for google cloud based OCR check below link:
https://developers.google.com/api-client-library/dotnet/apis/vision/v1
Also check this link, I donno if this is also cloud based or offline version.
GitHub - A9T9/Google-OCR-Vision-API-CSharp: Test app for the Google OCR features of the Google Vision API. Visual Studio C# project.[
OCR]
To build OCR you need to recognize each character its curves and its flow.
As my personal experience building an OCV system is easier than building an OCR.
So I suggest to use 3rd party DLL's.