><snip>
>
>>Hi Ed, Of course, for C or Assembler, these type of error always occur.
>>But as these part of code cannot be handled by programmer. The 4GL progr try all
>>possible case to trace what these error may occur?! much seriously!
>>
>>Besides, I wonder no need other function call! That is, it will cause error
>>even:
>>

>>= NVL(.NULL., _screen)
>>

>
>I ran this code, as well as:
>

>  FOR i = 1 to 100
>      =NVL(.NULL.,_SCREEN)
>  ENDFOR
>

>
>with no problems under VFP 5.0a, with no porblem to report.
>
>>
>>Maybe it is a weapon for fired progr to make a deadTime of program.

>>If Date() >= {2000/1/1}
>>  = NVL(.NULL., _screen)
>>Endif
>>

>>and hide it in any superclass Init procedure!
>
>Why?

It is surely if the Nvl() return value is putting to other user-defined function will cause error.

Strange?!
In Executable program, if you call:

 =NVL(.null.,_screen)

 messagebox(_screen.name + chr(9) + _screen.class)
* It will return other object value or Failure Error!!

I think the problem is the NVL() remap another object to system variable _screen, so there will be problem if any statement calling for _screen!

Such as _screen.paint()
Thus, it may error very fast or not depend on when the _screen object is called!