Exposing COM events to VBScript (ATL)

Question

I have built a COM server DLL in C++ with ATL by using the "ATL simple object" wizard. I followed Microsoft's ATLDLLCOMServer example. Everything works well except for one thing: I do not receive COM events in VBScript. I do receive the events in C#. I had events working in VBScript in an earlier MFC-based implementation as ActiveX control.

My control is defined like this:

class ATL_NO_VTABLE CSetACLCOMServer :
    public CComObjectRootEx<CComSingleThreadModel>,
    public CComCoClass<CSetACLCOMServer, &CLSID_SetACLCOMServer>,
    public IConnectionPointContainerImpl<CSetACLCOMServer>,
    public CProxy_ISetACLCOMServerEvents<CSetACLCOMServer>,
    public IDispatchImpl<ISetACLCOMServer, &IID_ISetACLCOMServer, &LIBID_SetACLCOMLibrary, /*wMajor =*/ 1, /*wMinor =*/ 0>,
    public IProvideClassInfo2Impl<&CLSID_SetACLCOMServer, &DIID__ISetACLCOMServerEvents, &LIBID_SetACLCOMLibrary>   /* Required for event support in VBS */
{
public:

BEGIN_COM_MAP(CSetACLCOMServer)
    COM_INTERFACE_ENTRY(ISetACLCOMServer)
    COM_INTERFACE_ENTRY(IDispatch)
    COM_INTERFACE_ENTRY(IConnectionPointContainer)
   COM_INTERFACE_ENTRY(IProvideClassInfo)
   COM_INTERFACE_ENTRY(IProvideClassInfo2)
END_COM_MAP()

BEGIN_CONNECTION_POINT_MAP(CSetACLCOMServer)
    CONNECTION_POINT_ENTRY(__uuidof(_ISetACLCOMServerEvents))
END_CONNECTION_POINT_MAP()

[...]

In VBScript I use the COM object like this:

set objSetACL = WScript.CreateObject("SetACL.SetACL", "SetACL_")

' Catch and print messages from the SetACL COM server which are passed (fired) as events.
' The name postfix of this function (MessageEvent) must be identical to the event name 
' as defined by SetACL.
' The prefix (SetACL_) can be set freely in the call to WScript.CreateObject
sub SetACL_MessageEvent (Message)
    WScript.Echo Message
end sub

The relevant parts of the IDL look like this:

[
    object,
    uuid(E1B57CA5-FD7F-4304-BC0A-8BEBDE231D53),
    dual,
    nonextensible,
    pointer_default(unique),
   helpstring("SetACL COM Interface")
]
interface ISetACLCOMServer : IDispatch
{
};

[
    uuid(00D4DCD3-02B9-4A71-AB61-2283504620C8),
    version(1.0),
   helpstring ("SetACL Type Library")
]
library SetACLCOMLibrary
{
    importlib("stdole2.tlb");

   [
        uuid(35F76182-7F52-4D6A-BD6E-1317345F98FB),
      helpstring ("SetACL Event Interface")
    ]
    dispinterface _ISetACLCOMServerEvents
    {
        properties:
        methods:
         [id(1), helpstring("Receives string messages that would appear on the screen in the command line version")]
         void MessageEvent([in] BSTR message);
   };

   [
        uuid(13379563-8F21-4579-8AC7-CBCD488735DB),
      helpstring ("SetACL COM Server"),
    ]
    coclass SetACLCOMServer
    {
        [default] interface ISetACLCOMServer;
        [default, source] dispinterface _ISetACLCOMServerEvents;
    };
};

The problem: SetACL_MessageEvent never gets called.

What I have tried:

• After reading this KB article I added the implementation of IProvideClassInfo2, but that did not help.
• This FAQ mentions that outgoing interface should not defined as a dual interfaces. I removed the "dual" from my interface definition, but that did not help.
• I found this SO question that seems to describe a similar problem. An answer was never given, only a workaround.

UPDATE 1:

I know now where it is failing, but not why: in the fire event method in _ISetACLCOMServerEvents_CP.h the call to Invoke fails with E_UNEXPECTED. This is the line:

hr = pConnection->Invoke(1, IID_NULL, LOCALE_USER_DEFAULT, DISPATCH_METHOD, &params, NULL, NULL, NULL);

Solution:

The problem was that I called the event firing function from a background thread in the COM object, in other words from the wrong apartment.

Answer 1

You can turn your VBScript function into IDispatch* pointers. For example the onreadystatechange handler in the Microsoft.XMLHTTP object.

Option Explicit
Dim xmlhttp
Function OnReadyStateChange
  WScript.Echo "ReadyState: " & xmlhttp.readyState
End Function
Set xmlhttp = CreateObject("Microsoft.XMLHTTP")
xmlhttp.onreadystatechange = GetRef("OnReadyStateChange")
xmlhttp.open "GET", "http://stackoverflow.com", True
xmlhttp.send
WScript.Echo "Waiting for 20 seconds to allow OnReadyStateChange events to finish."
WScript.Sleep 20000
WScript.Echo "Done!"

Outputs:

ReadyState: 1
Waiting for 20 seconds to allow OnReadyStateChange events to finish.
ReadyState: 2
ReadyState: 3
ReadyState: 4
Done!

Extracting the IDL for C:\Windows\System32\msxml3.dll shows we're setting an IDispatch* property:

[
  odl,
  uuid(ED8C108D-4349-11D2-91A4-00C04F7969E8),
  helpstring("IXMLHTTPRequest Interface"),
  dual,
  oleautomation
]
interface IXMLHTTPRequest : IDispatch {
    // ...
    [id(0x0000000e), propput, helpstring("Register a complete event handler")]
    HRESULT onreadystatechange([in] IDispatch* rhs);
};

When adapting this approach to your C++ application, your calling code will need to use IDispatch::Invoke with a DISPID of DISPID_VALUE, for example:

// IDispatch* pOnReadyStateChange <-- set from VBScript.
HRESULT hr = S_OK;
CComVariant vResult;
EXCEPINFO ei = { };
DISPPARAMS dispParams = { NULL, 0, 0, 0 };
hr = pOnReadyStateChange->Invoke(
    DISPID_VALUE,
    IID_NULL,
    0,
    DISPATCH_METHOD,
    &dispParams,
    &vResult,
    &ei,
    NULL);

Answer 2

Implementation of IProvideClassInfo2 should be sufficient (though I was under impression that it should work even without it). I created a minimalistic ATL project from template as a sample. This is ATL DLL + COM Class + a method and an event + IProvideClassInfo2:

Source code [SVN, Browser Friendly] + Win32 Binary.

Test .vbs is the following:

Function Test_Event(Text)
  WScript.Echo "Event: " + Text
End Function

Dim Test
Set Test = WScript.CreateObject("AlaxInfo.VbsEvents.Foo", "Test_")
Test.Method("Event Fun")

And the method itself is:

// IFoo
    STDMETHOD(Method)(BSTR sText) throw()
    {
        ATLTRACE(atlTraceCOM, 4, _T("sText \"%s\"\n"), CString(sText));
        ATLVERIFY(SUCCEEDED(Fire_Event(sText)));
        return S_OK;
    }

And having executed this, the output is:

I thought you might be having issues with events in your sample project if you are firing them from a background thread. You can also step your project with debugger and check if you have any errors in the Fire_ call.