VB.net - Adding to an ArrayList

Question

This is my first time using ArrayList. I have tried to add to BarcodeArray (as an arraylist) and execution is failing with the error message:

Object reference not set to an instance of an object.

My code is shown below:

'Populate the arrays        
BarcodeArray.Add(txt_Barcode.Text)
CategoryArray.Add(cmb_Categories.Text)
TitleArray.Add(txt_Title.Text)
DescriptionArray.Add(txt_Description.Text)
QuantityArray.Add(txt_Quantity.Text)
RRPArray.Add(txt_RRP.Text)
CostArray.Add(txt_Cost.Text)

This message appears when line 2 is executed. How do I add text to an ArrayList from a textbox without getting this error?

BarcodeArray is nothing. Just initialize it. Dim ar as new ArrayList() — kostas ch.
– kostas ch., Commented Feb 12, 2014 at 12:23
Just curious what your reason is for needing to maintain a separate ArrayList for each of the different values you list. From their names, it would seem better suited to something like a Product class with the properties of Barcode, Category, etc. Then you would populate a List(Of Product) which could contain all of those values for each product. — Aaron Palmer
– Aaron Palmer, Commented Feb 12, 2014 at 12:46

Aaron Palmer · Accepted Answer · 2014-02-12 12:54:39Z

2

This issue you're having is that you are not instantiating your ArrayLists before using them. You would need to do something like this to get your code to work:

Dim barcodeArray as New ArrayList()
barcodeArray.Add(txt_Barcode.Text)
... etc ...

But in your case, I think I would create a new class:

Public Class Product
    Public Property Barcode as String
    Public Property Category as String
    Public Property Title as String
    ... etc ...
End Class

Then I would use it in code like this:

Dim productList as New List(Of Product)()
productList.Add(new Product() With {
    .Barcode = txt_Barcode.Text,
    .Category = cmb_Categories.Text,
    .Title = txt_Title.Text,
    ... etc ...
})

This would let you use a single Product object rather than separate ArrayList objects, which would be a maintenance nightmare.

answered Feb 12, 2014 at 12:54

Aaron Palmer

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1 Comment

Fred Kerber Over a year ago

To add to Aaron's suggestion, I would add an "Public Overrides Function ToString() As String" to the class and set the Return to something like "return me.Title"; something that makes since when added to the list. It makes debugging much simpler and makes it so you can add the arrayed list to nearly any object and see the desired information.

Heinzi · Accepted Answer · 2014-02-12 12:17:45Z

2

In .NET, you need to instantiate objects before you can call methods on them. Example:

Dim a As ArrayList
a.Add(...)           ' Error: object reference `a` has not been set

The solution is to initialize the variable with a new ArrayList:

Dim a As ArrayList

a = New ArrayList()
a.Add(...)

or, alternatively:

Dim a As New ArrayList()
a.Add(...)

BTW, ArrayList is an old class that mainly exists for backwards compatibility. When you are starting a new project, use the generic List class instead:

Dim a As New List(Of String)()
a.Add(...)

answered Feb 12, 2014 at 12:17

Heinzi

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Comments

Saro Taşciyan · Accepted Answer · 2014-02-12 12:36:21Z

0

You need to instantiate it before you can use. The problem in line 2 is that it is null at that time (Nothing in VB.NET) since it is not created

Since all the values you want to add to the list has same type which is String i suggest you use a List(Of String) instead of ArrayList

Try the following:

Dim BarcodeArray as List(Of String) = New List(Of String)( { txt_Barcode.Text } )
Dim CategoryArray as List(Of String) = New List(Of String)( { cmb_Categories.Text } )
' ...
' Same for the other Lists you will need to use

answered Feb 12, 2014 at 12:36

Saro Taşciyan

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