Q175979: PRB: All Parameters of Choose Function Are Evaluated

Article: Q175979
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbGrpDSVB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, version 6.0 
- Microsoft Visual Basic Professional Edition for Windows, version 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
- Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 
- Microsoft Visual Basic Learning Edition for Windows, version 5.0 
- Microsoft Visual Basic Professional Edition for Windows, version 5.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 5.0 
- Microsoft Visual Basic Standard Edition for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 4.0 
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SYMPTOMS
========

When you specify functions as the Choice arguments in a Choose function, all
functions are executed.

CAUSE
=====

The Choose function allows you to select a value from a list of choices based on
the value passed as the index. For example, the following statement would result
in x being assigned the value "b":

     x = Choose(2, "a", "b", "c")

The parameters passed to the Choose function do not need to be static values as
in the above example. You can use function names instead:

     x = Choose(2, MyFunction1(), MyFunction2(), MyFunction3())

In this situation, all of the functions referenced are executed regardless of the
index specified. In this example, the functions MyFunction1, MyFunction2, and
MyFunction3 are all executed even though the result of MyFunction2 is the value
returned by Choose.

Choose is implemented as a function with a parameter array and every parameter
must be evaluated before it can be passed to the Choose function.

Visual Basic's implementation of the Choose function can be duplicated with the
following function:

     Public Function MyChoose(Index As Single, ParamArray Choice())
        On Error GoTo BadChoice
        If IsObject(Choice(Index - 1)) Then
           Set MyChoose = Choice(Index - 1)
        Else
           MyChoose = Choice(Index - 1)
        End If
        Exit Function
     BadChoice:
        MyChoose = Null
     End Function

RESOLUTION
==========

Because this behavior is by design, another approach must be taken to avoid all
parameters being evaluated. The logic of the Choose function can easily be
broken down into an equivalent "If...Then...ElseIf" statement or a "Select Case"
statement.

Sample Choose Statement
-----------------------

     result = Choose(MyIndex, MyFunction1(), MyFunction2(), MyFunction3())

Equivalent 'If...Then...ElseIf' Statement
-----------------------------------------

     If MyIndex = 1 Then
        result = MyFunction1()
     ElseIf MyIndex = 2 Then
        result = MyFunction2()
     ElseIf MyIndex = 3 Then
        result = MyFunction3()
     Else
        result = Null
     End If

Equivalent 'Select Case' Statement
----------------------------------

     Select Case MyIndex
        Case 1
           result = MyFunction1()
        Case 2
           result = MyFunction2()
        Case 3
           result = MyFunction3()
        Case Else
           result = Null
     End Select

STATUS
======

This behavior is by design.

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

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

1. Create a new Visual Basic project. Form1 is created by default.

2. Add a Command button to Form1.

3. Add the following code to Form1:

        Option Explicit

        Private Function MyFunction1() As Long
           Debug.Print "MyFunction1"
           MyFunction1 = 1
        End Function

        Private Function MyFunction2() As Long
           Debug.Print "MyFunction2"
           MyFunction2 = 2
        End Function

        Private Function MyFunction3() As Long
           Debug.Print "MyFunction3"
           MyFunction3 = 3
        End Function

        Private Sub Command1_Click()
           Dim result
           result = Choose(2, MyFunction1(), MyFunction2(), MyFunction3())
        End Sub

4. Save and run the project.

5. Click the CommandButton. Note that the "Debug.Print" statements from the
  three user-defined functions are executed even though the result of
  MyFunction2 is the returned value.

REFERENCES
==========

Visual Basic's Online Help for the Choose function.

Additional query words: kbVBp500 kbVBp600 kbVBp kbdsd kbDSupport kbVBp400 kbVBA

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Keywords          : kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbVB400Search kbVB400 kbZNotKeyword3
Version           : WINDOWS:4.0,5.0,6.0
Issue type        : kbprb

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