Private Sub KostenrechnungToolStripMenuItem_Click(ByVal sender As _
  System.Object, ByVal e As System.EventArgs) Handles _
  KostenrechnungToolStripMenuItem.Click
        Kostenrechnung.MdiParent = Me
        Kostenrechnung.Show()
    End Sub

...
          .Items.Add(chfName)
          AddHandler ContextMenuStrip1.Items(x).Click, AddressOf _
            ContextMenuStrip_Click
...
Private Sub ContextMenuStrip_Click(ByVal sender As Object, ByVal e As _
  System.EventArgs)
        Dim cmi As ToolStripMenuItem = CType(sender, ToolStripMenuItem)
        Dim i As Integer
        Dim x As String = cmi.Text
        For i = 0 To cmi.Owner.Items.Count - 1
            If x = cmi.Owner.Items(i).Text Then
                Me.MdiChildren(i).BringToFront()
                Exit For
            End If
        Next
    End Sub

Salzburger