ASP.NET MVC 2: Session or TempData in Master Page?

Question

I have a div in my master page that is only displayed if Session["message"] contains data:

<%
    if (!String.IsNullOrEmpty(Session["message"].ToString()))
    {
%>        
        <div id="sessionMessage" class="sessionMessage"><%:Session["message"].ToString()%></div>
<%
    }
%>  

I use it to pass general info to users for both action success and failure.

I tested it on failures, and it works great. Just set the Session["message"] in the catch block and return a new instance of the view:

{ 
   //Invalid - redisplay with errors
   Session.Add("message", "That object already exists.  Please try again.");
   return View(new DetailViewModel());            
}

Problem: However, for the success cases, which involve a RedirectToAction, the Session["message"] always gets cleared out by the time I get to the Master Page:

//Send message to view for user to see
Session.Add("message", "Object added.");

//Redirect to the details
return RedirectToAction("Details", new { id = viewModel.MyObject.ObjectId });

I considered switching to TempData, but it throws a nullreference error on initial page load. I assume this is because TempData is a member of the Controller class, and the Master Page has no controller per se?

Answer 1

It's just fine to use TempData in a master page. But if the TempData["message"] doesn't happen to contain anything, then calling .ToString() on it will give you a null reference exception. Session and TempData behave identically in this respect. This principle difference is that TempData is cleared when you read it, and Session is not. So you're more likely to see the problem with TempData, but your code has the same potential bug with either one.

Answer 2

I'd strongly encourage you to use TempData for all user alert and notification functionality.

Session can stick around forever while TempData clears after its read or if your using MVC 1 after the next page loads. Using Session means you could give the user a confusing error message that you forgot to clear.