No probé la solución rangitatanz, pero utilicé otro método y funcionó muy bien para mí.
private List<String> getOnlineUsers()
{
List<String> activeSessions = new List<String>();
object obj = typeof(HttpRuntime).GetProperty("CacheInternal", BindingFlags.NonPublic | BindingFlags.Static).GetValue(null, null);
object[] obj2 = (object[])obj.GetType().GetField("_caches", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(obj);
for (int i = 0; i < obj2.Length; i++)
{
Hashtable c2 = (Hashtable)obj2[i].GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(obj2[i]);
foreach (DictionaryEntry entry in c2)
{
object o1 = entry.Value.GetType().GetProperty("Value", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(entry.Value, null);
if (o1.GetType().ToString() == "System.Web.SessionState.InProcSessionState")
{
SessionStateItemCollection sess = (SessionStateItemCollection)o1.GetType().GetField("_sessionItems", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(o1);
if (sess != null)
{
if (sess["loggedInUserId"] != null)
{
activeSessions.Add(sess["loggedInUserId"].ToString());
}
}
}
}
}
return activeSessions;
}
posible duplicado de [Lista de todos activos ASP.NET Sesiones] (http://stackoverflow.com/questions/1470334/list-all-active-asp-net-sessions) –