Re: [comtypes-users] in,out parameters in comtypes 0.5 (trunk)

[Thomas H. <th...@ct...>]

MR Michael Robellard (5314) schrieb:
> Hello,
>  
> I am trying to implement the IURLSearchHook2 interface in comtypes and I
> am running into some issues.
>  

> which generate the following in the comtypes.gen .py files:
> class IURLSearchHook2(IURLSearchHook):
>     _case_insensitive_ = True
>     _iid_ = GUID('{5EE44DA4-6D32-46E3-86BC-07540DEDD0E0}')
>     _idlflags_ = []
> IURLSearchHook2._methods_ = [
>     COMMETHOD([], HRESULT, 'TranslateWithSearchContext',
>               ( ['in', 'out'], WSTRING, 'lpwszSearchURL' ),
>               ( ['in'], c_ulong, 'cchBufferSize' ),
>               ( ['in'], POINTER(ISearchContext), 'pSearchContext' )),
> ]

> I can get the data into the method and print it out fine.
> The problem is when I try and return a string out. It just plain hangs.
> If anyone has any thoughts. Is this even possible?
> I have tried both the "high level" and "low level" approach as discussed
> in Thomas' recent post

Mike, here are my thoughts on your problem.

The high level comtypes method implementation can only work for automation-compatible
argument types, and WSTRING is not one of those.

WSTRING is defined as c_wchar_p in the generated file.

c_wchar_p is interpreted as a zero-terminated wide character string,
but what COM really passes is a buffer with space for 'cchBufferSize' wide characters.
Which contains a zero-terminated string that you are supposed to change.

comtypes assumes c_wchar_p as 'const' strings which you cannot/should not change.

I think you should change the 'TranslateWithSearchContext' method definition
to something like this:


> IURLSearchHook2._methods_ = [
>     COMMETHOD([], HRESULT, 'TranslateWithSearchContext',
>               ( ['in', 'out'], POINTER(c_wchar), 'lpwszSearchURL' ),
>               ( ['in'], c_ulong, 'cchBufferSize' ),
>               ( ['in'], POINTER(ISearchContext), 'pSearchContext' )),
> ]

and write the (low-level) implementation like so:

    def TranslateWithSearchContext(self, this, lpwszSearchURL, cchBufferSize, pSearchContext):
        # lpwszSearchURL is a POINTER(c_wchar) instance.
        rawdata = lpwszSearchURL[:cchBufferSize]
        # rawdata is now a unicode string of length cchBufferSize, filled with NUL characters
        # at the end.  The [in] unicode string can probably be get by this:
        url = rawdata.split('\0')[0]

        # Maybe ctypes.wstring_at(lpwszSearchURL) would also work.

        # The [out] string must be written into the lpwszSearchURL buffer, maybe this works:
        lpwszSearchURL[:cchBufferSize] = '\0' * cchBufferSize # clear it
	urlout = "http://web.somewhere.whatdoIknow"
        lpwszSearchURL[:len(urlout)] = urlout
        return S_OK

Thomas