Why is System.Web.Mvc not listed in Add References?

Question

Using C#, Visual Studio 2010.

There is a namespace called System.Web.Mvc documented on MSDN. The documentation for all the types in that namespace says that they are

Answer 1

If you got this problem in Visual Studio 2017, chances are you're working with an MVC 4 project created in a previous version of VS with a reference hint path pointing to C:\Program Files (x86)\Microsoft ASP.NET. Visual Studio 2017 does not install this directory anymore.

We usually solve this by installing a copy of Visual Studio 2015 alongside our 2017 instance, and that installs the necessary libraries in the above path. Then we update all the references in the affected projects and we're good to go.