I am not experienced in MVC and have just created a regular web application to create a web project for an existing web page which we inherited and are 'reverse engineering.'

Which dll contains all the methods associated with the ModelState object? May I have a link to a download or at least a name to look for it? I have already added System.Web.MVC.

I would prefer not to create an entirely new project in MVC mode. I just want the application to compile.