Json.NET is a popular high-performance JSON framework for .NET

Online Manual: http://james.newtonking.com/projects/json/help/index.html?topic=html/T_Newtonsoft_Json_Linq_JToken.htm

Features
  • Flexible JSON serializer for converting between .NET objects and JSON
  • LINQ to JSON for manually reading and writing JSON
  • High performance, faster than .NET's built-in JSON serializers
  • Write indented, easy to read JSON
  • Convert JSON to and from XML
  • Supports .NET 2, .NET 3.5, .NET 4, Silverlight and Windows Phone
The serializer is a good choice when the JSON you are reading or writing maps closely to a .NET class.
LINQ to JSON is good for situations where you are only interested in getting values from JSON, you don't have a class to serialize or deserialize to, or the JSON is radically different from your class and you need to manually read and write from your objects.
Documentation
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Json.NET is a personal open source project. Started in 2006, I have put hundreds of hours adding, refining and tuning Json.NET with the goal to make it not just the best JSON serializer for .NET but the best serializer for any computer language. I need your help to achieve this.