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Can any one tell me how I can develop middle tier in vfp using vb as the front end?
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Compile your VFP business logic (with no interface) into a COM DLL that can be called from any front-end that can instantiate COM objects, including VB.

You can also download free fraweworks that do exactly that like COMCodebook or buy commercial VFP frameworks for N-Tier development.

For more info on frameworks you can check:
http://fox.wikis.com/wc.dll?Wiki~CategoryFrameworks