I was automating a scenario with a WSH script the other day that required interaction with a web server. So naturally I figured I’d make use of the WinHttpRequest automation object which wraps the WinHTTP API.
Those familiar with WSH may share my great distaste for the fact that when it functions as an automation controller, the developer is expected to hard-code enumeration constants and the like (as “vars” in JScript or “Consts” in VBScript). I first encountered this ridiculous limitation when a friend showed me how he translated C# code that automated Microsoft Word to VBScript, and had to look up the various constants by hand, with Visual Basic 6’s Object Browser, which functions as a convenient type library viewer. By default, the script engine only uses the automation object’s IDispatch interface, leaving the chore of constant resolution to the caller.
So I was relieved when I found out that .wsf files, which are XML files that wrap scripts executed by WSH, support referencing a type library for the purpose of making available the constants used with a controlled automation object. I was a little disappointed to find out about their not so ideal performance characteristics, but that was not problematic in my case.
I figured I’d introduce a reference of the following form to the script:
Not all was well, however. After introducing the change above, I noticed my script had stopped working on one of the systems. Invocation of the Windows Scripting Host failed, with WSH claiming to be unable to resolve the reference to the specified ProgID.
After looking into it I figured out the problem with that specific system was that it was running Windows Server 2003 rather than Windows XP. It seemed strange the newer Windows Server 2003 would have a regression like that. I continued investigating.
The first clue was that winhttp.dll, the DLL implementing the WinHTTP API, was MIA from Windows Server 2003’s system32 directory. Surely the API was not missing from the OS, MSDN clearly documents its presence. It was indeed there, albeit in a modified form: a native side by side assembly.
OK, so winhttp.dll is there, in an oddly named subdirectory somewhere in the winsxs store instead of system32. Still, I recalled from my previous interaction with SxS that side by side assemblies could expose COM objects to their clients. Examination of the manifest file for WinHTTP in Windows Server 2003’s winsxs store revealed that it was indeed doing so.
Microsoft documents that users of the flat WinHTTP C API under Windows Server 2003 should add winhttp.dll as a dependent assembly to the activation context of the client application, but this approach seemed inappropriate to me in the context of the WinHttpRequest automation object, since clients activate it by ProgID or GUID and do not load winhttp.dll directly. Them being made aware of this relationship would be a serious breach of COM’s encapsulation.
I proceeded to write a test application in C++. It initialized COM and proceeded to call CLSIDFromProgID to translate “WinHttp.WinHttpRequest.5.1″ to a GUID. Given success of this translation, it would call CoCreateInstance on returned GUID and if that worked out, QueryInterface for IDispatch and for IWinHttpRequest (defined in the Windows SDK’s httprequest.idl).
To my, I must admit, great surprise, the test application worked. The first surprising thing was that CLSIDFromProgID returned successfully, even though I specified a ProgID exposed by a SxS assembly. The ProgID was clearly absent from the HKEY_CLASSES_ROOT registry key in Windows Server 2003, in contrast to its presence there in Windows XP. Only if ole32.dll, the COM runtime, had specific knowledge of SxS and ability to perform a lookup in the winsxs store, would such a request be serviced successfully, I figured. However, no mention of this functionality could be found directly in CLSIDFromProgID’s documentation.
I was even more surprised that the CLSID returned by CLSIDFromProgID as the result of the lookup was NOT the CLSID of winhttp.dll! I couldn’t find the returned CLSID in the registry. However, when I promptly invoked CoCreateInstance, not only the activation request succeeded, I actually saw a Module Load event for winhttp.dll from the winsxs store in the debugger. I assume that the returned CLSID is part of some COM SxS integration magic.
OK, so my poor man’s automation controller implemented in C++ could obviously activate the WinHttpRequest object even in Windows Server 2003 with no knowledge of its new SxS semantics. It seemed odd that my script would fail to do same, since I assumed similar mechanics were behind its resolution process for locating the type library.
The next thing I did was to try and run my script on Windows Vista. I figured the change to WinHTTP making it a SxS assembly introduced in Windows Server 2003 was incorporated into Microsoft’s latest OS, as well. Continuing the previous chain of surprises, the script suddenly worked.
The first difference between Windows Server 2003 and Windows Vista that I observed was that Windows Script Host was updated to version 5.7 in the new OS. My first theory was that the new WSH had corrected whatever implementation issue that prevented WSH 5.6 from locating SxS type libraries.
I looked it up and found out that only days earlier Microsoft had actually made a release of the new Windows Script Host 5.7 to down-level platforms. Untypically for Microsoft nowadays, they even made a release for Windows 2000. So now I had a chance to test my theory. I installed WSH 5.7 on the Windows Server 2003 system and reran the script. In yet another surprise, it didn’t work, the type library reference giving the same error as before. It seems my instincts are really off about all of this.
So there must be a different reason for the different behavior of Windows Server 2003 and Windows Vista. After examining the Vista system, it appeared the whole thing was a lot simpler than I had originally thought. Windows Vista was a strange hybrid of the Windows XP and Windows Server 2003 behaviors, with winhttp.dll being present both as a SxS assembly in its winsxs store and as a regular DLL in system32. Indeed, examination of HKEY_CLASSES_ROOT in the Vista registry resulted in the discovery of plain old ProgID registration for the non-SxS winhttp.dll. This is most likely the reason that the type library lookup succeeds in the Windows Vista system.
With these details at hand, I was finally able to find a discussion of this issue in a newsgroup. In that newsgroup thread, Microsoft’s Biao Wang acknowledges WSH’s lack of support for SxS type library references. The thread being an old one, the possibility of a fix being introduced in Windows Server 2003 Service Pack 1 was mentioned. However, considering the issue presented itself on the Windows Server 2003 system that had Service Pack 2 installed and that the latest WSH 5.7 still doesn’t support this down-level, it appears that the issue ended up remaining unresolved, for whatever consideration Microsoft had made on the matter.
The thread does mention a satisfactory workaround: reference the SxS type library by GUID and version instead of by object ProgID and it seems to work. I tried referencing the type library by GUID when the ProgID approach didn’t work on Windows Server 2003 originally, but that reference didn’t work either since I left out the “version” directive. Another happy ending.
Lovers of type library constant imports, rejoice!