MSDN (https://msdn.microsoft.com/en-us/library/dd997357(v=vs.110).aspx):
In the .NET Framework 3.5 and earlier versions, ASP.NET provided an in-memory cache implementation in the System.Web.Caching namespace. In previous versions of the .NET Framework, caching was available only in theSystem.Web namespace and therefore required a dependency on ASP.NET classes. In the .NET Framework 4, the System.Runtime.Caching namespace contains APIs that are designed for both Web and non-Web applications.
The MemoryCache class is modeled on the ASP.NET cache object that is defined in the System.Web.Caching namespace. Therefore, the internal caching logic similar to the logic that was provided in earlier versions of ASP.NET.
If your application targets the .NET Framework 3.5 or earlier, you must use the caching classes that are defined in the System.Web.Caching namespace. For more information, see ASP.NET Caching Overview.
When you develop new applications, we recommend that you use the MemoryCache class. The API that is provided in the System.Runtime.Caching namespace is like the API that is provided in the Cache namespace. Therefore, the API will be familiar if you used caching in earlier versions of ASP.NET. For an example of how to use caching in ASP.NET applications, see Walkthrough: Caching Application Data in ASP.NET.