How to check empty array

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?

tinstaafl · Accepted Answer · 2013-06-21 07:19:05Z

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

Marcello Romani · Accepted Answer · 2013-06-21 06:21:17Z

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

Carlos Landeras · Accepted Answer · 2013-06-21 06:13:30Z

0

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

answered Jun 21, 2013 at 6:13

Carlos Landeras

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