PRB: Arguments Are Evaluated from Right to Left in Some Cases

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 16-bit and 32-bit, for Windows, version 4.0

SYMPTOMS

Visual Basic 4.0 allows you to pass a variable number of arguments to a subroutine or function by using a parameter array (ParamArray). Although, arguments passed to another function or subroutine are evaluated from right to left when a parameter array is used in the 32-bit version, the 16-bit version evaluates arguments from left to right when a parameter array is used.

Arguments are evaluated from left to right if a set number of arguments is passed to a function or subroutine in both the 16-bit and 32-bit versions.

Arguments are evaluated from right to left when using the switch function in both the 16-bit and 32-bit versions of Visual Basic.

WORKAROUND

Code that relies on the order of evaluation of arguments passed to another function is dangerous code in any language. To correct code that fails because of the order of evaluation, Microsoft recommends that arguments be explicitly calculated before being passed to a function or subroutine.

STATUS

This behavior is by design.

MORE INFORMATION

The following table summarizes the order in which arguments are evaluated:

Version   Parameter Array   Set Number of Arguments   Switch Function

16-bit    Left to Right     Left to Right             Right to Left
32-bit    Right to Left     Left to Right             Right to Left

Steps to Reproduce Behavior When Using a ParamArray Argument

This example was constructed in Visual Basic 4.0 using the 32-bit version, but the 16-bit or 32-bit version can be used.

1. Create a new project. On Form1 (the default form), place one command button (Command1).

2. In the Click event for the Command1 button, place this code:

      Private Sub Command1_Click()
         y = Test(inc(1), inc(2), inc(3))
         Debug.Print y
      End Sub
   
   

3. In the General section for Form1, place these two code segments:

   Function Test(ParamArray z()) As Integer

         Test = z(2)
   

   End Function

   Public Function inc(y As Integer) As Integer

         Static x As Integer
         x = x + 1
         inc = x + y
   

   End Function

4. Run the program by pressing the F5 key, and click the Command1 button.

   The debug window prints out the number 4. If the arguments were evaluated from left to right, the same code would print out the number 6 because x would be incremented twice before the third argument was evaluated in the inc function.

   If a set number of arguments are passed to the Test function, then the arguments are evaluated from left to right. This is illustrated by rewriting the function Test as follows:

   Function Test(z0, z1, z2) As Integer

         Test = z2
   

   End Function

   In this case, the debug window prints out 6.

   To fix the code, and still use a parameter array, you could change the code in the Click event of Command1 to evaluate the arguments first as in this example:

      Private Sub Command1_Click()
         X1 = inc(1)
         X2 = inc(2)
         x3 = inc(3)
         y = Test(X1, X2, X3)
         Debug.Print y
      End Sub
   
   

Steps to Reproduce Behavior When Using the Switch Function

1. Create a new project. On Form1 (the default form), place one command button (Command1).

2. In the Click event for the Command1 button, place this code:

      Private Sub Command1_Click()
         Test
      End Sub
   
   

3. In the General section for Form1, place these two code segments:

   Public Function inc(x As Integer, ByVal y As Integer) As Integer

         x = x + 1
         inc = x
         Debug.Print "x: "; x; "evaluated at position"; y
   

   End Function

   Public Sub Test()

         Dim x As Integer
         x = -1
         y = Switch(False, inc(x, 1), False, inc(x, 2), True, inc(x, 3))
         Debug.Print y
      End Sub
   
   

4. Run the program by pressing the F5 key, and click the Command1 button. From the output, you can see that inc(x,3) evaluates first, then inc(x,2) evaluates, and finally inc(x,1) evaluates.

Keywords          : PrgOther VB4ALL VB4WIN vbwin GnrlVb kbprg
Technology        : kbvba
Version           : WINDOWS:4.0
Platform          : NT WINDOWS
Issue type        : kbprb