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[QUOTE="spiketg, post: 142030, member: 37233"]
Hi there,
I recently started programming with Vb, been using JAVA for the last few years. Im trying to read/edit an XML document that will store information for this web service (a music playlist project)

I keep getting the following error when testing any of the functions.

Any help would be greatly appreciated. As long as I can get the file to work I can handle any other errors. 
Code is below.

Thanks in advance.
TG

System.NullReferenceException: Object reference not set to an instance of an object.   
at Service.getUserPlaylist(String userID) in C:\Users\Tommy\Documents\Visual Studio 2008\WebSites\WebSite3\App_Code\Service.vb:line 70   
at Service.InsertNewPlaylist(String userID) in C:\Users\Tommy\Documents\Visual Studio 2008\WebSites\WebSite3\App_Code\Service.vb:line 78   
at Service.CreateNewPlaylist(String UserID) in C:\Users\Tommy\Documents\Visual Studio 2008\WebSites\WebSite3\App_Code\Service.vb:line 168[CODE]Imports System.WebImports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.Xml

' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
' <System.Web.Script.Services.ScriptService()> _
<WebService(Namespace:="http://tempuri.org/")> _
<WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<Global.Microsoft.VisualBasic.CompilerServices.DesignerGenerated()> _
Public Class Service
     Inherits System.Web.Services.WebService

'This object will provide access to the entire
    ' XML tree of ToDOPlaylist items...
    Private _xmlData As New XmlDocument
    Private _fileName As String

' This function will always return a valid name for
    ' the data file...
    Private Function fileName() As String
        Return Server.MapPath("App_Data\AllData.xml")
    End Function

' This function makes sure that when we try to access the XML document, 
    ' it is loaded.  The process is called 'lazy instantiation'.  
    ' _xmlData is the entire data set, and should always be accessed via
    ' this function.  Two other functions (below) are used to load and 
    ' save the xml data to a file.  We must ensure that apart from these, 
    ' no other code accesses _xmlData directly...
    Private Function xmlData() As XmlDocument
        If _xmlData Is Nothing Then
            getDataFile()
        End If
        Return _xmlData
    End Function

' The XML data file will be stored in the App_Data folder.
    ' It makes sense to have a simple function to call to retrieve
    ' the XML from this file and another to save a modified version...
    Private Function getDataFile() As Boolean
        If IO.File.Exists("test.xml") Then
            _xmlData = New XmlDocument

_xmlData.Load("test.xml")
            Return True
        Else
            Return False
        End If
    End Function

' Save the XML document...
    Private Sub saveDataFile()

If _xmlData IsNot Nothing Then

_xmlData.Save("test.xml")
        End If
    End Sub

' This function returns a particular user's Playlist of to-do items.
    ' The XPath "//Playlist[@ID=user@domain.com]" selects the Playlist item
    ' that matches that user's ID (email address)
    Private Function getUserPlaylist(ByVal userID As String) As XmlElement
        Dim xmlItems As XmlElement
        ' The XPath says 'Get every Playlist element with the correct userID...
        Dim xPath As String = "//Playlist[@ID='" & userID & "']"
        ' There ought to be only one matching Playlist.
        xmlItems = xmlData.DocumentElement.SelectSingleNode(xPath)
        Return xmlItems
    End Function

' This function inserts a new user's empty <List /> element.
    ' Each <playlist /> element has an ID attribute which identifies 
    ' the user.  It returns False if a playlist with that ID already exists.
    Private Function InsertNewPlaylist(ByVal userID As String) As Boolean
        If getUserPlaylist(userID) IsNot Nothing Then
            Return False
        Else
            ' This code will create a new element of the form...
            '     <Playlist ID="whatever userID is"/>
            ' We can later add items to the playlist.
            Dim newPlaylist As XmlElement = xmlData.CreateElement("Playlist")
            Dim newAttr As XmlAttribute = xmlData.CreateAttribute("ID")
            newAttr.Value = userID
            newPlaylist.Attributes.Append(newAttr)
            xmlData.DocumentElement.FirstChild.AppendChild(newPlaylist)
            Return True
        End If
    End Function

' This function returns a new XML element, of the form:
    '     <ElementName>Element Value</Element Name> to the
    ' calling code.  Note that MSXML requires an XMLDocument 
    ' to generate new elements - hence the use of xmlData here.
    Private Function newElement(ByVal name As String, ByVal value As String) As XmlElement
        Dim el As XmlElement = xmlData.CreateElement(name)
        el.InnerText = value
        Return el
    End Function

' This function create a new XML <Track /> element, creates
    ' the Name attribute, Description and DateTime elements,
    ' and then inserts it into the identified user's <Playlist /> element...
    Private Function InsertNewTrack(ByVal userID As String, _
                                   ByVal trackArtist As String, _
                                   ByVal trackName As String, _
                                   ByVal trackAlbum As String, _
                                   ByVal trackGenre As String, _
                                   ByVal trackYear As Date) As Boolean
        Try
            'THE WHOLE PROCESS OF CREATING A NEW XML ELEMENT WITHIN A DOCUMENT
            '=================================================================
            ' 1. We need variables to refer to the various bits.
            '    This one is the new element to be added...
            Dim anElement As XmlElement
            '    This is the existing list of elements...
            Dim userPlaylist As XmlElement = getUserPlaylist(userID)
            '    Create the element (need to do so from an xmlDocument)...
            Dim newTrack As XmlElement = xmlData.CreateElement("track")
            '    Create an attribute...
            Dim newAttr As XmlAttribute = xmlData.CreateAttribute("name")
            Dim rating As Integer
            rating = 0
            '    Apply the attribute value...
            newAttr.Value = trackName
            '    Assign it to the newly created element...
            newTrack.Attributes.Append(newAttr)
            '    Now make a new sub-element (again from the xmlDocument)...
            anElement = newElement("artist", trackArtist)
            '    Add this to the element created earlier...
            newTrack.AppendChild(anElement)
            '    ...and again...
            anElement = newElement("genre", trackGenre)
            newTrack.AppendChild(anElement)
            '    ...this is a new element, so not yet completed...
            anElement = newElement("year", trackYear.ToString())
            newTrack.AppendChild(anElement)
            '    Finally add the new element to the list of elements...
            userPlaylist.AppendChild(newTrack)
            anElement = newElement("rating", rating)
            userPlaylist.AppendChild(anElement)
            Return True
        Catch ex As Exception
            Return False
        End Try
    End Function

<WebMethod()> _
    Public Function GetItems(ByVal userID As String) As String
        Return getUserPlaylist(userID).OuterXml
    End Function

<WebMethod()> _
    Public Function GetItem(ByVal userID As String, ByVal itemName As String) As String
        Dim items As XmlElement = getUserPlaylist(userID)
        Dim item As XmlElement = items.SelectSingleNode("//Item[@name='" & itemName & "']")
        If item Is Nothing Then
            Return "<Item />"
        Else
            Return item.OuterXml
        End If
    End Function

<WebMethod()> _
    Public Function CreateNewPlaylist(ByVal UserID As String) As Boolean
        If InsertNewPlaylist(UserID) Then
            saveDataFile()
            Return True
        Else
            Return False
        End If
    End Function

<WebMethod()> _
    Public Function ratePlaylist(ByVal userId As String, ByVal rating As String) As Boolean
        ' This should set the completed element of the specified item to True...
        Try
            Dim listToRate As XmlElement = getUserPlaylist(userId)
            ' Completed is the last child element...

listToRate.LastChild.InnerText = rating
            saveDataFile()
            Return True
        Catch ex As Exception
            Return False
        End Try
    End Function

End Class

[/CODE]
[/QUOTE]

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