PRB: Can't Set Formal Parameter When Setting Object Vars

- Microsoft Visual Basic programming system for Windows,

  versions 2.0 and 3.0

SYMPTOMS

When trying to use the Set keyword to assign a value to a formal parameter passed to a Visual Basic Sub or Function procedure, you may get a "Can't Set formal parameter" error at compile or run time.

Specifically, trying to use a Set statement on an object variable that is a formal parameter of a procedure results in this error.

CAUSE

This behavior is by design. You cannot use a procedure's formal parameter as the destination of a Set statement.

Object variables can be parameters of a Sub or Function procedure, but if an object variable is a parameter, its value cannot be changed inside the called procedure.

WORKAROUND

You can use a Set statement on an array of objects that is passed as a parameter. For example, you can modify the code shown in the "Steps to Reproduce Behavior" section to the following to prevent the error:

Sub Form_Load ()
   ReDim X(0) As Form
   Call MySub1(X())
End Sub

Sub MySub1 (FormalParameterX() As Form)
   Set FormalParameterX(0) = Form1
   FormalParameterX(0).Caption = "Success"    ' Form's caption will change.
End Sub

RESOLUTION

If you make the object variable Global instead of passing it as a parameter, you can use Set statements inside procedures.

STATUS

This behavior is by design.

MORE INFORMATION

Objects as parameters can be thought of as a copy of the structure that defines the object. If Set statements were allowed on these objects, this would change the value inside the routine, but upon returning from the routine the changes would be lost and the object variable would revert back to its original value.

Steps to Reproduce Behavior

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

2. Add the following code to the Form_Load procedure:

      Sub Form_Load ()
         Dim X As Form
         Call MySub1(X)
      End Sub
   
      Sub MySub1 (FormalParameterX As Form)
         Set FormalParameterX = Form1       ' Can't Set formal parameter error
         FormalParameterX.Caption = "Success"    ' This is never executed.
      End Sub
   
   

3. Attempt to run or compile the application to generate the "Can't Set formal parameter" error.

More Steps to Reproduce Behavior

Here is another set of steps that result in the error:

1. Start a new project in Visual Basic and add the following procedure to the application:

      Sub s (tb As Table)
         Set tb = Nothing
      End Sub
   
   

2. Press the F5 key to run the application. The error "Can't Set Formal Parameter" should occur immediately.

   Expected: Integer or Long or Single or Double or Currency or
   String or Variant.