>While not that big a problem in this case, I can envisage a point where there are lots of interfaces implemented in a VFP 7 application and you have to install lots of 3rd party software simply to compile it. This situation will be a problem for all developers. Is there another simple solution to this?

Lloyd,
PMFJI, but why do you see this as a problem? The same situation exists today with ActiveX controls. You can't build an application using them if you don't have them. You can't simply add a placeholder and say this should be the WidgetActiveX control.

One of the reasons the interface must exist on the machine during compilation is because VFP verifies that you have implemented all aspects of that interface (all methods/events). If not, VFP throws an error. If the interface doesn't exist, the verification fails.