WinDiff-type library?

I've googled around a little, and I can't find a library to use with WinDiff-like functionality (comparing text files, determining added, removed, or changed lines). I found some C source code to an old version of WinDiff itself at grigsoft.com, but the web site's author is in competition with WinDiff and has posted old code for it. I'm particularly looking for a .NET or COM interface, but I can always wrap up whatever I can find. Is there a good open source project I'm missing? ~BenDilts( void );