unexpected behaviour of object stored in web service Session

Question

Hi. I'm using Session variables inside a web service to maintain state between successive method calls by an external application called QBWC. I set this up by decorating my web service methods with this attribute:

[WebMethod(EnableSession = true)]

I'm using the Session variable to store an instance of a custom object called QueueManager. The QueueManager has a property called ChangeQueue which looks like this:

 [Serializable]
 public class QueueManager
 {
  ...
  public Queue<QBChange> ChangeQueue { get; set; }
  ...

where QBChange is a custom business object belonging to my web service.

Now, every time I get a call to a method in my web service, I use this code to retrieve my QueueManager object and access my queue:

QueueManager qm = (QueueManager)Session[ticket];

then I remove an object from the queue, using

qm.dequeue()

and then I save the modified query manager object (modified because it contains one less object in the queue) back to the Session variable, like so:

Session[ticket] = qm; 

ready for the next web service method call using the same ticket.

Now here's the thing: if I comment out this last line

//Session[ticket] = qm;

, then the web service behaves exactly the same way, reducing the size of the queue between method calls. Now why is that?

The web service seems to be updating a class contained in serialized form in a Session variable without being asked to. Why would it do that? When I deserialize my Queuemanager object, does the qm variable hold a reference to the serialized object inside the Session[ticket] variable?? This seems very unlikely.

Answer 1

All reference types (such as classes) are stored in memory as references and Session State is also (normally) stored in memory, but can also be stored to a file or a database. When storing reference types in Session, you should almost always ensure they are Serializable as well.

QueueManager is a reference type, so all the Session[ticket] is doing is holding a reference to it in memory. You do not need to re-assign to it as the Session variable itself IS the item that you're modifying.

This is just a simplified version of what you're doing:

Session["Foo"] = new Bar();
Bar rar = (Bar)Session["Foo"];
rar.Count = 1;

if (((Bar)Session["Foo"]).Count == 1)
{
    // Great success!
}