VB.net-添加到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?

Answer 1

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(...)           

Answer 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.

Answer 3

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