dotNetRDF is a free and open source API for working with RDF and SPARQL in the .Net stack. Originally started by Rob Vesse back in January 2009, and handed over to the wider community in 2016 the project is still going strong and has a small core development team and a growing user community around the world.
It provides support for most common RDF data formats and incorporates a fully compliant SPARQL 1.1 query and update engine. It also provides intergration with a variety of popular commercial and open source triple stores making it more than capable of being heavy duty production grade applications.
If you use NuGet you can install the libraries directly by searching for dotNetRDF in the Package Manager.
Source Code may be obtained through the GitHub repository.