Delegate Problem: Object Reference not set to an instance of an Object

Posted on 2006-05-16
Medium Priority
Last Modified: 2012-06-27
I am building a custom listbox control and I need to send notification to one control, from another when the Mouse Wheel is used.

I am new to delegates but I thought I had understood them this way:
If Class A needs to tell Class B the mouse wheel was used then...

Class A
protected m_Notify As OnThisMouseWheel

Public Delegate Sub OnThisMouseWheel(Str)

Public Sub NotifyOnMouseWheel(ByVal value As OnThisMouseWheel)
    m_Notify = value
End Sub

Private Sub MouseWheel(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles ...

    Catch ex As Exception

    End Try

End Sub

Class B

Private Sub AddItem(ByVal ob As CustomControl)
    ob.NotifyOnMouseWheel(New CustomControl.OnThisMouseWheel(AddressOf NotifyMe))

End Sub

Public Sub NotifyMe(ByVal s As String)
End Sub

If I invoke m_Notify from NotifyOnMouseWheel in Class A right after assigning it, then it works. The object assignment is working and the delegate communicates to the other control. But after that the m_Notify variable equals Nothing. I Imagine it has something to do with the Subroutine that sets up the controls and creates the delegate but I've tried a couple different ways to pass the variable and I still get the same result. Is this thread related?

It is rather urgent so high points.

Question by:MattCoatney
  • 2

Author Comment

ID: 16692295
To be very clear, the thread question is a subquestion. The Points will be awarded for helping me get the mousewheel event notification to the container control even if that means we don't use delegates.

Thanks in advance.
LVL 86

Accepted Solution

Mike Tomlinson earned 2000 total points
ID: 16693597
How about...

Public Class myListBox
    Inherits ListBox

    Protected m_Notify As OnThisMouseWheel

    Public Delegate Sub OnThisMouseWheel(ByVal Str As String)

    Public Sub NotifyOnMouseWheel(ByVal value As OnThisMouseWheel)
        m_Notify = value
    End Sub

    Protected Overrides Sub OnMouseWheel(ByVal e As System.Windows.Forms.MouseEventArgs)
        If Not (m_Notify Is Nothing) Then
        End If
    End Sub

End Class

Public Class myListBoxSubscriber

    Public Sub NotifyMe(ByVal s As String)
    End Sub

End Class

Public Class Form1
    Inherits System.Windows.Forms.Form

    Private mlbs As myListBoxSubscriber

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim mlb As New myListBox
        mlb.Location = New Point(0, 0)
        mlb.Size = New Size(50, 200)
        For i As Integer = 1 To 100
            mlb.Items.Add("Item" & i)

        mlbs = New myListBoxSubscriber
        mlb.NotifyOnMouseWheel(New myListBox.OnThisMouseWheel(AddressOf mlbs.NotifyMe))
    End Sub

End Class

Author Comment

ID: 16700476
Thanks a bundle.

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