Object array and refference

Hello ,

i write a class that stores data simple. the class was in the module and i import that module in the main form class.
Name of Class>> DataSource
Name of Object>> DS( )
i create an object array in the module by the following code

Public DD() As DataSource

and then i add the following line to initialize the object array in button click event.

DS(Index + 1) = New DataSource

Here index+1 show how many elements i want to be in object array.
but when i run the project it succesfull but clicking the button[that contains the code] gives program a break and show the following error.

An unhandled exception of type 'System.NullReferenceException' occurred in abc.exe
Additional information: Object reference not set to an instance of an object.

i initialize the object but it give the above error what i want is to create an object array containing index+1 number of elements. the class is public declare in module file and the object is also public declare in module file and initialization of object occurs in button click event when i know the total number of elements.

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Object array and refference

AJ_101
First of all thanks spotty and dustin to helping me out this mess
Index is an integer which calculated after some lopping. where i get the how many objects i need to initialize.
and i am using .net 2003 framework 1.1 and in system collection and i can't find generic's may be it was in 2005 edition of framework 2.0 i find the list array class.
Kiku
his is why the arraylist or list item is nicer than the array as you dont have to be concerned about how many elements it is - you can just add elements as you go.
ocanon

You could even use the generics list type to create a list which you simply add and removed items out of. As its using generics (new feature with VB 2005) it is type safe and will only allow storage of specific type rather than any object.

All you need to do to add an element is use the AList.add method and to remove an element is use AList.Remove

Public Class fred

End Class

Module Module1

Dim AList As New System.Collections.Generic.List(Of fred)

Sub Main()
AList.Add(New fred)
AList.Add(New fred)
End Sub

End Module

carzyR

You can also make an arraylist. They are handy if you don't really want to track the array bounds of your collection.

Public Class Form1
    Private DataCollection As ArrayList

    Public Sub Main()
        'Make new connection, and get it's position in the array.
        Dim position As Integer = DataCollection.Add(New Data.OleDb.OleDbConnection)

        'Modify the settings for the elodbconnection.
        With CType(DataCollection(position), Data.OleDb.OleDbCommand)
            .CommandText = "SELECT * FROM Something"
            .CommandTimeout = 30
        End With

        'All done work, remove the item from the array list.
        DataCollection.RemoveAt(position)

    End Sub
End Class

As you see, easier to manage.

Hope it helps!

Dustin.

joerage

You're not initializing your array....

From your code sample... do this..

Public DD() As DataSource
private SizeOfArray As Integer =100
Redim DD(SizeOfArray)

OR.....
You can dynamically increase the array as you go....

Public DD() As DataSource
dim SizeOfArray As Integer =-1

SizeOfArray += 1 'Increase the array Size.
Redim Preserve DD(SizeOfArray) 'Resize array to the new size.
DD(SizeOfArray) = New DataSource 'Create new DataSource.

Dustin.

dcritch
Probably a couple of reasons

1st - you dimension with variable DD and use DS in the call.

Public DD() As DataSource

DS(Index + 1) = New DataSource

2nd - you havent defined how many elements the array is going to hold. You have just defined it is an array. You could use something like Public DD(10) as datasource to define that the array will have 11 elements ( 0 - 10)

3rd - I dont see the code as to how Index is being established to define what values this will hold. So I have no idea of what values this is referring to.

This is why the arraylist or generics list collection is a nice approach as you dont have to be concerned about how many elements you need to establish - you are just adding elements to the collection.

RSatter

Heres an example that works.... When you want to add the additional element then you redim the array to accomodate a new element.

Public Class Fred

End Class

Public Class Form1
Dim NormalArray() As fred
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim INDEX As Integer

If NormalArray Is Nothing Then
INDEX = 0
Else
INDEX = NormalArray.GetUpperBound(0) + 1
End If

'//Redimension the array to add an addition element
ReDim Preserve NormalArray(INDEX)
NormalArray(INDEX) = New fred
End Sub
End Class

Or you could use an generic list of a specific type of class and just add the items to the list.

Freek Bos
Thanks spotty u give me two different solutions to resolve my problem but why i am getting object refference error mention above post was not answer.
I give it a try for lists thanks again.
Object array and refference

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