Public Class myWindow
 
    Public Sub New()
    ...
    DispatcherSaver.MainWindowDispatcher = Me.Dispatcher 'Den Dispatcher für 
    ' spätere Zugriffe speichern
    ...
    End Sub
 
    ...
 
End Class

Public Class mybox
 
    ...
 
    Delegate Sub CallErr(...)
 
    Public Sub ShowDialogThread(...)
        Dim del As New CallErr(AddressOf ShowDialog)
        ...
        MainWindowDispatcher.Invoke(del, objArr)
    End Sub
 
    ...
 
End Class
 
Public Module DispatcherSaver
    Public MainWindowDispatcher As Dispatcher 'Hier wird der Dispatcher 
    ' gespeichert
End Module

        If Me.Dispatcher.CheckAccess() Then
 
        End If
 
        'oder
 
        If Me.CheckAccess Then
 
        End If