PRB: All Parameters of Choose Function Are Evaluated (175979)

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

MORE INFORMATION

Steps to Reproduce Behavior

Create a new Visual Basic project. Form1 is created by default.
Add a Command button to Form1.

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

Save and run the project.
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.