Making a C# kill event for a vb6 app?

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Here is a pretty standard scheme for asynchronous background processing with VB6 clients and VB6 servers. (For instance it’s in Dan Appleman’s book and Microsoft’s VB6 samples.) I think it should work for a C# client too.

  • The VB6 ActiveX EXE object should expose an event CheckQuitDoStuff(). This takes a ByRef Boolean called Quit.
  • The client calls StartDoStuff in the ActiveX EXE object. This routine starts a Timer on a hidden form and immediately returns. This unblocks the calling thread. The Timer interval is very short so the Timer event fires quickly.
  • The Timer event handler disables the Timer, and then calls back into the ActiveX object DoStuff method. This begins the lengthy processing.
  • Periodically the DoStuff method raises the CheckQuitDoStuff event. The client’s event handler checks the special flag and sets Quit True if it’s necessary to abort. Then DoStuff aborts the calculation and returns early if Quit is True.

This scheme means that the client doesn’t actually need to be multi-threaded, since the calling thread doesn’t block while “DoStuff” is happening. The tricky part is making sure that DoStuff raises the events at appropriate intervals – too long, and you can’t quit when you want to: too short, and you are slowing down DoStuff unecessarily. Also, when DoStuff exits, it must unload the hidden form.

If DoStuff does actually manage to get all the stuff done before being aborted, you can raise a different event to tell the client that the job is finished.

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