How do I pass a variable from an ActionFilter to a Controller Action in C# MVC?

Question

Taking a simple action filter, which checks if the user is logged in and retrieves their user ID.

public class LoginFilter : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {    
        // Authenticate (somehow) and retrieve the ID
        int id = Authentication.SomeMethod();
        // Pass the ID through to the controller?
        .....
    }
}

How could I then pass this ID through to my controller action?

[LoginFilter]
public class Dashboard : Controller
{
    public ActionResult Index()
    {
        // I'd like to be able to use the ID from the LoginFilter here
        int id = ....
    }
}

Is there an equivalent to the ViewBag that would allow me to do this? Or some other technique that allows me to pass variables and objects between the filter and the controller action?

Answer 1

I believe ActionExecutingContext contains a reference to the calling controller. Using this mixed with a custom controller class derived from the base Controller class to then store the id as an instance variable of the controller would probably do it.

Custom controller

Public Class MyController : Controller
{
    Public int Id {get;set;}
}

LoginFilter

public class LoginFilter : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {    
        // Authenticate (somehow) and retrieve the ID
        int id = Authentication.SomeMethod();
        ((MyController)filterContext.Controller).Id = id; 
        //Assign the Id by casting the controller (you might want to add a if ... is MyController before casting)
    }
}

Controller

[LoginFilter]
public class Dashboard : MyController
{
    public ActionResult Index()
    {
        //Read the Id here
        int id = this.Id
    }
}