In my introduction post on the Open XML SDK I mentioned that the SDK is built on top of System.IO.Packaging. System.IO.Packaging is a set of APIs that are part of .NET 3.0, which allow developers to create and manipulate documents based on the Open Package Convention (OPC). Given that Open XML Formats are based on OPC, the SDK uses System.IO.Packaging APIs to open, edit, create, and save Open XML packages.
Some of you have left comments on this blog and on the Open XML SDK forum asking about native code APIs to manipulate Open XML files. I have some good news for you guys. Included in Windows 7 will be all new native code Win32 Packaging APIs. For more information on this new API, check out the OPC team blog. In particular check out the post comparing the managed version of the API with the native code version.