thanks for your reply. with your information how to call the execute method of the edit an create command correctly I was able to solve this problem. My workaround is not pretty, but it works. My appointments now have a boolean flag to indicate if they were created from drag or not. If the AppointmentCreating was raised by a drag event I simply fire the RadScheduleViewCommands.CreateAppointment.Execute command. The only problem I had after that was that the resources of the appointment were not set correctly anymore if AppointmentCreating was called the second time. To solve that I keep a reference to the appointment and set the resources again. Finally the Dialog appears correctly with two out of 3 resources set correctly. Now the user only has to select the third resource and then can press the OK button of the edit dialog. Here's my implementation of the AppointmentCreating event handler so you and others get a better imagination of this.
sender, AppointmentCreatingEventArgs e)
SchedulerAppointmentViewModel app = e.Appointment
m_AppointmentCeatedByDrag = app;
Anyway I'm looking forwarding for a more clean solution.