Two particularly useful Shims

As promised in my BriForum 2014 session, I will start a small series on Application fixes using shims and symbolic links. The first installment is on shims:

Shims can be really useful in just about any environment and I wish there were some better resources explaining how to use them. This is why I wrote this article explaining my two favorite shims.

As Microsoft releases new versions of their OS, folder structures and other key areas are bound to change. Or some applications may store personal user settings in a shared location or has a registry key that is best lived in HKCU instead of HKLM. This is where shimming or “Compatibility Fixes” come to the rescue.
A shim is installed at the system level and will essentially intercept an application call and change the default behavior of the application. The two main flavors of shims that I find extremely useful are CorrectFilePaths and VirtualRegistry. There are several hundred types of shims, but these two are the ones I find myself using a lot.

This shim is particularly useful when an application stores a file or folder in a directory that you want stored somewhere else. For example, say you install an application on an RDS server that stores a configuration file in ProgramData. This is not a good location if each user writes settings to it and get overwritten by other users. The CorrectFilePaths shim allows you to redirect the file call from the original path (e.g. C:\ProgramData\ABC Inc\UserConfig.ini) and replace it with a path that is user specific (e.g. %appdata%\ABC Inc\UserConfig.ini).

During shim creation you specify what process will be intercepted and redirected. This way when a user launches the process you specified and calls the file, it will be redirected to the new location. Now every user will have their own configuration file instead that doesn’t conflict with other users.

Much like CorrectFilePaths, the VirtualRegistry shim allows you to redirect a registry call to a different location. An example where this would be useful is when an application stores a database server value in HKLM. This value may tell the application what server to establish a connection to. Now, say you have two groups of users – Group A & Group B. Group A connects to database server 1 and Group B connects to database server 2. Normally you would build a separate RDS server for each group to accommodate the multiple database values, but what if you wanted to have both user groups use the same server? How would you be able to accommodate that?

Create a new VirtualRegistry shim! You can redirect the HKLM values to HKCU and set those values with a logon script. This way Group A gets its registry key for database server 1 and Group B gets theirs for database server 2.

Shims sound great and they really are, but they can be a pain in the rear to get configured. I’ll post an article on both shims with examples in a few days.

If you want to get started Shimming, download the Application Compatibility toolkit from Microsoft here: