<sessionState
   cookieless="UseCookies"
   regenerateExpiredSessionId="true"
   mode="InProc"
   timeout="20"/>

<pages enableSessionState="true">

<pages enableSessionState="true">
 <namespaces>
   <add namespace="..."/>
 </namespaces>
</pages>

Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
    ' Fires when the session is started
End Sub

Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
    ' Fires when the session ends
 End Sub

public class MySession
 {
    public static void Add(string key, object value)
    {
        if (HttpContext.Current.Session != null && (HttpContext.Current.Session.Keys == null || (HttpContext.Current.Session.Keys != null && HttpContext.Current.Session[key] == null)))
        {
            HttpContext.Current.Session.Add(key, value);
        }
    }

    public static void Remove(string key)
    {
        ...
    }

    public static void Update(string key, object value)
    {
        ...
    }

    public static object GetValue(string key)
    {
        ...
    }
}