First step: porting Matroska's JEBML (a Java parser for EBML) to C#. EBML is the "binary xml" format that is the basis for the Matroska (thus WebM) container.
I tried to convert libebml2 (written in C) to C#, but it is too "unobjectifiable" and so I searched a bit more for some easier path. Didn't look at some of the C++ parsers available, but JEBML although looking a bit abandoned of late seems to model the main concepts and surely is a good starting point. JEBML is LGPL-licensed which should not compromise the whole effort.
Going with renaming .java files to .cs, and doing wholesale Find&Replace, but have to stop now, while it doesn't even compile yet. Tomorrow I hope to fix it into a somewhat compilable state, and then, I'll need to move to .NET system classes, and use generics to slim down the whole thing.
The sources so far were uploaded into my Managed.Commons group, later I'll decide if add a project on Google Code, or GitHub.