A primary interop assembly is an assembly that is already prepared by the vendor of .theCOM component.

This makes it easier to use the COM component.

A primary interop assembly is a runtime-callable wrapper that might differ from an automatically generated RCW.

You can find primary interop assemblies in the directory <program files>\Microsoft .NET\ Primary Interop Assemblies.

Aprimary interop assembly already exists for the use of ADO from within .NET.

If you add a reference to the COM library Microsoft ActiveX Data Objects 2.7 Library, no wrapper class is created because a primary interop assembly already exists; the primary interop assembly is referenced instead.