TITLE: "FOR EACH Enumeration of VFP Collections Does Not Provide True Object References"

SEVERITY: High

IMPORTANCE: Moderate (workaround available)

STEPS TO REPRODUCE: Run the sample code that appears below. It creates an object with a collection member and adds two items to the collection. Then it iterates in two ways: first using FOR EACH, second using using the implied Item() method. Results of several checks are written to _screen.

OBSERVED: When iterating, as in the first run, using FOR EACH <obj> IN THIS.MyApplications, the <obj> that is provided is not a true object reference to the underlying VFP object, but rather some sort of COM proxy. When you isntead iterate, as in the second run, using the (implied) Item method, the object references are as expected. The following differences between the the two runs points this out:

a) A COMPOBJ() check between the object reference provided by the enumerator and the actual original object itself returns .F., while it returns .T. in the second case.

b) Calling AMEMBERS() on the object reference provided by the iterator provides the wrong result in two ways: many properties are missing, and some *methods* are included in what should be a property list.

c) As you happen to reference additional proeprties directly in your code, the result of AMEMBERS grows, indicating that some discovery, via COM, seems to go on.

d) If you reference an invalid property name, instead of the expected error 1734 ("property is not known"), you get a 1426 COM error ("Unknown name").

e) NOTE: If you expand the object reference in the Watch/Local window, you do see all the properties, so something deeper than AMEMBERS is used internally there.

EXPECTED: To get the true object reference that was placed into the collection in the first place.

DANGERS: This is a particular problem when using AMEMBERS. Aside from getting the wrong result, AMEMBERS calls are often followed up by TYPE() checks of each property (such as when testing for array proeprties). If the result from AMEMBERS includes some *method* names, using TYPE() causes these methods to get *run*. In the instance where I discovered this bug, that running created recursion that eventually blew up VFP with "DO nesting too deep".

WORKAROUND: Avoid using FOR EACH with the VFP Collection base class, preferring the second form of iteration shown in the example code. This is mandatory if using AMEMBERS() or COMPOBJ().

SAMPLE CODE:

_SCREEN.FontName = "Courier New"
CLEAR
PRIVATE poAppManager
poAppManager = CREATEOBJECT("TheApplicationManager")
poAppManager.ProcessRequest()
RELEASE poAppManager

poAppManager = CREATEOBJECT("TheApplicationManager")
poAppManager.ProcessRequest2()

DEFINE CLASS TheApplicationManager AS Custom
  ADD OBJECT MyApplications AS Collection
  FUNCTION init
    this.Setup()
  ENDFUNC
  FUNCTION Setup
    this.AddApplication(CREATEOBJECT("AnApplication"), "App_1")
    this.AddApplication(CREATEOBJECT("AnApplication"), "App_2")
  ENDFUNC
  FUNCTION ProcessRequest()
    PRIVATE goL7App
    FOR EACH goL7App IN THIS.MyApplications
      IF m.goL7App.IsMyItem()
        goL7App.ProcessItem("Iteration using FOR EACH.")
        EXIT 
      ENDIF
    ENDFOR 
  ENDFUNC
  FUNCTION ProcessRequest2()
    PRIVATE goL7App
    LOCAL ii
    FOR ii = 1 TO THIS.MyApplications.Count
      goL7App = THIS.MyApplications[m.ii]
      IF m.goL7App.IsMyItem()
        goL7App.ProcessItem("Iteration using Count property.")
        EXIT 
      ENDIF
    ENDFOR 
  ENDFUNC
  FUNCTION AddApplication(loApp, lcKey)
    this.MyApplications.Add(loApp, lcKey)
  ENDFUNC 
ENDDEFINE

DEFINE CLASS AnApplication AS Session
  Prop1 = "ABC"
  Prop2 = 3.14
  Prop3 = .T.
  FUNCTION INIT
  ENDFUNC
  FUNCTION IsMyItem
    RETURN .t. && try a break point here also
  ENDFUNC
  FUNCTION ProcessItem(lcMsg)
    THIS.CheckConsistency(m.lcMsg)
  ENDFUNC 
  FUNCTION CheckConsistency(lcMsg)
    LOCAL aaa[1], lcProps, ii, jj
    ? m.lcMsg
    ? REPLICATE("=", LEN(m.lcMsg))
    ? "     COMPOBJ(THIS, m.goL7App):", COMPOBJ(THIS, m.goL7App)
    ? "          True property count:", TRANSFORM(AMEMBERS(aaa, THIS))
    FOR jj = 1 TO 2
      ? "Properties count per AMEMBERS:", TRANSFORM(AMEMBERS(aaa, m.goL7App))
      lcProps = ""
      FOR ii = 1 TO ALEN(aaa)
        lcProps = lcProps + aaa[m.ii] + CHR(13) + CHR(10)
      ENDFOR
      ? "          Properties reported:", m.lcProps FUNCTION "V30"
      ? "         Check value of Prop2:", TRANSFORM(m.goL7App.Prop2)
    ENDFOR
    TRY
      = goL7App.Unknown
    CATCH TO loExc
      ? "    Error on unknown property:", TRANSFORM(loExc.ErrorNo), loExc.Message
    ENDTRY
    ?
  ENDFUNC
ENDDEFINE