We ship a product as a merge module. The msm installs a network driver via a custom action. If the driver is already installed then the CA must not be run -- it kills networking to install it twice, and unfortunately the CA itself can't check. So I'm using a condition on the CA in the InstallExecuteSequence.



The condition uses AppSearch to check the registry reference count of the driver file, in case it has already been installed by another application, so the condition looks like this:

$C_filt.sys.(guid) = 3 AND FILT_REFCOUNT.(guid) = "blank"

(The FILT_REFCOUNT property has a default value "blank".)



A customer has hit a bug in my merge module during upgrade. With RemoveExistingProducts between InstallValidate and InstallInitialize the driver is uninstalled during RemoveExistingProducts, but is not reinstalled because the reference count was nonzero when AppSeach ran. (The problem does not occur when RemoveExistingProducts is after InstallFinalize, of course.)



In an MSI I could extend the condition to check an ActionProperty set in the Upgrade table, e.g.:

$C_filt.sys.(guid) = 3 AND (UPGRADE OR FILT_REFCOUNT.(guid) = "blank")

This is not ideal; it will allow the upgrade to succeed only if no other application has installed the driver, but unless someone has a clever idea it will do for now.)



However, the extra condition is not possible in a merge module -- the ActionProperty name isn't known to the merge module. And I'd prefer not to have to ask end users to edit the merge module conditions (there are twentyfour CAs, for different OSes etc., making it easy to break) So is there any other way to detect that an upgrade is occurring? Or any other way to change the conditions on the custom actions to add/remove the driver?