How to check empty array - arrays

Question

I am working in VB.net where i have class like below:
Public Class vertex
Public wasVisited As Boolean
Public name, type As String
Public x_pos, y_pos As Double
Public Sub New(ByVal x_pos As Double, ByVal y_pos As Double, ByVal name As Integer, ByVal type As String)
Me.x_pos = x_pos
Me.y_pos = y_pos
Me.name = name
Me.type = type
wasVisited = False
End Sub
End Class
I have object of some other class named as "graph" where in constructor of graph class I am calling constructor of vertex class.
I have array of vertex class: Public vertices() As vertex
And redim vertices(2000): resizing array again for some reason.
Now, when i loop the array to check empty value it throws an error:
Object reference not set to an instance of an object. (Since value contains "nothing")
even though i am checking like this,
If (vertices(i).name) Is Nothing Then
Exit For
End If
How can i check empty element of array?

Answer 1

Since you seem to want your collection be dynamic, a List(Of vertex) would serve you better. that along with a default New() constructor and you can add, remove, sort, search, whatever you need. To check for any empty value you can use If Vertices(i).name = "" then
Public Class vertex
Public wasVisited As Boolean
Public name, type As String
Public x_pos, y_pos As Double
Public Sub New()
wasVisited = False
name = ""
type = ""
x_pos = 0
y_pos = 0
End Sub
Public Sub New(ByVal x_pos As Double, ByVal y_pos As Double, ByVal name As String, ByVal type As String)
Me.x_pos = x_pos
Me.y_pos = y_pos
Me.name = name
Me.type = type
wasVisited = False
End Sub
End Class
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
Dim Vertices As New List(Of vertex)
For I = 0 To 99
Vertices.Add(New vertex())
Vertices(I).name = "Test" + I.ToString
Next
End Sub

Answer 2

What's the size of vertices() before the redim operation ? If it's less than 2000, then the added elements will be Nothing right after the array enlargement, therefore when you try to access the name property of vertices(i) for values of i that go beyond the initial array size you're actually trying to dereference a null object reference.
You either need to check that vertices(i) IsNot Nothing before testing for the value of its properties or make sure every element of the array is assigned a new vertex object.
If vertices(i) Is Nothing OrElse vertices(i).name Is Nothing Then
Exit For
End If
Here's a thread on vbforums about a similar problem: http://www.vbforums.com/showthread.php?546668-RESOLVED-Redim-array-of-objects

Answer 3

Have you tried:
If Not vertices Is Nothing AndAlso Not vertices(i) Is Nothing _
AndAlso Not vertices(i).name Is Nothing Then
Dim value as string= vertices(i).name
End If