You need to use OpenCV, or more specifically, a C# wrapper library for OpenCV.
I think you can implement some (very) basic face detection using OpenCVDotNet, although you should be aware that I don't think it's actively being developed. I just noticed I'm still on the list as a project committer, but I haven't done any work on it for a long time (if ever, can't remember why I got added in the first place).
You should probably go with Emgu CV, which is a much better C# wrapper for OpenCV. It is actively developed, and is compatible with OpenCV 2.0, and has a wider community surrounding it, and the documentation is good enough that you can easily get face detection working pretty quickly.