What should I do if the current ASP.NET session is null?

Yes, the Session object might be null, but only in certain circumstances, which you will only rarely run into:

• If you have disabled the SessionState http module, disabling sessions altogether
• If your code runs before the HttpApplication.AcquireRequestState event.
• Your code runs in an IHttpHandler, that does not specify either the IRequiresSessionState or IReadOnlySessionState interface.

If you only have code in pages, you won't run into this. Most of my ASP .NET code uses Session without checking for null repeatedly. It is, however, something to think about if you are developing an IHttpModule or otherwise is down in the grittier details of ASP .NET.

Edit

In answer to the comment: Whether or not session state is available depends on whether the AcquireRequestState event has run for the request. This is where the session state module does it's work by reading the session cookie and finding the appropiate set of session variables for you.

AcquireRequestState runs before control is handed to your Page. So if you are calling other functionality, including static classes, from your page, you should be fine.

If you have some classes doing initialization logic during startup, for example on the Application_Start event or by using a static constructor, Session state might not be available. It all boils down to whether there is a current request and AcquireRequestState has been run.

Also, should the client have disabled cookies, the Session object will still be available - but on the next request, the user will return with a new empty Session. This is because the client is given a Session statebag if he does not have one already. If the client does not transport the session cookie, we have no way of identifying the client as the same, so he will be handed a new session again and again.

If your Session instance is null and your in an 'ashx' file, just implement the 'IRequiresSessionState' interface.

This interface doesn't have any members so you just need to add the interface name after the class declaration (C#):

public class MyAshxClass : IHttpHandler, IRequiresSessionState

The following statement is not entirely accurate:

"So if you are calling other functionality, including static classes, from your page, you should be fine"

I am calling a static method that references the session through HttpContext.Current.Session and it is null. However, I am calling the method via a webservice method through ajax using jQuery.

As I found out here you can fix the problem with a simple attribute on the method, or use the web service session object:

There’s a trick though, in order to access the session state within a web method, you must enable the session state management like so:

[WebMethod(EnableSession = true)]

By specifying the EnableSession value, you will now have a managed session to play with. If you don’t specify this value, you will get a null Session object, and more than likely run into null reference exceptions whilst trying to access the session object.

Thanks to Matthew Cosier for the solution.

Just thought I'd add my two cents.

Ed