Visual Basic

Public Class MyClass
    'class level declaration of control to load on the background worker
    Private _addresses As uscAddresses = Nothing

    Public Function Addresses() As uscAddresses
        If _addresses Is Nothing Then 'if its not been loaded we should put it on the top of the stack
            'don't worry if it is already on the stack as it will not get loaded twice
            _loadQueue.Push(New ComponentToLoad(GetType(uscAddresses), _addresses, Nothing, tpAddresses))
            'if we are already running the background worker no need to start it a second time
            If Not bgwLoader.IsBusy Then bgwLoader.RunWorkerAsync()
            'ideally won't need this next bit in the final solution
            While _addresses Is Nothing
                'Application.DoEvents()
                'Threading.Thread.Sleep(100)
            End While
        End If
        Return _addresses
    End Function

    Structure ComponentToLoad
        Public Type As System.Type 'type of object that we want to load
        '**************************************************************
        Public Target As UserControl 'the class level reference to use for the created object
        '**************************************************************
        Public Data As Object 'any data to pass to the object
        Public Parent As Control 'the control to add the new object to after its created

        Public Sub New(ByVal inType As System.Type, ByVal inTarget As UserControl, ByVal inData As Object, ByVal inParent As Control)
            Type = inType
            Target = inTarget
            Data = inData
            Parent = inParent
        End Sub
    End Structure

    'runs as a stack so most recent request is delt with first
    Private Shared _loadQueue As New Stack(Of ComponentToLoad)
    'could use another stack but a queue will do, user should see little difference
    Private Shared _controlsToAdd As New Queue(Of ComponentToLoad)

    Private Sub MatterManager_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        'queue objects to load here if any are needed but not immediatly at form load
    End Sub

    'handles all loading, including checks that the object is not already loaded
    Public Sub LoadControl(ByVal onUIThread As Boolean, ByRef inComponent As ComponentToLoad)
        If onUIThread Then
            'only add the control if its not there already
            If Not DirectCast(inComponent.Parent, TabPage).Controls.Contains(inComponent.Target) Then
                DirectCast(inComponent.Parent, TabPage).Controls.Add(inComponent.Target)
            End If
        Else
            'only make the object if its not already made
            If inComponent.Target Is Nothing Then
                inComponent.Target = Activator.CreateInstance(inComponent.Type)
                inComponent.Target.Dock = DockStyle.Fill
            End If
            'could pass the data in to the object here
        End If
    End Sub

    Private Sub bgwLoader_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bgwLoader.DoWork
        'check there is something in the queue
        If _loadQueue.Count > 0 Then
            'make reference to what we are loading so we can add it to the ui queue
            Dim toLoad As ComponentToLoad = _loadQueue.Pop
            'load it
            LoadControl(False, toLoad)
            'stick it on the ui addition queue
            _controlsToAdd.Enqueue(toLoad)
        End If
    End Sub

    Private Sub bgwLoader_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles bgwLoader.RunWorkerCompleted
        While _controlsToAdd.Count > 0
            'add all loaded controls
            LoadControl(True, _controlsToAdd.Dequeue())
        End While
        If _loadQueue.Count > 0 AndAlso Not bgwLoader.IsBusy Then
            'load the next object
            bgwLoader.RunWorkerAsync()
        End If
    End Sub
End Class