VB.NET: Dynamic Usage of Event Handlers

By Thomas Kaloyani 

With Events and Handles clause requires form us to declare the object variable and the event handler as we write our code, so linkage is created upon compilation. On the other hand, with AddHandler and RemoveHandler, linkage is created and removed at runtime, which is more flexible.

Let's assume that we want to load several MDI child forms, allowing each of them to be loaded only once, and of course to know when one of the child forms is closed. Since we have several forms to load we would like to use the AddHandler and RemoveHandler keywords so we can be flexible and write the minimal code we can.

Let's get dirty.

1. In each MDI child form we have to declare a public event.
Public Event FormClosed(ByVal f As Form)

2. In each MDI child form we have to use the Form_Closed method which handles the MyBase.Closed class and raise the FormClosed event.

Private Sub Form1_Closed(ByVal sender As Object, ByVal e As System.EventArgs)  _
   Handles MyBase.Closed
   RaiseEvent FormClosed(Me)
End Sub

3. On our MDI form we need to declare two member variables. The first's of type Form and the second's type is ArrayList.
Private m_f(0) as Form
Private m_sLoadedChildForms As New ArrayList

4. We need to implement a method the will search the MDI child forms that are loaded. We'll also use this method when we unload the MDI child forms.

Private Function SearchChildForm(ByVal strSearchForm As String, _
Optional ByVal idxEventHandler As Long = -1) As Long
   Dim i As Long = 0


   For i = 0 To m_sLoadedForms.Count - 1
      If m_sLoadedForms.Item(i) = strSearchForm Then
         Dim j As Long = 0
         For j = m_f.GetLowerBound(0) To m_f.GetUpperBound(0)
            If m_f(j).Name = strSearchForm Then idxEventHandler = j
         Next j
         Return i
      End If
   Next
   Return -1
End Function

5. We need to implement a method to load the mdi child forms and use the SearchChildForm method in order not to load the same mdi child form second time.

Private Sub LoadChildForms(ByVal f As Form)
   If m_f.GetUpperBound(0) > 0 Then
      ReDim Preserve m_f(m_f.GetUpperBound(0) + 1)
   End If
   m_f(m_f.GetUpperBound(0)) = f


   If Not SearchChildForm(m_f(m_f.GetUpperBound(0)).Name()) >= 0 Then
      m_f(m_f.GetUpperBound(0)).MdiParent = Me


      AddHandler m_f(m_f.GetUpperBound(0)).Closed, _
         AddressOf UnloadChildForm
      m_f(m_f.GetUpperBound(0)).Show()


      m_sLoadedChildForms.Add(m_f(m_f.GetUpperBound(0)).Name)
   Else
      If m_f.GetUpperBound(0) > 0 Then
         ReDim Preserve m_f(m_f.GetUpperBound(0) - 1)
      End If
   End If
End Sub

6. At last we need to implement a method to take out our mdi child form from the array list so we can load it again if we want.

Private Sub UnloadForm(ByVal sender As System.Object, ByVal e As System.EventArgs)
   Dim i As Long
   Dim s As String = sender.GetType().Name
   Dim IndexForEventHandler = -1
   i = SearchChildForm(s, IndexForEventHandler)


   If i >= 0 Then m_sLoadedForms.RemoveAt(i)


   If IndexForEventHandler >= 0 Then
      RemoveHandler m_f(IndexForEventHandler).Closed, AddressOf UnloadForm
      m_f(IndexForEventHandler) = Nothing
   End If


End Sub

Thomas is an experienced Visual Basic developer, with expertise of 7+ years developing especially financial applications. His main IT skills are VB, SQL, Crystal Reports - should you need a Visual Basic developer for your projects feel free to contact Thomas through his personal website Kaloyani.com or through VBprofs - the newest Visual Basic and VB.NET resources portal.

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