How to have your class track its instances?

[mrdippy]

In other languages I used this pattern to keep tabs on all the instances of my class within the class itself, just take a ref to the instance within the constructor and push it onto a static array.

When I try creating an instance of this bogus lil class I get the "Object reference not set to an instance of the object" on the records.add(Me) line.

I need a clue... Is this just not a fit for VB .NET coding style? I have a couple C# books, its kinda hard to find a nefarious question like this in their index or by flippin through. Thanks!

  Public Class Record
        Public Shared records As List(Of Record)

        Dim _artist As String
        Dim _album As String

        Public Sub New()
            records.Add(Me)
        End Sub

    End Class

 

[InertiaM]

Firstly, the reason you are getting the error is that you havent initialised the list (ie New List) - see my posts 7 and 8 when I did the same thing

However, if I was writing this, I'd do something like :-

Option Strict On

Public Class cRecordCollection
    Inherits System.Collections.ObjectModel.Collection(Of Record)
End Class

Public Class Record

    Private _artist As String
    Private _album As String

    Public Sub New(ByVal Artist As String, ByVal Album As String)
        _artist = Artist
        _album = Album
    End Sub


    Public Property Artist() As String
        Get
            Return _artist
        End Get
        Set(ByVal value As String)
            _artist = value
        End Set
    End Property

    Public Property Album() As String
        Get
            Return _album
        End Get
        Set(ByVal value As String)
            _album = value
        End Set
    End Property

End Class

and

        Dim RecordCollection As New cRecordCollection
        RecordCollection.Add(New Record("Enya", "Watermark"))
        RecordCollection.Add(New Record("Queen", "Greatest Hits"))

I'm sure someone will correct me if that's not the best way to do it

 

[mrdippy]

Oh my... I feel like a newbie, well I am. Your approach of using properties is right on with my plans - I kept the example bare bones to the krux of my issue. I just had to add the word "New" and all fixed.

I think a generic List is all I need... and I think the most efficient means of storage too. Time will tell, thanks for your answer and approach. I'm still learning the VB .Net style. I thought there were some special rules I was oblivious to about constructors or the "Me" variable - now i'm embarassed, feel like I forgot a semicolon.

Building on this lil example within my Form1 form/class I have a DataGridView1 and set its "DataSource" to record.records, so the gridview displays all my objects as rows by accessing the List of objects through the shared List. Kinda a lightweight dataset of objects if you will. Whether thats good vb style or not its what I'm used to coding and seems like a good deal. (e.g. DataGridView1.DataSource=Record.records)