System.NullReferenceException When checking if != null

Question

I'm using an ASHX handler, i want the handler to check if Session != null.

if (context.Session["Username"] != null)

And i get this error pointing this line:

System.NullReferenceException: Object reference not set to an instance of an object.

What's the problem?

BrokenGlass · Accepted Answer · 2011-04-25 00:47:40Z

11

if (context.Session["Username"] != null)

Does your handler implement IRequiresSessionState? Otherwise Session might not be available.

From MSDN:

Specifies that the target HTTP handler requires read and write access to session-state values. This is a marker interface and has no methods.

answered Apr 25, 2011 at 0:47

BrokenGlass

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1 Comment

Danpe Over a year ago

Oh stupid me, I should have add IReadOnlySessionState... I found this link: hanselman.com/blog/…

Teoman Soygul · Accepted Answer · 2011-04-25 12:27:53Z

4

Use it like this. One of the encapsulating objects may be already null:

if (context != null)
  if (context.Session != null)
    if (context.Session["Username"] != null) {
      // Do stuff
}

edited Apr 25, 2011 at 12:27

answered Apr 25, 2011 at 0:48

Teoman Soygul

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3 Comments

Teoman Soygul Over a year ago

It is always safer to stack null checks like the one above, so that you can pinpoint the exact source of the problem. Seems like a chore but pays off.

BrokenGlass Over a year ago

this approach is reasonable if there is an alternate code path if Session is not available - on the other hand if it is a hard assumption to expect the Session to be there and your code depends on this assumption, the fail fast approach would be to just let it throw an exception, otherwise chances are you are masking this problem with a null check.

Teoman Soygul Over a year ago

Well of course nested if statements should be complemented with nested else's, where it would be suitable to deal with a null object (which shouldn't be null in the first place) and throw a proper exception with a proper message.

John Batdorf · Accepted Answer · 2011-04-25 00:48:32Z

1

Yeah I'd say that check to see if the context is not null first.

answered Apr 25, 2011 at 0:48

John Batdorf

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Comments

Jonathan Stanton · Accepted Answer · 2013-04-01 18:25:43Z

0

I had a similar problem with an .ashx file. The solution was that the handler has to implement IReadOnlySessionState (for read-only access) or IRequiresSessionState (for read-write access). eg:

public class myModule: IHttpHandler, IRequiresSessionState { ... }

These Interfaces do not need any additional code but act as markers for the framework.

Hope that this helps.

Jonathan

answered Apr 1, 2013 at 18:25

Jonathan Stanton

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