I need to set a variable to a session variable (if that's what you call it)
like this:

dim ds as dataset = HttpContext.Current.Session("CustDataSet")

But I get an exception if this variable in the current session hasn't been
set yet so I need to test it. I tried:

If HttpContext.Current.Session("CustDataSet") = nothing then
HttpContext.Current.Session("CustDataSet") = GetCustDataSet()
End If

But I get an exception on this also and the exception message is:

"Object reference not set to an instance of an object."

How can I test for this?

you are comparing reference types, so use is instead of =

        If HttpContext.Current.Session("CustDataSet") Is Nothing Then
            HttpContext.Current.Session("CustDataSet") = GetCustDataSet()
        End If

if you still have problems  . we need to see  "GetCustDataSet" implementation , and where did you palced that code ?

can you add a break point and make sure that the HttpContext is not null(nothing ) ?

the above code was a sample lines to get an idea

here is my real code which is royal pain to me:

public class PageRoles : MyBasePage  //mybasepage is inheriting from  System.Web.UI.Page
 {
        public PageRoles() { }
        public static ArrayList LandingPage()
        {
            if (HttpContext.Current.Session["LPage"] == null)  //<<<<throwing error
            {
                    ArrayList roleNames = new ArrayList();
                    roleNames.Add(RoleNames.GUARD);
                    roleNames.Add(RoleNames.GUARD1);


                   HttpContext.Current.Session["LPage"] = (ArrayList)roleNames;
                    return roleNames;
                }
             
            return (ArrayList)HttpContext.Current.Session["LPage"];
        }
        }
    }

you are trying to access the httpContext before its being initialized,

the solution is to exit the function if the HttpContext.currnet is null

       public static ArrayList LandingPage()
        {
           if( HttpContext.Current==null) return new Arraylist();
            if (HttpContext.Current.Session["LPage"] == null)  //<<<<throwing error
            {
                    ArrayList roleNames = new ArrayList();
                    roleNames.Add(RoleNames.GUARD);
                    roleNames.Add(RoleNames.GUARD1);


                   HttpContext.Current.Session["LPage"] = (ArrayList)roleNames;
                    return roleNames;
                }
              
            return (ArrayList)HttpContext.Current.Session["LPage"]; 
        }
        }

i'm getting the error in:

Yes, we must check the sesion also ,

if (HttpContext.Current == null || HttpContext.Current.Session==null) return new ArrayList();
  if (HttpContext.Current.Session["LPage"] == null)
.....

Anas:

your approach exists the moethd if the HttpContext.Current == null and i'm not looking for that

what i'm looking is, if the HttpContext.Current.Session["LPage"] == null then create a ArrayList and add that ArrayList to a Session variable called "LPage"

 thanks for looking.

shukran

The problem is that the line of code you say is erroring shouldn't be causing a problem. I can execute this code without an error:

if (HttpContext.Current.Session["LPage"] == null)
{
    HttpContext.Current.Session["LPage"] = "SOME VARIABLE";
}

i have no idea why this is not setting... anyway

here is my full code

public partial class Default2 : MyBasePage
{
    ArrayList roleNames = PageRoles.LandingPage();
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsUserAuthenticatedForPage(roleNames))
        {
            Response.Write("welcome!");
        }
        else
        {
            Response.Write("you do not have access!");
        }

    }
}

  public class PageRoles : MyBasePage
    {
        public PageRoles() { }

        public static ArrayList LandingPage()
        {
            //if (HttpContext.Current == null || HttpContext.Current.Session == null) return new ArrayList();
            //if (HttpContext.Current.Session["LPage"] == null)
            //if (HttpContext.Current == null) return new ArrayList();
            if (HttpContext.Current.Session["LPage"] == null)  //<<<<throwing error
            {
                ArrayList roleNames = new ArrayList();
                roleNames.Add(RoleNames.GUARD);
                roleNames.Add(RoleNames.GUARD1);
                HttpContext.Current.Session["LPage"] = (ArrayList)roleNames;
                return roleNames;
            }
            return (ArrayList)HttpContext.Current.Session["LPage"];
        }
}

 public class MyBasePage : System.Web.UI.Page
    {
        public MyBasePage()
        {
            //this.Load += new EventHandler(BasePage_Load);
        }

        public enum RoleNames
        {
            GUARD,
            GUARD1,
            GUARD2
        }
}

 WEB.CONFIG

 <system.web>
  <pages enableSessionState="true" enableEventValidation="false" >

I think anas was right (he usually is). You're calling LandingPage() before the Session has been set up. If you debug and look at HttpContext.Current.Session you'll see it's null, which is why you're getting an error. What you can do is move the call to LandingPage() into your Page_Load by which time the Session should be available, e.g.

public partial class Default2 : MyBasePage
{
    ArrayList roleNames;
    protected void Page_Load(object sender, EventArgs e)
    {
        roleNames = PageRoles.LandingPage();
        if (IsUserAuthenticatedForPage(roleNames))
        {
            Response.Write("welcome!");
        }
        else
        {
            Response.Write("you do not have access!");
        }
    }
}

oops... i never paid attention to when i'm setting the session

thank you so much Anas and Ramblor for helping me to understand.