Q214483: Implements Statement Requires Interface Derived from IDispatch

Article: Q214483
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0; winnt:5.0,6.0
Operating System(s): 
Keyword(s): kbVBp kbVBp500 kbVBp600 kbVC kbVC500 kbVC600 kbGrpDSVB
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

If you create an interface, which is derived from an interface other than
IUnknown or IDispatch, this interface cannot be implemented using the Implements
statement in Visual Basic. Doing so causes the following compile error:

  Compiler error
  Bad interface for Implements: interface is derived from another pure interface
  with non-restricted methods.

CAUSE
=====

In the Visual Basic documentation for "Implements Statement", the following is
stated:

  "Visual Basic does not implement derived classes or interfaces."

RESOLUTION
==========

Change your interface definition of the class, deriving it from either IDispatch
or IUnknown.

STATUS
======

This behavior is by design.

MORE INFORMATION
================

Steps to Reproduce the Behavior
-------------------------------

Create an ATL COM project in Visual C++:

1. Create a new Visual C++ project named ATLTest specifying the project category
  to be ATL COM AppWizard.

2. In the ATL COM AppWizard dialog box, select "Dynamic Link Library" as the
  server type, and then click Finish.

3. From the Visual C++ Insert menu, select New Class.

4. In the New Class dialog box, type "CMyobj" as the class name, specify "Dual"
  as the interface type, and increase the "Number of interfaces" to 2. Two
  interfaces named IMyobj and IMyobjInt2 appear in the interface Names dialog
  box. Click OK.

5. Select the ClassView tab, expand the ATLTest classes node, right-click the
  "IMyobj" interface, and select Add Method. In the Add Method to Interface
  dialog box, type "Test" in the Method Name field and click OK.

6. Right-click the "IMyobjInt2" interface and select Add Method. In the Add
  Method to Interface dialog box, type "Bar" in the Method Name field and click
  the OK button.

7. Click the FileView tab, expand the ATLTest files node, and then expand the
  Source Files node. Open the ATLTest.idl file and locate the interface
  definition for IMyobjInt2. Delete the existing interface and replace it with
  the following:

     interface IMyobjInt2 : IMyobj
        {
           [id(2), helpstring("method Bar")] HRESULT Bar();
        };

8. Expand the Header Files node and open the Myobj.h file. Comment out the
  following line from the "CMyobj" class definition:

     public IDispatchImpl<IMyobj, &IID_IMyobj, &LIBID_ATLTESTLib>,

9. From the Build menu, choose Rebuild All to create the DLL.

Create a Visual Basic Client Program:

1. Create a Visual Basic Standard EXE project. Form1 is created by default.

2. From the Project menu, choose References, and add a reference to ATLTest.

3. From the Project menu, choose Add Class Module to add Class1 to the project.

4. Paste the following code in the General Declarations section of Class1:

     Implements IMyobjInt2

5. Run the project. The following error occurs:

  Compiler error
  Bad interface for Implements: interface is derived from another pure interface
  with non-restricted methods.

Steps to Resolve the Problem:

1. Close the Visual Basic client program.

2. Open the ATLtest project in Visual C++.

3. In the FileView of the project, open the ATLTest.idl file, find the interface
  "IMyobjInt2" definition, and replace the reference to IMyobj with IDispatch.

4. Open the Myobj.h file and remove the comment you added to the "CMyobj" class
  definition:

5. From the Build menu, choose Build All.

6. Go back to the Visual Basic client program. If you attempt to run the project
  at this point you will receive the following error message:

  Compiler error
  Object module needs to implement 'Bar' for interface 'IMyobjInt2'

7. Drop down the left hand combo box of Class1's Code window and select
  IMyobjInt2 to implement the Bar method. Then the sample no longer generates
  an error.

Additional query words:

======================================================================
Keywords          : kbVBp kbVBp500 kbVBp600 kbVC kbVC500 kbVC600 kbGrpDSVB 
Technology        : kbVCsearch kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600 kbVC500 kbVC600 kbVC32bitSearch kbVC500Search
Version           : WINDOWS:5.0,6.0; winnt:5.0,6.0
Issue type        : kbprb

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