Session is null in IRouteHandler.GetHttpHandler with Asp.net routing

Question

I'm trying to get Session enabled in the GettHttpHandler method of my IRouteHandler classes but session is always null. Could someone tell me what I'm doing wrong?

In global.asax I have

RouteTable.Routes.Add("All", new Route("{*page}", new MyRouteHandler()));

The MyRouteHandler class where Session is null looks like this:

public class MyRouteHandler : IRouteHandler, IRequiresSessionState
{
    public System.Web.IHttpHandler GetHttpHandler(RequestContext requestContext)
    {
        string test = HttpContext.Current.Session["test"].ToString();
        return BuildManager.CreateInstanceFromVirtualPath("~/Page.aspx", typeof(Page)) as Page;
    }
}

I made a small test app that shows the problem.

Could someone tell me what I'm doing wrong?

_{Edited to add:}

Yes, I really need session data in the route handler. There are many reasons but one easily explainable is when the user can switch to browse the site in preview mode.

The site consists of a hierarchy of dynamic pages (/page1/page2...) in the database that can be published normally or to preview. A content producer browsing the site can choose to view just normal pages or also those published to preview. The browsing mode is stored in the user's session so therefor the route handler needs to know the browsing mode to be able to solve the requested page.

So I really need the session already at that stage.

Answer 1

I have explained reason behind this problem in this answer. And now I have found a solution to the problem!

1. You create a custom HttpHandler class:

   class MyHttpHandler : IHttpHandler, IRequiresSessionState
   {
     public MyRequestHandler RequestHandler;
     public RequestContext Context;
     public MyHttpHandler(MyRequestHandler routeHandler, RequestContext context)
     {
       RequestHandler = routeHandler;
       Context = context;
     }
   
     public void ProcessRequest(HttpContext context)
     {
       throw new NotImplementedException();
     }
   
     public bool IsReusable
     {
       get { throw new NotImplementedException(); }
     }
   }
   

It is important to add IRequiresSessionState interface, otherwise IIS does not load session for this request. We do not need to implement logic of ProcessRequest and IsReusable, but class must implement the IHttpHandler interface.

2. You change your RouteHandler implementation:

   public class MyRequestHandler : IRouteHandler
   {
     public IHttpHandler GetHttpHandler(RequestContext requestContext)
     {
         return new MyHttpHandler(this, requestContext);
     }
   
     public IHttpHandler DelayedGetHttpHandler(RequestContext requestContext)
     {
         // your custom routing logic comes here...
     }
   }
   

You simply move your original, Session dependent logic to DelayedGetHttpHandler function and in the GetHttphandler function you return an instance of the helping MyHttpHandler class.

3. Then, you hook your handling logic to the HttpApplication.PostAcquireRequestState event, e.g. in the Global.asax:

   public class Global : HttpApplication
   {
       public override void Init()
       {
           base.Init();
           PostAcquireRequestState += Global_PostAcquireRequestState;
       }
    }
   

For more reference, check this page: https://msdn.microsoft.com/en-us/library/bb470252(v=vs.140).aspx. It explains the request lifecycle and why I use the PostAcquireRequestState event.

4. In the event handler, you invoke your custom RouteHandling function:

   void Global_PostAcquireRequestState(object sender, EventArgs e)
   {
      if (HttpContext.Current.Handler is MyHttpHandler) {
        var handler = HttpContext.Current.Handler as MyHttpHandler;
        HttpContext.Current.Handler = handler.RouteHandler.DelayedGetHttpHandler(handler.Context);
      }
   }
   

And that's it. Works for me.

Answer 2

I am not sure that you can do this (although I may be wrong). My recollection is that IRequiresSessionState indicates that an HttpHandler needs session state, rather than the route handler (which is responsible for giving you a handler appropriate to the route).

Do you really need the session in the route handler itself and not the handler it gives out?