After upgrading a project to ASP.NET 4.5, a warning appeared next to the System.Web.Http reference. Visual Studio could not find the related DLL anymore.
Starting from Visual Studio 2017, this DLL is no longer available in the GAC but should be added through NuGet.
You can find the corrent assembly in the Web Api Core NuGet package.
Remark: Don’t be confused by the Core in the name. This package is not meant for ASP.NET Core.