Probably, the most popular REST API client library for .NET
JSON, XML and custom serialization and deserialization
Sync and Async
Variety of overloads to make synchronous and asynchronous HTTP calls
Almost every aspect of an HTTP call can be customized
Basic, OAuth 1, OAuth 2, JWT, NTLM are supported. Not enough? Write your own!
Forms, request body, and files
Send objects as the request body in JSON or XML, or as a form. Upload and download files as bytes or as streams.
Add query, URL segment, body, form or header parameter using an easy and fluent API
RestSharp is probably the most popular HTTP client library for .NET. Featuring automatic serialization and deserialization, request and response type detection, variety of authentications and other useful features, it is being used by hundreds of thousands of projects.
RestSharp passed over 32 million downloads on NuGet, with average daily download count of 10,000. It's being used by many popular OSS projects, including Roslyn and Swagger.