Hi All,
I have master page on which I have written a javascript which open ups a new window when user clicks on menu item placed on the left side of master page. The javascript follows
 function OpenWind()
       {
         <% if( HttpContext.Current.Session["Id"]==null) { %>
                alert('Session Expired! Redirecting to login page!');
                location.href= 'logout.aspx';
         <% } %>
         var SessionId = <%= HttpContext.Current.Session["Id"].ToString() %>;        
         window.open('adduser.aspx?Mode=E&&Id='+SessionId+'','AddUser',"resizable,scrollbars,menubar,location,status,toolbar,titlebar,scrollbar"); 
       }
and the calling mark up is this 

<a href="#" onclick="javascript:OpenWind();"><asp:Label runat="server" ID="lbl_MyProfile" Text='My Profile'></asp:Label></a>
But this fails gives me object reference not set to an instance error when HttpContext.Current.Session["Id"] is null.
 

from where you are assigning HttpContext.Current.Session["Id"].ToString()  the value ?

get the values if not null
<% if( HttpContext.Current.Session["Id"] !=null) { %>
                var SessionId = <%= HttpContext.Current.Session["Id"].ToString() %>;   
         <% } %>

     
"Never underestimate the power of stupid people in large groups"

Hello
Assign Session variable like this

Session["Id"] = Context.Session.LCID;
then check as ur condition
<% if( HttpContext.Current.Session["Id"]==null) { %>
                alert('Session Expired! Redirecting to login page!');
                location.href= 'logout.aspx';
         <% } %>
var SessionId = <%= HttpContext.Current.Session["Id"].ToString() %>;  

You can't call a method on an object that is not null, so you can't perform a (null).ToString().  I always prefer to maximise the amount of logic in the code-behind wherever possible, so I would prefer to put a method in my code-behind to produce the information needed, but if you want to use inline code do something like this (note that I always code in C#, but you might be able to use an inline IF in VB):

var SessionId = <%= (HttpContext.Current.Session["Id"] == null) ? string.Empty : HttpContext.Current.Session["Id"].ToString() %>;
Andrew  
blog.andrewrivers.co.uk

Thanks All,
And sory for relayed reponse.
the code is changed as
 function OpenWind()
       {
         <% if( HttpContext.Current.Session["Id"]==null) { %>
                alert('Session Expired! Redirecting to login page!');
                location.href= 'logout.aspx';
         <% } else { %>
         var SessionId = <%= HttpContext.Current.Session["Id"].ToString() %>;        
         window.open('adduser.aspx?Mode=E&&Id='+SessionId+'','AddUser',"resizable,scrollbars,menubar,location,status,toolbar,titlebar,scrollbar"); 
         <%  }  %>
 }
 And Finally Solved. Thanks All.