DOCUMENT:Q191616  24-AUG-1998  [vbwin]
TITLE   :BUG: Order of Evaluation Change Between Native Code and P-Code
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:
OPER/SYS:WINDOWS
KEYWORDS:

======================================================================
---------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Visual Basic Learning, Professional, and Enterprise Editions
   for Windows, versions 5.0, 6.0
---------------------------------------------------------------------

SYMPTOMS
========

A program appears to be incorrectly evaluating expressions, or evaluates
differently between P-Code and Native Code with optimizations.

CAUSE
=====

Visual Basic does not guarantee the order of evaluation in expressions when
programs are compiled to native code with optimizations.

RESOLUTION
==========

In a no-optimization case, the expression is evaluated left to right. In a
optimized case, Visual Basic may change the order in which it evaluates
expressions. A change in logic may be required to achieve the desired
results.

STATUS
======

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

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

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

1. Create a new Standard EXE project in Microsoft Visual Basic. Form1 is
   created by default.

2. Select Add Module from the Project menu to add a standard code module
   (Module1) to the project.

3. Copy and Paste the following code into Module1:

      Dim Sample As String

      Public Sub Main()

         Dim Temp1 As String

         While (get_String) And Not (Sample Like "*/*/*")
            MsgBox "In loop --> " & Sample & " " & Not _
            (Sample Like "*/*/*")
         Wend
         MsgBox "after --> " & Sample
      End Sub

      Function get_String() As Boolean
         Sample = "09/19/97"
         get_String = True
      End Function

4. Select Project Properties from the Project menu. Change the Startup
   Object to "Sub Main."

5. Select the Compile tab and ensure the project is set to compile to
   Native Code.

6. Create the EXE by selecting Make Project1.EXE from the File menu.

7. Run the EXE and note the results.

8. Recompile the EXE by changing the project compile properties so that it
   compiles to P-Code.

9. Run the EXE again and note the different results. When compiled to P-
   Code, the expression 'While (get_String) And Not (Sample Like "*/*/*")'
   is evaluated as expected. However, when compiled to Native Code with
   optimizations, the (Sample Like "*/*/*") will be evaluated first.

Additional query words: kbDSupport kbVBp600bug kbCompiler kbdss kbVBp
kbVBp500bug
======================================================================
Platform          : WINDOWS
Issue type        : kbbug
Solution Type     : kbpending

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

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Copyright Microsoft Corporation 1998.