Zitat:

Ich würde denken, Deine Windows laufen im Oberflächenthread

Private Sub MainWindow_Loaded(...) Handles Me.Loaded
    mjsMsgBox.Initialize()
End Sub

mjsMsgBox.ShowBoxDialog("Hallo Welt")

Partial Public Class mjsMsgBox
    Private Shared STA_Dispatcher As Dispatcher
    Private Delegate Sub CallBox(Text As String)
 
    Public Shared Sub Initialize()
        STA_Dispatcher = Application.Current.Dispatcher
    End Sub
 
    Public Shared ShowBoxDialog(Text As String)
        Dim c As New CallBox(AddressOf Caller)
        STA_Dispatcher.Invoke(c, Text)
    End Sub
 
    Private Shared Caller(Text As String)
        Dim myBox As New mjsMsgBox
        myBox.ShowDialog(Text)
    End Sub
End Class