Getting a NullReferenceException when adding something to a list

Question

I get a null reference exception when I try to use this webservice I'm working on. I have two fields in the object ipadarticle called fullname and tags, which are declared to be lists, so that ipadarticle can return multiple tags and authors. The null reference exception points to

ipadarticle2.FullName.Add(a_var.firstname + " " + a_var.lastname)
ipadarticle2.Tag.Add(a_var.tagtext)

I'm pretty new to vb programming, so I'm not really to sure what is causing this. To clarify, what is going on is that this stored procedure is fetching entries from a db, which has a list of articles with -among other things- tags and authors associated with it. Since articles have multiple tags and authors there are multiple entries for each article. When I return the info in the web service I am trying to make it so that only one ipadarticle object is returned for reach article, and then that contains a list of the multiple tags and authors associated with each article. I'm having a headache trying to figure this out.

            Dim lq As New lqDFDataContext
            Dim var = lq.mobile_IpadGetSavedArticlesAR(simpuser.UserID, tempParamKW(0), tempParamKW(1), tempParamKW(2), tempParamKW(3), tempParamKW(4), pageNum, pageLen)
            Dim ipadarticle2 As New ipadArticle()

        For Each a_var In var



            If a_var.articleID <> temp Then

                If flag = 0 Then
                    result.add(ipadarticle2)
                    Dim ipadarticle1 As New ipadArticle()
                    ipadarticle2 = ipadarticle1
                End If

                ipadarticle2.ArticleID = a_var.articleID
                ipadarticle2.PublishedOn = a_var.publicationdate
                ipadarticle2.Title = a_var.title
                ipadarticle2.MedAbbr = a_var.medabbr.Replace(" ", "-").ToLower()
                ipadarticle2.FullName.Add(a_var.firstname + " " + a_var.lastname)
                ipadarticle2.Tag.Add(a_var.tagtext)

                flag = 1

            Else
                ipadarticle2.Tag.Add(a_var.tagtext)
                ipadarticle2.FullName.Add(a_var.firstname + " " + a_var.lastname)
                flag = 0
            End If

            temp = a_var.articleID

        Next
    End If
    Return result

ipadArticle class:

Imports Microsoft.VisualBasic

Public Class ipadArticle
    Inherits SimpleObject

    Public Sub New()

    End Sub



    Private _ArticleID As Integer
    Public Property ArticleID() As Integer
        Get
            Return _ArticleID
        End Get
        Set(ByVal value As Integer)
            _ArticleID = value
        End Set
    End Property


    Private _Title As String
    Public Property Title() As String
        Get
            Return _Title
        End Get
        Set(ByVal value As String)
            _Title = value
        End Set
    End Property
    Private _PublishedOn As String
    Public Property PublishedOn() As String
        Get
            Return _PublishedOn
        End Get
        Set(ByVal value As String)
            _PublishedOn = value
        End Set
    End Property


    Private _MedAbbr As String
    Public Property MedAbbr() As String
        Get
            Return _MedAbbr
        End Get
        Set(ByVal value As String)
            _MedAbbr = value
        End Set
    End Property

    Private _Tag As List(Of String)
    Public Property Tag() As List(Of String)
        Get
            Return _Tag
        End Get
        Set(ByVal value As List(Of String))
            _Tag = value
        End Set
    End Property

    Private _FullName As List(Of String)
    Public Property FullName() As List(Of String)
        Get
            Return _FullName
        End Get
        Set(ByVal value As List(Of String))
            _FullName = value
        End Set
    End Property


End Class

competent_tech · Accepted Answer · 2012-08-03 18:52:42Z

1

The most likely cause is that the objects FullName and Tag have not been created (are Nothing) in ipadarticle2. These should most likely be created as new objects in the class constructor.

EDIT:

Based on the posted class, the above assumption was correct: FullName and Tag are defined as List(Of String), but the backing members are never created.

This can be fixed in a couple of ways:

1) Instantiate the backing member variables directly in their definition, i.e.:

Private _FullName As New List(Of String)

2) Instantiate the backing member variables in the constructor:

Public Sub New()
  _FullName = New List(Of String)
  _Tag = New List(Of String)
End Sub

3) Instantiate the backing member variable in the getter if it is nothing:

Public Property Tag() As List(Of String) Get If _Tag Is Nothing Then _Tag = New List(Of String) End If Return _Tag End Get

Basically, any variable types other than simple data types must be instantiated before they can be used (unless you test them for Nothingness).

edited Aug 3, 2012 at 18:52

answered Aug 3, 2012 at 18:21

competent_tech

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2 Comments

xxyyxx Over a year ago

FullName and Tag are just members of the object ipadArticle, and are a list of strings not objects themselves. Is that not allowed? I'll edit my post with what the ipadArticle class looks like.

competent_tech Over a year ago

Ah, that's what I thought. I've updated my answer with some information and guidance.

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