RESTful API Server in C#

If you are using .NET Framework 4.5 you can use Web API

If you are using .NET Framework 3.5 I highly recommend ServiceStack

If you are using .NET Core then you can use ASP.NET Core Web API

Nancy is a free REST framework for C#.

If you are working in .NET 4.0 or higher and looking for a pre-existing REST server solution that you can plug into (which it sounds like you are), you might want to check out Grapevine. You can get it using NuGet, and the project wiki has lots of sample code.

I am the project author, and I had a similar need as the one you described. Using resources I found here and elsewhere, I built Grapevine so that I would have a solution in my back pocket whenever I needed it again (DRY).