System.Web.Http missing after .net 4.5 upgrade

Just run the following nuget command:

Install-Package Microsoft.AspNet.WebApi.Core

As for VS2017 - as said in the asp.net forums Microsoft has moved the namespace to a Nuget package called Web Api Core. In order to install it you need to typeInstall-Package Microsoft.AspNet.WebApi.Core in the Nuget Package Manager Console.

Have you looked in the Extensions part of the "Add new reference", instead of Framework ?

If it's not there, it's in the Microsoft.AspNet.WebApi.Core package on NuGet.