Q208212: PRB: Error When Using Assign Method to Assign Object References

Article: Q208212
Product(s): Microsoft FoxPro
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbOOP kbvfp600 kbGrpDSFox kbDSupport kbCodeSnippet
Last Modified: 18-NOV-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
========

The following error message appears when using an assign method to store object
references in a property and then trying to access properties of that object
through the property:

  Unknown member <property name>

CAUSE
=====

The assign method fires twice when assigning an object reference to a property.
The first time it fires, it sets the value of the property containing the object
reference to NULL and then it runs again and assigns the property the object
reference.

RESOLUTION
==========

In the assign method, check to see if the new value is not NULL before assigning
it to the property.

In the program code in the "Steps to Reproduce Behavior" section below, look in
the class definition of the Test class and uncomment the IF...ENDIF block code
as shown to check the new value. It is in the Procedure obj_assign:

  *   IF !ISNULL(m.vNewVal)        && uncomment this to make things work.
         THIS.obj     = m.vNewVal
  *   ENDIF

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

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

The code in the steps below defines a class named Test that has two properties,
obj and prevobj. The obj property has an assign method on it. The assign method
takes the value in the obj property and assigns it to prevobj before the new
value of obj is assigned.

The code in the program creates an object reference to the Test class and then
instantiates an instance of a class based on the custom base class and assigns
this object reference to the obj property. It does this three times. When the
reference to the custom class is assigned to the obj property, the assign method
fires.

The code outputs with ? commands to the Visual FoxPro desktop the values in the
obj and prevobj properties at various times. Note that the ? command in the
assign method fires twice the second and third times the new object reference to
the custom class is assigned to the obj property.

The error message

  Unknown member PREVOBJ.

appears the third time when oTest.prevobj is not checked to see if it is NULL
before accessing the oTest.prevobj.name property.

1. Copy the following code into a new program:

  CLEAR

  LOCAL oTest, oCustom
  oTest = CREATEOBJECT("test")

  WITH oTest
      oCustom   = NEWOBJECT("custom")
      .obj = oCustom
      .obj.NAME = "one"
      ? "obj     = ", .obj.NAME
      * oTest.prevobj is null the first time through.
      ? "prevobj = ", IIF(ISNULL(.prevobj),"Null", .prevobj.NAME)    
      ?

      RELEASE oCustom
      ? "Second Code Block"
      oCustom = NEWOBJECT("custom")
      .obj = oCustom
      .obj.NAME = "two"
      ? "obj = ", .obj.NAME
      ? "prevobj = ", IIF(ISNULL(.prevobj),"Null", .prevobj.NAME)
      ?

      RELEASE oCustom
      ? "Third Code Block"
      oCustom = NEWOBJECT("custom")
      .obj = oCustom
      .obj.NAME = "three"
      ? "obj = ", .obj.NAME
      ? "prevobj = ", IIF(ISNULL(.prevobj),"Null", .prevobj.NAME)
      ? "prevobj = ", .prevobj.NAME   && this causes error
      ?
  ENDWITH

  oTest = NULL
  oCustom  = NULL

  RETURN

  DEFINE CLASS test AS CUSTOM
      obj     = NULL
      prevobj = NULL

      PROCEDURE obj_assign
      LPARAMETERS vNewVal
      THIS.prevobj = THIS.obj
      ? "Assign Method: The value of this.prevobj is " + IIF(ISNULL(THIS.prevobj), 'Null', 'Not Null')

  *   IF !ISNULL(m.vNewVal)        && uncomment this to make things work.
         THIS.obj     = m.vNewVal
  *   ENDIF
  ENDPROC
  ENDDEFINE

2. Save and run the code. Note the output on the Visual FoxPro desktop.

3. Uncomment the commented IF...ENDIF block in the obj_assign procedure in the
  Test class definition and re-run the code.

4. Note that this time the output in the second and third code blocks for the
  prevobj property is "one" and "two" instead of NULL. The error message does
  not appear either.

The assign method only fires once when assigning other data types to a property.
Code to demonstrate this is not included in this article.

Additional query words: kbDSE

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Keywords          : kbOOP kbvfp600 kbGrpDSFox kbDSupport kbCodeSnippet 
Technology        : kbVFPsearch kbAudDeveloper kbVFP600
Version           : WINDOWS:6.0
Issue type        : kbprb
Solution Type     : kbpending

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