How To Avoid GPF When Using a VB6 DLL with a VB5 UserControl

MORE INFORMATION

A PropertyBag object is passed into an object through the ReadProperties and WriteProperties events in order to save and restore the state of the object. The problem described in this article occurs when a Visual Basic 5.0 PropertyBag interface is passed into a Visual Basic 6.0 server, which then attempts to de-reference a non-existent VTable entry for the Contents property.

This occurs because the Contents property of the PropertyBag interface in version 6.0 did not exist in version 5.0.

To avoid binary incompatibility when inter-mixing Visual Basic 5.0 and Visual Basic 6.0 components, use the PropertyBag_VB5 interface in Visual Basic 6.0 projects and do not use the Contents property.

Steps to Reproduce Behavior

1. Create a new Visual Basic 6.0 ActiveX DLL project. A project named Project1 and a class module named Class1 are created by default. From the Properties Page, change the project Name property to PropHandler2.
2. From the Class1(Code) window, enter the following code:

         Public Sub Test(obj As PropertyBag)
            Dim vnt As Variant
            vnt = obj.Contents
            obj.Contents = vnt
         End Sub
   
   						

3. From the File menu, choose Make PropHandler2.dll to build PropHandler2.dll. In the Make Project dialog, make a note of the DLL file name and directory where the DLL is saved, then choose OK.
4. Create a new Visual Basic 5.0 ActiveX Control project. UserControl1 is created by default.
5. The reference to the DLL built in step 3 now needs to be added to the ActiveX Control project. From the Project menu, choose References, and then add the reference to PropHandler2.dll. To do this, scroll down the list of available references to find PropHandler2 and check its checkbox. Then choose OK.
   
   NOTE: If PropHandler2 is not found, click on Browse and go to the directory in which PropHandler2.dll was built. Enter PropHandler2.dll as the File name, then choose Open.
6. From the UserControl1(Code) window, enter the following code:

         Private Sub UserControl_WriteProperties(PropBag As PropertyBag)
            Dim obj As New Class1
            obj.Test PropBag
         End Sub
   
   						

7. Close the control's design window. To do this, from the Project window, double-click UserControl1 to bring its designer to the front, then press CTRL+F4 to close the window. Closing the designer's window puts the UserControl1 control in run mode. As soon as the control is in run mode, its icon (the default toolbox icon for a user control) is enabled in the Toolbox.
8. From the File menu, choose Add Project. Add a new Standard EXE project named Project2. Form1 is created by default.
9. From the Toolbox, place the UserControl on Form1.
10. From the File menu, choose Save Project2. At the Save File As dialog, choose Save. Note that the above compile error will be received.