Just like you can use _AppStart.cshtml to write code before pages in the site run, you can write code that runs before (and after) any page in a particular folder run. This is useful for things like setting the same layout page for all the pages in a folder, or for checking that a user is logged in before running a page in the folder.
For pages in particular folders, you can create code in a file named _PageStart.cshtml. The following diagram shows how the_PageStart.cshtml page works. When a request comes in for a page, ASP.NET first checks for a _AppStart.cshtml page and runs that. Then ASP.NET checks whether there's a _PageStart.cshtml page, and if so, runs that. It then runs the requested page.
Inside the _PageStart.cshtml page, you can specify where during processing you want the requested page to run by including a RunPagemethod. This lets you run code before the requested page runs and then again after it. If you don't include RunPage, all the code in_PageStart.cshtml runs, and then the requested page runs automatically.
http://www.asp.net/web-pages/overview/ui-layouts-and-themes/18-customizing-site-wide-behavior