object reference not set to an instance of an object

Question

1.00/5 (1 vote)

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This is my code..

C#

protected void Button2_Click(object sender, EventArgs e)
   {
       Examination exam = (Examination)Session["questions"];

       try
       {


           var conString = System.Configuration.
                  ConfigurationManager.ConnectionStrings["examConnectionString"];
           string strConnString = conString.ConnectionString;
           SqlConnection con = new SqlConnection(strConnString);

           // SqlConnection con = new SqlConnection("uid=sa;database=exam");
           con.Open();
           SqlCommand cmd = new SqlCommand("select isnull( max(examid),0) + 1 from oe_essay", con);
           int examid = (Int32)cmd.ExecuteScalar();
           cmd.CommandText = "insert into oe_esay values(@examid,@mid,@fullname,@sid,@noq,@ans,@stdate,getdate())";
           cmd.Parameters.Add("@examid", SqlDbType.Int).Value = examid;
           cmd.Parameters.Add("@mid", SqlDbType.Int).Value = exam.mid;
           cmd.Parameters.Add("@fullname", SqlDbType.VarChar).Value = exam.fullname;
           cmd.Parameters.Add("@sid", SqlDbType.Int).Value = exam.sid;
           cmd.Parameters.Add("@noq", SqlDbType.Int).Value = exam.SIZE;
           cmd.Parameters.Add("@ans", SqlDbType.NVarChar).Value = TextBox1.Text;
           cmd.Parameters.Add("@stdate", SqlDbType.DateTime).Value = exam.StartTime;
           cmd.ExecuteNonQuery();

       }
       catch (Exception ex)
       {
           Label1.Text = "Message" + ex;
       }


       //con.Close();
   }

im getting the error at exam.mid;

Error:

VB.NET

object reference not set to an instance of an object

system.nullreferenceexception

Posted 24-Nov-15 1:11am

Nagaraju Kukkudala

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Comments

aarif moh shaikh 24-Nov-15 7:18am

Check your session,connection String ... One of them have null value

Nagaraju Kukkudala 24-Nov-15 7:20am

Sorry , I didnt get u.. Can u please tell me in detail

Kornfeld Eliyahu Peter 24-Nov-15 7:25am

Check this alone cmd.Parameters.Add("@examid", SqlDbType.Int)
See what returns...

Nagaraju Kukkudala 24-Nov-15 7:33am

its taking exam id as 1
when coming to next line
exam=null
examid=1

Raje_ 24-Nov-15 7:38am

It means your Session["questions"] have null value. Check where are you setting Session.

Raje_ 24-Nov-15 7:32am

Please learn to debug your code. It's very easy to find out which line throws this error. :)

Nagaraju Kukkudala 24-Nov-15 7:48am

Thank you Guys.. For ur support... Problem Solved... :)

Gautham Prabhu K 24-Nov-15 8:45am

Please share the stack trace.

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OriginalGriff · Answer 1 · 2015-11-24T01:30:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!