[c#] Cannot find JavaScriptSerializer in .Net 4.0

For those who seem to be following the answers above but still have the problem (e.g., see the first comment on the poster's question):

You are probably working in a solution with many projects. The project you appear to be working in references other projects, but you are actually modifying a file from one of the other projects. For example:

  • project A references System.Web.Extensions
  • project A references project B

But if the file you are modifying to use System.Web.Script.Serialization is in project B, then you will need to add a reference to System.Web.Extension in project B as well.