I\'ve just upgraded my solution to .net 4.5 using Target Framework Migrator, and then the package manager console command:
Update-Package -Reinstall -IgnoreDe
Just run the following nuget command:
Install-Package Microsoft.AspNet.WebApi.Core
I do this
Update-Package -Reinstall
and it helped to my project be well done
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.
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.