How do I catch session null exception error for all pages in ASP.NET?

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hello all,
i am new to learning asp.net and c#

I am using Session["variables"] i my project to store user details for some time
but sometime while checking in the page_load like

C#

if (string.IsNullOrEmpty(Session["Username"] as string))
{
     //Some Stuff to initialize the variable
}

I get error

C#

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

please tell me

Q1.how do i catch these error and ignore it OR initialize it and proceed?
or
Q2.How can i initialize Session["variables"] every time when website loads into client machine so that i can initialize it and don't get error?

please help
thanks

What I have tried:

tried storing data in as global variables but it is not working perfectly.

Posted 8-Feb-17 7:07am

Palash Sachan

Updated 8-Feb-17 7:46am

v4

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Comments

Richard Deeming 8-Feb-17 13:33pm

You need to start by working out which object is null.

Based on the code you've posted, you would only get a NullReferenceException if Session is null. But the Page.Session property will never return null - if session state is disabled for the page, it would throw an HttpException instead.

So it seems highly unlikely that the line of code you've posted is the source of the exception. Double-check the stack trace of the exception, and also check the surrounding lines in case the line number is wrong.

Palash Sachan 8-Feb-17 13:41pm

i know which session objects are NULL when the page loads and so i am checking it that if its null then do some stuff..but it gives error
do u know how can i initialize the session variable when the website loads on client machine?

Richard Deeming 8-Feb-17 13:58pm

As I said, the code you've posted will not generate a NullReferenceException. You might get an HttpException if session state is disabled, but you won't get a NullReferenceException.

(There was a bug back in .NET 2.0 RTM where string.IsNullOrEmpty would throw a NullReferenceException; but that was fixed in SP1, which was released in 2011.)

As I said, the code you've shown is not responsible for the exception you're seeing. You need to track down the code that is responsible, and fix that.

Palash Sachan 8-Feb-17 14:04pm

yeah but its showing error..i don't know why its showing error when everything is fine..anyway thanks for the help :)

Richard Deeming 8-Feb-17 14:07pm

As I said, check the stack trace of the exception. That should tell you what line in what file caused the exception. With ASP.NET, the line number can sometimes be a couple of lines off, so check the surrounding lines as well.

If you can reproduce the error, debug your code. Visual Studio will break on the line where the error occurred, and you will be able to inspect the variables to see what's null when it shouldn't be.

F-ES Sitecore 9-Feb-17 4:17am

If your concern is uninitialised session variables then a solution is to initialise them using the Session_Start event in the global.asax. That way if someone has been idle for a while, then continues to browse your site it will trigger the session start again and the variables will be initialised. Alternatively you simply handle it every time you access a session variable;

if (Session["xyz"] == null)
{
// variable not initialised
}

Palash Sachan 9-Feb-17 7:06am

OMG.."Session_Start" in Global.asax worked
i added a new session like this there

protected void Session_Start(object sender, EventArgs e)
{
Session.Add("UsernameAvailable", "false");
}
and then checked in the content page page_load
thanks a lot sir..its working now :)

1 ques - when does this session_start trigger and when not??
pls tell

F-ES Sitecore 9-Feb-17 8:03am

It triggers when the user requests an aspx page from your site and they don't have an active session. So the first time they view the site, and when they return to the site after the session expiry period has passed.

Palash Sachan 9-Feb-17 12:41pm

okay..understood..thanks a lot for the help :)

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