NullReferenceException while getting properties from session object

Question

I have created a class with name Examination, in this class I have a method with name Get Question(), in take exam when i am creating object of Examination and Run Application it gives following error.

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

Source Error: 


Line 18:         Examination e = (Examination)Session["questions"];
Line 19:         // display data
Line 20:         lblSubject.Text = e.sname;
Line 21:         lblQno.Text = e.curpos + 1 + "/" + e.SIZE;
Line 22:         lblCtime.Text = DateTime.Now.ToString();

Seems like e is null, first check whether Session["questions"] contains data and if it is possible to cast it to Examination. — Abbas
– Abbas, Commented Dec 10, 2013 at 13:02
You know that line 18 does not create an object but reads it from the session, right? — Krisztián Balla
– Krisztián Balla, Commented Dec 10, 2013 at 13:05
@JennyO'Reilly i am creating object of Examination But Give red color ErrorLine without discription Examination e=new Examination(); — Nitin Turankar
– Nitin Turankar, Commented Dec 10, 2013 at 13:16

C.Evenhuis · Accepted Answer · 2013-12-10 13:02:54Z

3

Most probably Session["questions"] does not contain a value, and returns null. It is perfectly valid to cast null to a reference type, but the exception occurs where you try to access properties of it.

You should test if it's null, for instance:

Examination e = (Examination)Session["questions"];
if (e == null)
{
    lblSubject.Text = "Your session has expired";
}

(If I'm wrong, and e actually contains a reference to an Examination object, then it's lblSubject that's null)

answered Dec 10, 2013 at 13:02

C.Evenhuis

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3 Comments

Nitin Turankar Over a year ago

if e null then what to do

Krisztián Balla Over a year ago

When your Session (20 minutes per default iirc) times out, e is gong to be null. In this case, you need to reinitalize the session. There are event handlers for this: msdn.microsoft.com/en-us/library/ms178583.aspx

Adarsh Shah Over a year ago

As mentioned Session["questions"] is probably null. Its always a good practice to check for null before using any object specially if its returned from a different class/source (even if it can never be null). What if the implementation of the other class/source changes in future? Its just safe to check.

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