stephan.smolek wrote: The clients can all be started seperatly by the user, or the master application starts them automaticly to have the standard user interaction of the client programs and also automated interaction / control with the master program. The only way to start the other applications is by using the Process class to launch them. stephan.smolek wrote: also automated interaction / control with the master program You cannot just expose public methods and expect to be able to call them acrossed AppDomain boundries. You're going to have to learn about .NET Remoting, or Windows Communication Foundation, and setup remoting servers and clients in your applications to handle the communication and you'll also have to carefully design your applications to expose their functionality through various object models, kind of like how Word and Excel do it. A guide to posting questions on CodeProject[^] Dave Kreskowiak Microsoft MVP Visual Developer - Visual Basic      2006, 2007

Thanks for your answer, I found another solution myself today.:) Stephan