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Q194202: PRB: Datatype Error Accessing Property With _Access Method

Article: Q194202
Product(s): Microsoft FoxPro
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): 
Last Modified: 11-DEC-1999

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The information in this article applies to:

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

Returning an invalid datatype in the _Access method of an object's property
results in the following error when you reference that property:

  Datatype is invalid for this property.

RESOLUTION
==========

To avoid this error, make sure that the datatype returned is valid for the
property being accessed.

It is recommended that the return value of the _Access method be the property's
value.

Example:

     PROCEDURE object1.INTERVAL_Access
        RETURN THIS.INTERVAL
     ENDPROC

In the rare instance where you need to return a value other than the property's
value, changing the value of the property, and returning the property ensures
that you are storing the value for future comparison.

NOTE: You should consider changing the value of a property in the property's
_Access method an extreme case, and make sure you fully document the change.

STATUS
======

This behavior is by design.

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

NOTE: The example we provide in this article is not the only place this behavior
occurs. Any access method that tries to return an invalid datatype for the
property fails with the error.

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

Run the following sample code from a program file.

Notice that the RETURN statement in the object1.INTERVAL_access method returns
nothing explicitly. In Visual FoxPro, unless you specify what to return, FoxPro
returns a logical true or false (.T. or .F.). This is not a valid setting for
the Interval property of the Timer object. This results in the error "Datatype
is invalid for this property," and Visual FoxPro cannot instantiate the object.

        *******Begin Sample Code************

        PUBLIC oDS
        oDS=CREATEOBJECT("test")
        oDS.SHOW
        READ EVENTS
        USE
        RELEASE oDS

        DEFINE CLASS test AS FORM
           DOCREATE = .T.
           CAPTION = "Now Testing Access for Object TIMER"
           ADD OBJECT object1 AS TIMER
           ADD OBJECT btnClose AS COMMANDBUTTON
           PROCEDURE object1.INTERVAL_access
              RETURN
           ENDPROC
           PROCEDURE btnClose.CLICK
              CLEAR EVENTS
              RELE THISFORM
           ENDPROC
        ENDDEFINE

        *********End Sample Code********

Additional query words: kbVFp600 kbOOP

======================================================================
Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP600
Version           : WINDOWS:6.0
Issue type        : kbprb

=============================================================================

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