I believe you are looking for the "Common SpellerApplication Programming Interface". Here is the official link about it: The Microsoft Common Speller API (CSAPI) for Office spell checking

As you see, it's not really public, but the actual specification can be downloaded from here: http://spell.com.ru/dicts/csapi.pdf

If you're looking for a sample, google for "Interfacing an Irish Spellchecker to microsoft word 2000".

It's not a .NET interface, it's a DLL interface, sou you'll have to use some C/C++ or other language capable of exposing DLL exports somewhere in your program to be able to build a proxy to access .NET.