Wednesday 23 September 2009

The Perils of Intellisense

Work on my WMI library has been a little erratic during my recent time off, but I came across a bug recently which I lay squarely at the door of Intellisense. Of course, it's really a case of user error, but Intellisense lulls you into a false sense of security...

I had just started the library and added the initial unit test for creating a connection. I decided that I could get away with talking to the WMI service on the local machine as it's pretty much a standard piece of Windows technology and the unit tests would still run quickly. I started the implementation in the Connection class by adding the some private typedefs and members:-

typedef WCL::ComPtr IWbemLocatorPtr;
typedef WCL::ComPtr IWbemServicesPtr;

IWbemLocatorPtr m_locator;
IWbemServicesPtr m_services;

As one would expect, Intellisense did it's thing during the typing of the typedefs by showing me a list of types, and after typing just "IWbem", I could see the ones I wanted listed and so went with them.

In the implementation file I proceeded to write the Connection::open() method by declaring local variables for the underlying Locator and Connection using the typedefs declared earlier. The WMI Locator is the root object and is a singleton so requires no arguments during or after its construction. Once again Intellisense shows its productivity enhancing ability by showing me a list of relevant CLSID's after I type the initial portion, which I know to be "CLSID_Wbem". Seeing a CLSID that has 'Wbem' and 'Locator' in it's name I dutifully click it and get on with the hard part of actually writing the method logic - after all I "Lean on the Compiler" as it's much better at spotting errors than me...

IWbemServicesPtr services;
IWbemLocatorPtr locator(CLSID_WbemAdministrativeLocator);
WCL::ComStr path(host + nmspace);

HRESULT result = locator->ConnectServer(path.Get(), nullptr, nullptr, nullptr, 0, nullptr, nullptr, AttachTo(services));

if (FAILED(result))
throw Exception(result, locator, TXT("Failed to connect to the local WMI provider"));

m_locator = locator;
m_services = services;

This works a treat - my unit test passes. I can open and close a connection, so without delay I get on with writing the next set of tests and code to perform a simple WMI query.

After a few distractions, such as going on holiday, I decide to start implementing my WMICmd tool which is a simple command line tool for executing WMI queries. It will also do very nicely as a vehicle for thoroughly testing the WMI library. I get the shell of the application up and running and add support for running a query. The first query is the same as the unit test, and it works. I do some work on the output format and try a few other simple queries for good measure. It's working a treat and so I move onto more the more useful features like being able to query a remote computer. I add the command line support for providing a remote host and give it a whirl...

It fails. The error is "Invalid Parameter". I read the MSDN help and surmise that maybe you can't use 'localhost' as it suggests you use '.' for local connections. I need to allow a separate login and password to be provided anyway so I skip straight onto the full solution by refactoring the Connection::open() code to allow a username and password to be provided. I don't have any unit tests for this (for obvious reasons) but the other tests pass, so I know I haven't broken anything. It sill fails. Huh? "Invalid Parameter" again...

I guess that I've forgotten something COM security related, perhaps I need to call CoInitialiseSecurity() - but I'm sure I don't. I know it can't be the call to CoSetProxyBlanket() as that comes after. I pull Keith Brown's "Programming Windows Security" off the bookshelf in search of enlightenment. Nothing obvious. I try a few 'random tweaks' in the hope of getting a different error. Still nothing. I go over it in my head again and again - "Invalid Parameter" means I must have got one of the arguments to the ConnectServer() method wrong. So I carefully read the documentation a number of times and this raises a few questions about my assumptions in my implementation. But none of them are the cause.

I stare at the code for ages trying to work out what to do next. I compare it line for line with the example code in the WMI SDK documentation. Only I don't. I've been skipping the 'trivial' initialisation code before the call to ConnectServer(). Finally I decide to double-check the CLSID for the Locator COM object and I spot a difference... It's not CLSID_WbemAdministrativeLocator in the example, it's CLSID_WbemLocator! I make the relevant code change...

IWbemServicesPtr services;
IWbemLocatorPtr locator(CLSID_WbemLocator);

I run the unit tests. Good, they still pass. I run my WMICmd tool, and bingo, it now works. I go back and try 'localhost' to prove to myself that I was obviously wrong with my assumption about having to use "." for a local query, and of course it also works.

So, by accident, I've been instantiating the wrong COM object, and that object just happens to implement the interface I need - IWbemLocator! As Harry Hill would say "What are the chances of that happening?". I'm blaming Intellisense for the hair that I've torn out trying to fix this issue, but that's not exactly fair. I could have cut-and-pasted the code from the WMI documentation, and we all know how fallible documentation is. I'm sure the fact that I've implemented something like this before for a client some years ago lead me to become complacent. And the unit tests, which constantly passed, deflected me away from the existing code and instead lead me to believe I was missing something else. Useful though unit testing is, I need to remind myself that they are not a panacea.

I've not forgiven the Intellisense window yet, but at least we're on speaking terms again...

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