show Online users using c#.net

About Global.aspx:

The Global.asax file, also known as the ASP.NET application file, is an optional file that contains code for responding to application-level events raised by ASP.NET or by HttpModules. The Global.asax file resides in the root directory of an ASP.NET-based application.
At run time, Global.asax is parsed and compiled into a dynamically generated .NET Framework class derived from the HttpApplication base class. The Global.asax file itself is configured so that any direct URL request for it is automatically rejected; external users cannot download or view the code written within it.
The ASP.NET Global.asax file can coexist with the ASP Global.asax file. You can create a Global.asax file either in a WYSIWYG designer, in Notepad, or as a compiled class that you deploy in your application's \Bin directory as an assembly. However, in the latter case, you still need a Global.asax file that refers to the assembly.
The Global.asax file is optional. If you do not define the file, the ASP.NET page framework assumes that you have not defined any application or session event handlers.
When you save changes to an active Global.asax file, the ASP.NET page framework detects that the file has been changed. It completes all current requests for the application, sends the Application_OnEnd event to any listeners, and restarts the application domain. In effect, this reboots the application, closing all browser sessions and flushing all state information.
<%@ Application Language="C#" %>

<script runat="server">

    void Application_Start(object sender, EventArgs e)
    {
        int count = 0;
        // Code that runs on application startup
        Application["myCount"] = count;
        Application["OnlineUsers"] = 0;       
    }
   
    void Application_End(object sender, EventArgs e)
    {
        //  Code that runs on application shutdown
       
    }
       
    void Application_Error(object sender, EventArgs e)
    {
        // Code that runs when an unhandled error occurs      
        Response.Redirect("Error.aspx");
    }

    void Session_Start(object sender, EventArgs e)
    {
        // Code that runs when a new session is started
        int count = 0;
        count = Convert.ToInt32(Application["myCount"]); Application["myCount"] = count + 1;
        Application["OnlineUsers"] = (int)Application["OnlineUsers"] + 1;
        Application.UnLock();
    }

    void Session_End(object sender, EventArgs e)
    {
        // Code that runs when a session ends.
        // Note: The Session_End event is raised only when the sessionstate mode
        // is set to InProc in the Web.config file. If session mode is set to StateServer
        // or SQLServer, the event is not raised.
        Application.Lock();
        Application["OnlineUsers"] = (int)Application["OnlineUsers"] - 1;
        Application.UnLock();
    }
      
</script>
open Default.aspx type Online Visitors: <%= Application["OnlineUsers"].ToString() %>