KB194368 - FIX: Operators Do Not Recursively Call Object Default Properties

Article ID: 194368

Article Last Modified on 5/13/2003

Microsoft Visual Basic 6.0 Learning Edition
Microsoft Visual Basic 6.0 Professional Edition
Microsoft Visual Basic 6.0 Enterprise Edition

This article was previously published under Q194368

SYMPTOMS

In Microsoft Visual Basic 5.0, the following code calls the default property of obj:

Dim obj As Object
If obj = 5 Then

If the Object's default property returns an object, it recursively calls the default property of that object until it finally finds a non-object property.

Prior to Visual Basic 6.0 Service Pack 3 (SP3), Visual Basic 6.0 only invoked the default property of the second level, at most. If the default property is also an object, Visual Basic 6.0 generates the following error message:

Type Mismatch

CAUSE

The way the comparison operators (=, >, <, etc.) evaluate objects was changed in Visual Basic 6.0. However, the behavior was changed back in Visual Basic 6.0 SP3.

NOTE: Visual Basic 5.0 behavior for this case is undocumented and programmers should not rely on this feature.

RESOLUTION

Install Visual Basic 6.0 SP3 or later.

-or-

Use the following workaround:

Add the following function to your project:

      Public Function EvaluateObjToLong(obj As Object) As Long
            EvaluateObjToLong = obj
         End Function

Replace the following line:

      if obj = 5 then

with:

      if EvaluateObjToLong(obj) = 5 then

NOTE: This is only a workaround for migrating from Visual Basic 5.0 to Visual Basic 6.0. Programmers should not rely on this feature. The recursive evaluation of the default property of the assignment operator may change in future versions.

The use of default properties, especially non-parameterized default properties, is discouraged for the following reasons:

The code is harder to read because the person who reads your code must know exactly which property is the default to understand what you are really doing.
The code might not work the way you think it should and the behavior might change between different versions of Visual Basic.
If you forget to write the property name when you use an object that has a default property, Visual Basic considers that you are using the default property and you will not get an error message when you compile the code. However, if your object does not support a non-parameterized default property, the compiler generates an error message.

STATUS

This bug was corrected in Visual Studio 6.0 Service Pack 3.

For more information about Visual Studio 6.0 Service Packs, please see the following articles in the Microsoft Knowledge Base:

194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

NOTE: This behavior change is by design. The reasons for this are:

This change makes the behavior of late bound objects more closely match that of early bound objects.
The behavior of Visual Basic 5.0 permits the possibility of an infinite loop, which was determined to be much worse than this restriction.

MORE INFORMATION

Steps to Reproduce Behavior

Create a new ActiveX DLL project in Visual Basic 5. Class1 is created by default.
From the Project menu, add two additional classes named Class2 and Class3 to the project.

Add following code to Class1:

      Public Property Get DefProp() As Object
         Set DefProp = New Class2
      End Property

From the Tools menu, select Procedure Attributes. In the Procedure Attributes dialog box, click the Advanced >> command button, and then change the Procedure ID to "<Default>."

Add the following code to Class2:

      Public Property Get DefProp() As Object
         Set DefProp = New Class3
      End Property

Set the DefProp of Class2 to be the default property, as in step 4.

Add the following code to Class3:

      Public Property Get DefProp() As Long
         DefProp = 5
      End Property

Set the DefProp of Class3 to be the default property, as in step 4.
From the File menu, add a Standard EXE project to the project group as Project2. Form1 is created by default. Set Project2 as the start up project.

Add the following code to the General Declarations section of Form1:

      Public Function EvaluateObjToLong(obj As Object) As Long
            EvaluateObjToLong = obj
         End Function

         Private Sub Form_Load()
            Dim obj As Object
            Set obj = CreateObject("project1.Class1")
            If obj = 5 Then MsgBox "work"
         End Sub

Press the F5 key to run the project, and note that a message box displays the word "work."
Stop the project and save the project group.
Open the project group in Visual Basic 6.0 (without Service Pack 3) and run the project. The following error message displays:
Run Time Error '13':
Type Mismatch

Change the line:

      If obj = 5 Then MsgBox "work"

to:

       If EvaluateObjToLong(obj) = 5 Then MsgBox "work"

Run the project again, and note that the Message box displays "work."

Additional query words: kbDSupport kbDSD kbVBp kbVBp600 kbActiveX kbVBp500

Keywords: kbbug kbfix kbvs600sp3fix KB194368