Global Structure array gives a NullReferenceException

Question

Here is the definition of the structure

Structure Ct

    Public name As String

    Structure Pt
        Public identity As String
    End Structure

    Public Pty() As Pt

End Structure


Public Cty() As Main.Ct

This is declared in a module called main.

Then this piece of code, located in a subroutine in another class is run

Dim i As Integer = 1
    For Each item As String In cataList.Items
        'cataList is a listbox
        Cty(i).name = item
        i += 1
    Next

It throws a nullReferenceException.

What is missing? I need the structure to be global.

Duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Bjørn-Roger Kringsjå, Dec 27 '14 at 09:49

score 1 · Answer 1 · answered Dec 27 '14 at 07:08

cataList.Items is made up of listviewitems instead of strings. That may be causing problems. In addition, Cty has no members when it is declared.

Try this:

ReDim Cty(cataList.Items.Count-1)

For Each item As ListViewItem In cataList.Items
    'cataList is a listbox
    Cty(i).name = item.Text
    i += 1
Next

You could also use a list (with .add) instead of an array to avoid the redim.

score 1 · Accepted Answer · answered Dec 27 '14 at 13:28

Your array is declared but not instanced as the linked dupe describes. But an array of structures is not the most efficient way to do what you are trying.

Friend Class Ct                     ' cries out for a meaningful name        

    Public Property Name As String
    Private _identities As New List(of String)    ' ie "Pty"

    ' a ctor to create with the name prop
    Public Sub New(n As String)
        Name = n
    End Sub

    Public Sub AddIdentity(id As String)
         _identities.Add(id)
    End Sub

    ' get one
    Public Function GetIdentity(index As Integer) As String
         Return _identities(index)
    End Sub

    ' get all
    Public Function Identities As String()
        Return _identities.ToArray
    End If

    ' and so on for Count, Clear...  
End Class

Then a list of these things:

' as (elegantly) described in the NRE link, NEW creates an instance:
Friend Cty As New List(Of Ct)

Then fill the List from the ListBox:

For Each s As String In cataList.Items
    Cty.Add(New CT(s))             ' create CT with name and add to list
Next

The more you work with Lists and Collections, the more you will appreciate how much more flexible and powerful they are. For instance, there might not be a need to manually populate the ListBox at all. Use the DataSource property to bind the list to the listbox; this is more efficient because you map rather than copy the data to the UI control:

' tell the listbox which property on your class (Type) to display
cataList.DisplayMember = "Name"
cataList.DataSource = Cty

For cases where you want/need to copy the data:

For n As Integer = 0 to Cty.Count-1
    cataList.Items.Add(Cty(n).Name)
Next

Or:

For Each item As Ct In cty          
    cataList.Items.Add(item.Name)
Next

Global Structure array gives a NullReferenceException

2 Answers2

Linked