Calvin Hsia posted a code sample on the MS site, http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnfoxgen7/html/vfpandcom.asp , regarding "VFP Callback Design" (at the bottom). I've run the code in VFP7 SP1 and it works fine. However, I don't understand *how* it works. Specifically, in the definition of the cCallback class there's the line

implements iDemoEvents in idemo.dll

I can't see where the iDemoEvents interface is defined. And, in the sample IDEMO.PRG, the cCallBack class is instantiated before the iDemo COM server.

For reference I've pasted the code sample below:

CLEAR ALL
IF PROGRAM() != "IDEMO"
   ?"this file MUST BE NAMED 'idemo.prg'"
   RETURN
ENDIF
IF .t.
   IF FILE("idemo.dll")
      DECLARE integer DllUnregisterServer IN idemo.dll
      DllUnregisterServer()
      CLEAR DLLS
   ENDIF
   BUILD PROJECT idemo FROM idemo
   BUILD DLL idemo from idemo recomp
endif
clear

oCallback = NEWOBJECT("cCallback")    && the event callback
ostock=newOBJECT("idemo.idemo")       && the business COM obj
ostock.setcallback(oCallBack)         && like BindEvents
?ostock.BuyStock("MSFT",10000 )       && invoke a method
ostock.setcallback(.null.)            && like UnBindEvents
?ostock.BuyStock("MSFT",20000 )       && this one doesn't fire events

*This is the actual implementation of the event interface
DEFINE CLASS cCallback as session
   implements iDemoEvents in idemo.dll
   procedure iDemoEvents_BeforeBuyStock(cStock as String, qty AS Number)
      ?program(),cstock,qty
   procedure iDemoEvents_AfterBuyStock(cStock as String, qty AS Number)
      ?program(),cstock,qty

enddefine
*the rest of this file is used in the COM server
DEFINE CLASS idemo as session olepublic
   oc = .null.
   PROCEDURE init
      this.SetCallBack(.null.)    && set callback to default
   PROCEDURE setcallback(oC as Variant)
      IF ISNULL(oc)
         && dummy instance that does nothing: virtual function
         this.oc = NEWOBJECT("DemoEvents")
      else
         IF VARTYPE(oc) != 'O'
            COMRETURNERROR(PROGRAM(),"callback must be obj")
         ENDIF
         this.oc = GETINTERFACE(oC,"iDemoEvents","idemo.idemo")
      endif
   procedure MyDoCmd(cCmd as String) as Variant
      &cCmd
   procedure MyEval(cExpr as String) as Variant
      return &cExpr
   procedure BuyStock(cStock as String, qty AS Number) as Boolean
      this.oc.BeforeBuyStock(cStock, qty)
      *here we buy the stock
      this.oc.AfterBuyStock(cStock, qty)
   FUNCTION Error(nError, cMethod, nLine)
      COMreturnerror(cMethod+'  err#='+str(nError,5)+'  line='+;
         str(nline,6)+' '+message(),_VFP.ServerName)
      && this line is never executed

enddefine

*Just an interface definition that should be implemented by outside callers
DEFINE CLASS DemoEvents as session olepublic
   procedure BeforeBuyStock(cStock as String, qty AS Number)
   procedure AfterBuyStock(cStock as String, qty AS Number)
enddefine