Extract from an MVC controller which authenticates to SharePoint and uses .NET CSOM code..

AppForSharePoint_HomeController.cs

// code taken from an MVC controller which authenticates to SharePoint and uses .NET CSOM code. The SharePointContext and
// TokenHelper helper classes are added by Visual Studio..

[SharePointContextFilter]
public ActionResult Index()
{
    User spUser = null;
    // this method calls various methods on the TokenHelper class which is also added..
    var spContext = SharePointContextProvider.Current.GetSharePointContext(HttpContext);

    using (var clientContext = spContext.CreateUserClientContextForSPHost())
    {
        if (clientContext != null)
        {
            spUser = clientContext.Web.CurrentUser;
            clientContext.Load(spUser, user => user.Title);
            clientContext.ExecuteQuery();

            ViewBag.UserName = spUser.Title;
        }
    }

    return View();
}