Namespace System.Web.Http.Results not found within asp.net web api application

I have an asp.net web api application in which I have this line :

using System.Web.Http.Results;

My problem is that the namespace isn't recognized, I verified the dll system.Web.Http exists in the bin folder !!

So I need to know:

• What is the reason of this problem?
• How can I fix it?

You need reinstalling the NuGet package, which corrects broken dependencies. Open package manager and run Update-Package Microsoft.AspNet.WebApi -reinstall.