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Paul Patterson
20
Singleton Pattern On Specific MDI Children
posted

Is there a more efficient way of implementing a singleton pattern on mdi child forms? Here is what I have...

I created a custom form class that inherits from a Form...

Imports System.Windows.Forms
Imports System.ComponentModel

Public Class CustomMDIChildForm   
        Inherits Form    

Private _singletonOnly As Boolean = False
     <Browsable(True)> _

Public Property SingletonOnly() As Boolean
        Get
            Return _singletonOnly
        End Get
        Set(ByVal value As Boolean)
            _singletonOnly = value
        End Set
    End Property 

End Class

I inherit this when creating a form I know will be an MDI child...

Public Class frmSomeMDIChildForm
    Inherits CustomMDIChildForm
  ...
  ..
  .
 End Class

Then in my parent form, I have this method....

Public Sub LoadMDIForm(ByRef theForm As CustomMDIChildForm)

        Me.Cursor = Cursors.WaitCursor
       
        Dim sStatus = "Loading " & theForm.Text & " functionality..."
       
        ToolStripStatusMessages.Text = sStatus
       
        Me.Refresh()
       
        Dim bFoundSingleton As Boolean = False
        Dim sForm As CustomMDIChildForm = Nothing
        
        ' If the form is only supposed to be a singleton.
        If theForm.SingletonOnly Then
             ' Check first if one is already loaded.
            For Each sForm In Me.MdiChildren
                '   If found, make it selected.
                If sForm.Name = theForm.Name Then
                    bFoundSingleton = True
                    Exit For
                End If
            Next
            If bFoundSingleton Then
                sForm.Activate()
                theForm = sForm
            Else
                With theForm
                    .MdiParent = Me
                    .Dock = DockStyle.Fill
                    .Show()
                End With
            End If
        Else
            '   Not a singleton, so load up a new instance of the form.
            With theForm
                .MdiParent = Me
                .Dock = DockStyle.Fill
                .Show()
            End With
        End If
         Me.Cursor = Cursors.Default
        ToolStripStatusMessages.Text = String.Empty

   End Sub

This works great, but not sure if this is a good approach.

BTW: I'm using ByRef because I have other funky stuff I am doing for specific forms, like selecting a specific datagrid row on a singleton form.

Any feedback is appreciated,

Paul

  • 2334
    posted
    My only suggestion would be to hash the singleton forms as you add them. If you throw a singleton form in the a hashtable as you add it you can then look it up more efficiently that by looping over the MDIChildren.