What are shims?
All Office applications are unmanaged while all Add-in Express .NET based add-ins are managed class libraries. Therefore, there must be some software located between Office applications and your add-ins. Otherwise, Office applications will not be able to learn about your .NET add-ins and other Office extensions. That software is called a shim. Shims are unmanaged DLLs that isolate your add-ins in a separate application domain. When you install your add-in, the registry settings for the add-in will point to the shim. And the shim will be the first DLL examined by the host application when it runs your add-in or smart tags.
