FIX: Optional Argument Default Value Breaks Binary Compatibility (176164)


The information in this article applies to:

  • Microsoft Visual Basic Learning Edition for Windows 5.0
  • Microsoft Visual Basic Professional Edition for Windows 5.0
  • Microsoft Visual Basic Enterprise Edition for Windows 5.0
This article was previously published under Q176164

SYMPTOMS

Setting the default value of an Optional String argument to vbNullString causes the following error when the project is compiled with Binary Compatibility:
"<procedure> in the <class> class module has arguments and/or a return
type that is incompatible with a similar declaration in the
version-compatible component"

RESOLUTION

Use quotation marks instead of vbNullString. For example: 
   Optional myString as String = ""

You may also see the error:
"Variable uses an automation type not supported in Visual Basic (Error 458)."


This occurs when you attempt to set the default value of a Variant optional argument to Null. Avoid using any of the null-related constants (vbNull, vbNullString, etc.) when providing a default value for an optional argument.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been fixed in Visual Basic 6.0.

MORE INFORMATION

A project containing the specification of this default value returns an error stating that the method for which the argument is defined is not Binary Compatible. This problem is specific to the use of vbNullString. It does not affect default values for Optional arguments in general.

Steps to Reproduce Behavior

  1. Create a new ActiveX DLL project.
  2. Insert the following code into Class1: 
          Public Function meth1(pf1 As String, Optional pf2 As String = _
          vbNullString) As Boolean
         'Sample method
      End Function
  3. From the File menu, select Make Project1.DLL.
  4. From the Project menu, select Project1 Properties.
  5. Click the Component Tab.
  6. Set the Version Compatibility to Binary Compatibility.
  7. Verify that TestBinaryProject1.dll is listed in the File Location Box at the bottom of the form.
  8. Close the Project Properties form, and select "Make TestBinary Project1.DLL" from the File menu. Note that the error occurs at this point.
  9. Select "Make Project1.dll" from the File menu. Change the path to the DLL so that it is in a different location than the originally compiled version of project1.dll and click OK. The error occurs at this point.
Modification Type:MajorLast Reviewed:11/18/2003
Keywords:kbbug kbfix KB176164
©2003 Microsoft Corporation. All rights reserved.