>I want to create a VFP DLL that takes data from a calling program and then processes it. The software that is doing the talking allows for "plugins".
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>What general principals should I be aware of?
>Mike

Often, "plug ins" are programs that must conform to a specific set of interfaces, depending on the host software. For a host program to use a VFP dll, it must be able to create and manipulate COM objects; that's what VFP dlls are.