmlNode.InnerText & " "
End If
Next
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:OfferSummary/amz:TotalUsed", MyXmlNamespaceManager)
ResponseMessage += "TotalUsed = " & MyXmlNode.InnerText
ResponseMessage += vbCrLf
MyXmlNode = ItemXmlNode.SelectSingleNode("amz:OfferSummary/amz:LowestUsedPrice/amz:FormattedPrice", MyXmlNamespaceManager)
ResponseMessage += "LowestUsedPrice = "
If IsNothing(MyXmlNode) = False Then
ResponseMessage += "" & MyXmlNode.InnerText
Else
ResponseMessage += "0"
End If
ResponseMessage += vbCrLf
'MyXmlNode = ItemXmlNode.SelectSingleNode("amz:SalesRank", MyXmlNamespaceManager)
'ResponseMessage += "SalesRank = " & MyXmlNode.InnerText
'ResponseMessage += vbCrLf
Next
End If
DialogOutput.Text = "Success"
DialogOutput.TextBoxOutput.Text += vbCrLf & ResponseMessage & vbCrLf
Else
ResponseMessage = FormatLogData(MyREST.RequestURL, MyREST.RequestMethod, MyREST.RequestHeaders, MyREST.ResponseStatusCode, MyREST.ResponseStatusDescription, MyREST.ResponseHeaders, MyREST.ResponseStringFormatted, MyREST.ErrorNumber, MyREST.ErrorDescription)
DialogOutput.Text = "Error"
DialogOutput.TextBoxOutput.Text = ResponseMessage
' DialogOutput.ShowDialog(Me)
' DialogOutput.Dispose()
End If
i = i + 1
End Sub)
'Next
DialogOutput.ShowDialog(Me)
ClearTextBox(Me)
sender.Enabled = True
End Sub