Visual Basic

Public Module Module1
    Public mps As New List(Of frmMdiParent)
End Module

Partial Friend Class MyApplication
        Private Sub MyApplication_Startup(sender As Object, e As StartupEventArgs) Handles Me.Startup

            For i As Integer = 0 To Screen.AllScreens.Count - 1
                Dim p As New frmMdiParent
                p.Location = Screen.AllScreens(i).Bounds.Location

                p.StartPosition = FormStartPosition.Manual
                p.WindowState = FormWindowState.Maximized
                p.Show()
                mps.Add(p)
            Next i

        End Sub 
End Class

Public Class frmMdiParent

    Private Sub frmMdiParent_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        Try
            mps.Remove(Me)
        Catch : End Try
    End Sub

    Private Sub frmMdiParent_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim f As New frmChild
        f.MdiParent = Me
        f.Show()
    End Sub
End Class

Private Sub btnSwitch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSwitch.Click
    'not very refined, first shot will be a dumb shot. 
    'Known, just a very rough test for testing the method
    If Index<= mps.Count - 2 Then
        Index += 1
    Else
        Index = 0
    End If

    Me.MdiParent = mps(Index)
End Sub

Public Class frmChild
    Dim index As Integer = 0
    Private Sub child_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        For i As Integer = 0 To mps.Count - 1
            If mps(i) Is Me.MdiParent Then index = i
        Next i
        Me.Text = Environment.TickCount  'Just for visual discrimination

    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

        'Added a second button to do something which references the parent.
        'as long as the form has just been created on startup in its original parent, 
        'everything works. Then the parent is alway snothing (even if returning to original one)
        '
        Me.MdiParent.WindowState = FormWindowState.Normal   '<-NullReferenceException 

    End Sub

    Private Sub btnSwitch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSwitch.Click

        'not very refined, first shot will be a dumb shot. 
        'Known, just a very rough test for testing the method, not a refined app
        If index <= mps.Count - 2 Then
            index += 1
        Else
            index = 0
        End If

        'Checked in debug, it's NEVER nothing. 
        'Just put a stop clause in case I was blind! Never Stopping, as expected
        If mps(index) Is Nothing Then Stop
        Me.MdiParent = mps(index)

        'Application.DoEvents()  //brought no benefits!

        'Just do prove myself I'm not getting confused. 
        'Yes, I just set the parent to something NOT noting and 
        'the mdiParent IS nothing! WTF??!!
        If Me.MdiParent Is Nothing Then Stop

        'AND THE FUN PART IS: the form actually keeps moving from one MDIContainer to another!!!

        'Show me visually whether it is nothing or not!
        Me.Label1.Text = If(Me.MdiParent Is Nothing, "NOTHING", "OK") 'After first click always displays NOTHING
        Me.Label2.Text = index   'always displaying correct index 

    End Sub
End Class