ASP.NET Web API Now Uses Json.NET, Gets Several Enhancements
Notably, Json.NET now becomes the default serializer for Web API which means we don’t need to swap the default implementation any more – for instance, to be able to work with dates (Json.NET 4.5 uses the ISO 8601 date format by default). As a result, the redundant System.Json.dll has been removed. As Microsoft puts it -
Json.NET provides the flexibility and performance required for a modern web framework.
HttpClient now derives from a new HttpMessageInvoker and the latter can be used for unit testing Http Message handlers, rather than using former directly which are heavier. Also the IAPIExplorer API has now been included into the release bits. The complete list of changes are included in the release notes.
Ronny Kohavi Dec 12, 2013