ASP.NET NullReferenceException for get_Session()

Question

public class MessageHelper : System.Web.UI.MasterPage
{
    public MessageHelper()
    {

    }

    public string Message
    {
        set { Session["Message"] = value; }
        get
        {
            if (Session["Message"] != null)
            {
                var msg = Session["Message"] as string;
                Session["Message"] = "";
                return msg;
            }
            return "";
        }
    }

    public string ErrorMsg
    {
        set { Session["Error"] = value; }
        get
        {
            if (Session["Error"] != null)
            {
                var err = Session["Error"] as string;
                Session["Error"] = "";
                return err;
            }
            return "";
        }
    }
}


[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.UI.UserControl.get_Session() +15
   WebApplication1.MessageHelper.get_ErrorMsg() in ..file.master.cs:71

where line 71 is: if (Session["Error"] != null)

what am I doing wrong here?!

EDIT (transcribed from original author):

@David,

here is AdminMaster.master.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.SessionState;

namespace WebApplication1
{
    public partial class AdminMaster : System.Web.UI.MasterPage
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            MessageHelper msg = new MessageHelper();

            if (msg.ErrorMsg != "")
            {
                // do something
            }
            if (msg.ErrorMsg != "")
            {
                // do something
            }
        }
    }
    public class MessageHelper : System.Web.UI.MasterPage
    {
        public MessageHelper()
        {

        }

        public string Message
        {
            set { System.Web.HttpContext.Current.Session["Message"] = value; }
            get
            {
                if (System.Web.HttpContext.Current.Session["Message"] != null)
                {
                    var msg = System.Web.HttpContext.Current.Session["Message"] as string;
                    System.Web.HttpContext.Current.Session["Message"] = "";
                    return msg;
                }
                return "";
            }
        }

        public string ErrorMsg
        {
            set { System.Web.HttpContext.Current.Session["Error"] = value; }
            get
            {
                if (System.Web.HttpContext.Current.Session["Error"] != null)
                {
                    var err = System.Web.HttpContext.Current.Session["Error"] as string;
                    System.Web.HttpContext.Current.Session["Error"] = "";
                    return err;
                }
                return "";
            }
        }
    }
}

so it does inherit from System.Web.UI.MasterPage, my bad.

i want the MessageHelper to be accessed from different pages on the site. all of my pages use the Master file, that's why i put the MessageHelper in the master file.

what is wrong here?

@Marcie, indeed. @NATTO, where is the code that is invoking/referencing .ErrorMsg? — Kirk Woll
– Kirk Woll, Commented Dec 3, 2010 at 17:50
its protected void Page_Load(object sender, EventArgs e) { ..... — natiz
– natiz, Commented Dec 3, 2010 at 17:53

David · Accepted Answer · 2010-12-04 01:23:38Z

4

During debugging, can you confirm that Session is not null? Try referencing it fully-qualified as System.Web.HttpContext.Current.Session within this class and see if that helps any.

Edit: In response to the non-answer answer that you posted...

It's not recommended to put that helper class in the same file as your master page. Put it in its own file named for the class. (There's probably debate over whether every class should have its own file, but in this particular case it's clear that the two classes in this one file are very much unrelated and shouldn't be together.)

The class can have its own namespace, such as WebApplication1.Helpers (though I recommend in the future using something more descriptive than WebApplication1, but don't try to change it here because it'll cause errors elsewhere in the project), and other class files can reference that namespace with using WebApplication1.Helpers in order to use that class.

Separating classes into an intuitive structure in the project (or multiple projects, as things grow in complexity) will make it easier to support in the future.

And, seeing the whole file, the helper class definitely should not inherit from MasterPage. It doesn't need to, and doing so adds things to that class that shouldn't be there.

edited Dec 4, 2010 at 1:23

answered Dec 3, 2010 at 17:55

David

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8 Comments

natiz Over a year ago

Okay, that seemed to solve the problem.. what could cause this? should I inherit from... where?!?

David Over a year ago

@NATTO: Not sure about all the details off the top of my head, but HttpContext.Current is something that pages have pre-referenced for you. You're creating a separate class, and though it inherits from MasterPage (why, by the way? it's strange to extend a class as something that doesn't do what it's meant to do), it doesn't seem to be aware of the HttpContext.Current singleton and is instead just referencing an empty Session.

natiz Over a year ago

well, I put it in a separate class because I want it to be accessed from every place, meaning the class sits on the master file, and could be accesses from different pages, am I wrong?

David Over a year ago

@NATTO: A separate class is good, and for the right reason. What I don't see from this code is why that class inherits from MasterPage. Is that MessageHelper class actually being used as a master page? If not, it shouldn't inherit from it.

natiz Over a year ago

it doesn't inherit from it.. its on the master.cs file, under the same namespace, and its defined: public class MessageHelper : System.Web.UI.MasterPage, so it inherit from System.Web.UI.MasterPage..

|

user528573 · Accepted Answer · 2010-12-03 22:46:45Z

0

I'm a bit confused by what you're trying to achieve with the MessageHelper class.

If it is code common to your master pages then you should surely be inheriting AdminMaster from MessageHelper.

eg.

public partial class AdminMaster : MessageHelper

If not, I don't understand why MessageHelper needs to inherit from MasterPage?

answered Dec 3, 2010 at 22:46

user528573

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