Q253879: BUG: Forms Cannot Find Class Library When Calling .Exe File

Article: Q253879
Product(s): Microsoft FoxPro
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbProjManager kbvfp600 kbvfp600bug kbXBase kbGrpDSFox kbDSupport kbCodeSnippet
Last Modified: 13-MAY-2002

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 6.0 
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SYMPTOMS
========

When attempting to run a form contained in an application (.app) file that is
called from an executable (.exe) file, an error message is generated:

  Error instantiating class. Cannot find ...

CAUSE
=====

Visual FoxPro cannot find the class libraries for the form even though they may
already be in memory. Because the libraries were excluded from the .app file,
the form is looking for them on disk, and they cannot be found. Visual FoxPro
does not search memory to find these missing classes.

RESOLUTION
==========

The resolution to the problem is to clear the ClassLoc field for all records in
the form.scx file and load the class libraries that are in the main .exe file.

To clear the Classloc field in the form, issue the following code.

NOTE: Ensure that you make a backup of the form or forms that are being updated.

  UPDATE MyForm.SCX SET ClassLoc = ""

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  http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

Copy the following code to a program (.prg) file called "SetFormClasses" (without
the quotation marks):

  *!*	Forms in VFP6 cannot find the classlibs used in the Form Designer if they
  *!*	are excluded from the APP project but are loaded in the calling EXE.  This
  *!*	program (SetFormClass) will load necessary classlibs in an APP called
  *!*	from an EXE and is dependant upon the classlibs in the EXE because they
  *!*	are excluded in the APP.
  *!*
  *!*	One important aspect to note is that the classlib field in the SCX file
  *!*	must be cleared.  Be certain to make a copy of the form before clearing
  *!*	the classloc field, or you may not be able to open the form in the Form
  *!*	Designer.  The classlibs needed for the form will be loaded in in this
  *!*	program if they are passed as parameters.
  *!*
  *!*	Parameters:
  *!*		tcClasslib1
  *!*		tcClasslib2
  *!*		tcClasslib3
  *!*		...
  *!*		tcClasslibN
  *!*
  *!*	Syntax:
  *!*		DO SetFormClasses WITH "MyClass1", "MyClass2", "MyClass3", ... "MyClassN"
  *!*
  *!*	This program must be run from the main program in the APP.
  *!*	The variable lcCurrentAPP is assuming that this program is called
  *!*	from the main program.

  *!*	If an APP has a classlib loaded and it is unloaded here, then
  *!*	returning to that APP will cause errors.  Need to release and reload the
  *!*	classes in MAIN, but only if they are not loaded with the IN clause.

  #DEFINE SFC_ADDITIVE .T.
  *!*	Add as many parameters as necessary to cover all necessary classlibs
  *!*	used in this module
  LPARAMETERS tcClassLib1, tcClassLib2, tcClassLib3, tcClassLib4, tcClassLib5, ;
     tcClassLib6, tcClassLib7  && Etc.

  LOCAL lcClass, lnClass, lnIndex, nPos1, nPos2, nChars, nChar0, nChar1, lnParams
  LOCAL ARRAY laClass[1], laParams[1]
  LOCAL lcMainEXE, lcCurrentAPP
  lnParams = PCOUNT()

  *!*	Load the parameters into an array for quick reference
  DIMENSION laParams[lnParams]
  FOR lnIndex = 1 TO lnParams
     laParams[lnIndex] = UPPER(EVAL("tcClassLib" + TRANSFORM(lnIndex)))
  ENDFOR

  lcMainEXE = SYS(16, 1)  && Main EXE
  lcCurrentAPP = SYS(16, PROGRAM(-1) - 1)

  lcClass = SET("CLASSLIB")
  IF EMPTY(lcClass)
     *!*	This is the first time in.  In addition, no class libraries have been
     *!*	set anywhere.  Set them all here.  All classes will come from params
     = ACOPY(laParams, laClass)
     lnClass = lnParams

     FOR lnIndex = 1 TO lnClass
        laClass[lnIndex] = laParams[lnIndex]
        DO SetClassLib WITH laClass, lnIndex, lcMainEXE, SFC_ADDITIVE
     ENDFOR
  ELSE
     *!*	Class libraries have been set at some point in time.  We don't know
     *!*	where they have been set, or if they are needed.  So, go through and
     *!*	try to add each class library back into the the system.  Need to check
     *!*	if it is already loaded "IN" somewhere.  If it is, then do not touch it.
     *!*	If it is not, compare it with the required classlibs passed in by
     *!*	parameters.  If it is not in the parameter list, leave it alone.  If it
     *!*	is in the list and loaded "IN" already, then don't process them again.
     *!*	Also need to check for new class libraries being added by this module.
     *!*	If they are not found, add them in.

     lnClass = OCCURS(",", lcClass) + 1  && Number of libs in memory
     DIMENSION laClass[lnClass, 3]
     laClass = .T.
     *!*	laClass[1,1] is the class
     *!*	laClass[1,2] is the class with full path
     *!*	laClass[1,3] is a flag indicating this needs to be loaded

     nPos1 = 1
     *!*	Place the stem (filename only) into an array for resetting later
     FOR lnIndex = 1 TO lnClass
        nPos2 = AT(",", lcClass, lnIndex)  && Locate the end of the class string
        nChars0 = IIF(nPos2 < 1, LEN(lcClass), nPos2 - nPos1)

        *!*	Need to make sure that you don't include any IN clauses in the substring,
        *!*	1) they will throw off the JUSTSTEM function
        *!*	2) FoxPro already knows where to find them.
        nChars1 = AT(".VCX IN ", SUBSTR(lcClass, nPos1, nChars0))
        IF nChars1 > 0
           *!*	VFP already knows where to find this classlib.  Do not reload this classlib
           laClass[lnIndex,3] = .F.
        ENDIF

        nChars = IIF(nChars1 = 0, nChars0, nChars1)

        laClass[lnIndex,1] = JUSTSTEM(SUBSTR(lcClass, nPos1, nChars))
        laClass[lnIndex,2] = SUBSTR(lcClass, nPos1, nChars0)

        nPos1 = nPos2 + 2 && New starting position
     ENDFOR

     *!*	If the loaded classlibs are also parameters,
     *!*	mark them for release is not already marked for release
     FOR lnIndex = 1 TO lnClass
        IF ( laClass[lnIndex,3] = .T. )
           *!*	Still marked for reload.  Passed the "IN" clause
           IF ( ASCAN(laParams, laClass[lnIndex,1]) = 0 )
              *!*	This module does not know about this class, don't unload it
              laClass[lnIndex,3] = .F.
           ELSE
              *!*	This will be reloaded later
              RELEASE CLASSLIB &laClass[lnIndex,2]
           ENDIF
        ENDIF
     ENDFOR

     *!*	Load all eligible classlibs
     FOR lnIndex = 1 TO lnClass
        IF laClass[lnIndex,3] = .T.
           *!*	Classlib was previously released, add it back in
           DO SetClassLib WITH laClass, lnIndex, lcMainEXE, SFC_ADDITIVE
        ENDIF
     ENDFOR

     *!*	Now try to load all the classes that were passed in as parameters.
     *!*	Those that don't load here will be loaded next
     = ACOPY(laParams, laClass1)
     lnClass = lnParams

     FOR lnIndex = 1 TO lnClass
        laClass1[lnIndex] = laParams[lnIndex]
        DO SetClassLib WITH laClass1, lnIndex, lcMainEXE, SFC_ADDITIVE
     ENDFOR

  ENDIF

  *!*	Check the parameters array for any classes that need to be loaded 
  *!*	for this module.  Add them if necessary.
  FOR lnIndex = 1 TO lnParams
     IF ASCAN(laClass, laParams[lnIndex]) = 0 AND ;
           ASCAN(laClass1, laParams[lnIndex]) = 0  && Add the new library
        SET CLASSLIB TO (laParams[lnIndex]) IN (lcCurrentAPP) ADDITIVE
     ENDIF
  ENDFOR

  PROCEDURE ClearElement
     *!*	If there is an error setting the classlib, assume that it is because
     *!*	it cannot be found.  Clear the reference to that element so the
     *!*	local classes will not be messed up when loading.
     LPARAMETERS taClass, tnElement
     taClass[tnElement] = CHR(255)
     RETURN

  PROCEDURE SetClassLib
     LPARAMETERS taClass, tnElement, tcMainEXE, tlAdditive
     EXTERNAL ARRAY taClass  && Avoid compile time errors
     LOCAL lcOnError
     lcOnError = ON('ERROR')

     ON ERROR DO ClearElement WITH taClass, tnElement

     IF tlAdditive
        SET CLASSLIB TO (taClass[tnElement]) IN (tcMainEXE) ADDITIVE
     ELSE
        SET CLASSLIB TO (taClass[tnElement]) IN (tcMainEXE)
     ENDIF
     ON ERROR &lcOnError
     RETURN

In the main program of the .app file, place a call to SetFormClasses with the
following code:

  DO SetFormClasses WITH "MyClass1", "MyClass2", ... "MyClassN"

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

MORE INFORMATION
================

There may be situations when an application (.app) with multiple modules is
created. The modules may be updated, but the base .exe is not updated at the
same time. The functionality of the base classes in the .exe files is still
needed until the .exe is updated, but not any new functionality in the classes
in development. The only way to do this is to exclude the classes from the .app.
However, by doing so, Visual FoxPro is no longer able to find the class.

Steps to Reproduce Behavior
---------------------------

1. Create a project and save it as "MAIN" (without the quotation marks).

   - Create a program (.prg) file and save it as "Main" (without the quotation
     marks). Add the following code:

  DO FORM Main
  READ EVENTS
  CLEAR EVENTS
  SET CLASSLIB TO

   - Create a new class named MyForm based on Form and store it in MyClass.
   - Add a class named MyCommandButton based on CommandButton and store it in
     MyClass.
   - Create a form based on the new class MyForm. Remove the original form and
     the form set. Place the following code in the Destroy event of the form:

  CLEAR EVENTS

     Save the form as "Main" (without the quotation marks).
   - Drag and drop two command buttons based on MyCommandButton onto the form.
   - In the click event of the first command button, add the following code.
     Give this button the caption of Module1Form.

  DO module1.APP WITH "Module1Form"

   - In the click event of the second command button, add the following code.
     Give this button the caption of Cancel.

  THISFORM.RELEASE()

   - Build an .exe file.

2. Create a second project and save it as "Module1" (without the quotation
  marks).

   - Create a .prg file and call it "Module1Main" (without the quotation
     marks). Add the following code:

  LPARAMETERS cForm
  *!* Uncomment this to see the workaround in action
  *!*DO setformclasses WITH "MyClass"
  DO FORM Module1Form

   - Add the class library MyClass to the project.
   - Create a new form based on the class MyForm and save it as Module1Form.
     Remove the original form and the form set.
   - Drag and drop one command button based on MyCommandButton from MyClass
     onto the form. Add the following code to the click event:

  THISFORM.RELEASE()

   - Build an APP.

3. Copy the .app file and the .exe file to a new folder.

4. Run the .exe and click on Module1Form. Note that the following error message
  appears:

  Error instantiating class. Cannot find ...

Additional query words:

======================================================================
Keywords          : kbProjManager kbvfp600 kbvfp600bug kbXBase kbGrpDSFox kbDSupport kbCodeSnippet 
Technology        : kbVFPsearch kbAudDeveloper kbVFP600
Version           : WINDOWS:6.0
Issue type        : kbbug
Solution Type     : kbpending

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