You should probably widen your search into C++ or Java libraries as well as developers want to create code that can easily be adapted to different platforms.
C# is pretty limited to Windows. (I know there is a Linux version, but I don't it runs on Android yet)
With Platform Invoke you can fairly easy create a c# wrapper for accessing C++ DLLs.
A quick search using the phrase "OCR library open source" gives quite a few results including:
http://www.findbestopensource.com/tagged/ocr[
^]
Then it is up to you to find the one that is best for your needs.
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