PRB: Error When Using Assign Method to Assign Object References (208212)

*   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 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.