[python-win32] How to write a COM Server implementing interfaces from type lib?

Thu Mar 22 13:51:06 CET 2012

Hi Mark,

thanks for your reply.

> That's E_FAIL which is pretty generic.  Doesn't sound like a simple 
> failure to QI for the correct interface.

Yeah. The client says something like "Unknown Error" which makes it hard to tell what the problem actually is.

> win32com should be able to do this given there is a tlb - see the 
> "pippo" samples.  Further, using the debug facilities and 
> win32traceutil, you should be able to see the creation of the object, 
> the QIs on the object and any methods actually called.  But as 
> mentioned, I doubt it is a QI failure.

Yeah, there is a type library (OPC DA). Actually there are even two type libraries, each of them contains two Interface which my server needs to implement.

I've had a look into policy.py and found that:

  def _build_typeinfos_(self):
    # Can only ever be one for now.
    (...)

which means using _typelib_guid_ allows only one type lib, right? I could try to patch your code or do you think there is a general problem that would make it impossible having more than one typelib in your framework?

> Have you tried contacting 
> the author of the object?

I am the author... I only use the 3rd party typelibs. I am writing the server that must support the interfaces so that other 3rd party clients can access the server component. 

I can reproduce the error using python the COMView tool trying to instanciate the Server. The problem is, that I don't see why. I've enabled debugging mode when registering the server. When using one of the 3rd party clients, rhe python trace collector shows the following:

Object with win32trace dispatcher created (object=None)
Entering constructor
in <PyOPCComServer.EttexOPCServer instance at 0x0213B058>._QueryInterface_ with unsupported IID {00000003-0000-0000-C000-000000000046} ({00000003-0000-0000-C000-000000000046})
in <PyOPCComServer.EttexOPCServer instance at 0x0213B058>._QueryInterface_ with unsupported IID {0000001B-0000-0000-C000-000000000046} ({0000001B-0000-0000-C000-000000000046})
in <PyOPCComServer.EttexOPCServer instance at 0x0213B058>._QueryInterface_ with unsupported IID {00000018-0000-0000-C000-000000000046} ({00000018-0000-0000-C000-000000000046})
in <PyOPCComServer.EttexOPCServer instance at 0x0213B058>._QueryInterface_ with unsupported IID {4C1E39E1-E3E3-4296-AA86-EC938D896E92} ({4C1E39E1-E3E3-4296-AA86-EC938D896E92})
in <PyOPCComServer.EttexOPCServer instance at 0x0213B058>._InvokeEx_-AddConnection(1, 0) [1,0,None]
Connection added. Active connections: 1
in <PyOPCComServer.EttexOPCServer instance at 0x0213B058>._InvokeEx_-ReleaseConnection(1, 0, 1) [1,0,None]
Connection released. Active connections: 0

The QI calls are standard COM calls, not application specific (AFAIK). You can see the "Entering constructor" message which is in my python constructor of the server. Just for fun, I implemented the IExternalConnection interface to see if the methods are called. "Connection added. ..." is my output. But I can't see an further requests that shows what the problem is. Are there any calls in pythoncom that could fail and does not give debug output?

My pythoncom server starts like that:

class EttexOPCServer:
    _reg_progid_ = "Ettex.OPC.Automation"
    _reg_desc_ = "ettex OPC DA Server"
    _reg_clsid_ = "{13B51E8D-4BC2-4DED-8D4E-4614692F88E6}"
    _reg_catids_ = [ '{63D5F432-CFE4-11D1-B2C8-0060083BA1FB}' ]
    _com_interfaces_ = [
        '{00000019-0000-0000-C000-000000000046}', # IExternalConnection (Is it really mandatory?)
        '{39C13A4D-011E-11D0-9675-0020AFD8ADB3}', # IOPCServer
        '{F31DFDE2-07B6-11D2-B2D8-0060083BA1FB}', # IOPCCommon
        '{39C13A72-011E-11D0-9675-0020AFD8ADB3}', # IOPCItemProperties
        '{B196B284-BAB4-101A-B69C-00AA00341D07}'  # IConnectionPointContainer
    ]
    _typelib_guid_ = "{3B540B51-0378-4551-ADCC-EA9B104302BF}"
    _typelib_version_ = (3, 0)
    _reg_clsctx_ = pythoncom.CLSCTX_LOCAL_SERVER

    _public_methods_ =  [ 'Connect', 'Disconnect', 'GetErrorString' ]
    ## IExternalConnection methods
    _public_methods_ += [ 'AddConnection', 'ReleaseConnection' ]

    #################################################################
    ## COM Class Attributes
    #################################################################
    _public_attrs_ = [ 'ClientName', 'OPCGroups' ]

    #################################################################
    ## COM Class Read-Only Attributes
    #################################################################
    _readonly_attrs_ = [ 'OPCGroups' ]

I don't have all interface methods implemented at the time but I guess that doesn't matter as long as not all type libs have been loaded, does it?

If its not possible with pythoncom, is it possible with comtypes? I've tried that as well with nearly the same result ("Unknown error"):

# OPC Data Access 3.00 Type Library
opc_da_tl = comtypes.GUID("{3B540B51-0378-4551-ADCC-EA9B104302BF}")
# OPC Common 1.10 Type Library
opc_com_tl = comtypes.GUID("{B28EEDB1-AC6F-11D1-84D5-00608CB8A7E9}")

GetModule((opc_da_tl, 3, 0))
GetModule((opc_com_tl, 1, 0))

import comtypes.gen.OPCDA as OpcDa
import comtypes.gen.OPCCOMN as OpcCommon

class EttexOPCServer2(OpcCommon.IOPCCommon, OpcCommon.IConnectionPointContainer, OpcDa.IOPCServer, OpcDa.IOPCItemProperties):
    _reg_progid_ = "Ettex.OPC.Automation2"
    _reg_desc_ = "ettex OPC DA Server 2"
    _reg_novers_progid_ = _reg_progid_
    _reg_clsid_ = "{80A2B8F7-792E-43F4-95F8-CD6BB4B413AD}"
    _reg_catids_ = [ '{63D5F432-CFE4-11D1-B2C8-0060083BA1FB}' ]
    _reg_clsctx_ = comtypes.CLSCTX_LOCAL_SERVER
    _regcls_ = comtypes.server.localserver.REGCLS_MULTIPLEUSE

    _com_interfaces_ = [OpcCommon.IOPCCommon, OpcCommon.IConnectionPointContainer, OpcDa.IOPCServer, OpcDa.IOPCItemProperties]

I don't know what to do. Is there anything more I can do to debug to find out WHAT exactly causes the error? Because as you can see, the pythoncom debug output doesn't show this error that is returned by the client.

Thanks a lot!

//Jan

----- Originalnachricht -----
Von: "Mark Hammond" <skippy.hammond at gmail.com>
Gesendet: Don, 3/22/2012 12:29pm
An: "Jan Wedel" <Jan.Wedel at ettex.de>
Cc: python-win32 at python.org
Betreff: Re: [python-win32] How to write a COM Server implementing interfaces from type lib?

On 22/03/2012 2:53 AM, Jan Wedel wrote:
> Hi,
>
> I'm currently having trouble to write a COM-Server that has some special
> requirements:
> - It needs to derive from IUnknown
> - It needs to implement multiple interface from two different
> proprietary typelibs (dlls)
> - It needs to implement a custom category
>
> At first I started with pythoncom. I used the attribute _reg_catids_ to
> specify the category and _com_interfaces_ to specify the interfaces I
> want to implement. The client (proprietary 3rd party sw, no source) sees
> the server but throws some 0x80004005 error on CoCreateInstance.

That's E_FAIL which is pretty generic.  Doesn't sound like a simple 
failure to QI for the correct interface.

win32com should be able to do this given there is a tlb - see the 
"pippo" samples.  Further, using the debug facilities and 
win32traceutil, you should be able to see the creation of the object, 
the QIs on the object and any methods actually called.  But as 
mentioned, I doubt it is a QI failure.

> I was hoping that I can just tell the COM dispatcher, "yes, I have these
> interface implemented" and implement the methods without really having
> the interface classes available.
>
> I read, that pythoncom can only create components that use IDispatch so
> I guess the _com_interfaces_ idea won't work, will it?

As above, you should be able to fully implement them so long as makepy 
has been run.

> Then I did some further research and found comtypes. I tried to write a
> server stub again. I used GetModule to load the type library containing
> the Interfaces, importing the generated interface classes and let the
> main server class extend these interfaces.
>
> The first problem was, that comtypes did not support the _reg_catids_
> attribute or anything similar so I had to add the Implemented Categories
> key manually to the registry.
> Then, I was able to see the server through the client, which obviously
> filters by categories, but it still shows the same error as before.
>
> So, what is the correct/best way to implement a server that needs to
> implement custom interfaces and categories? Or is it possible at all
> using python?

The fact you get the same error there implies something else is going 
wrong, but it is impossible to guess what.  Have you tried contacting 
the author of the object?

Mark

>
> Thanks!
>
> //Jan
>
>
>
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