To programmatically detect the users connected to Windows Server, you have to use PInvoke to call the Windows Terminal Services API. Alternatively, you can use a .NET library called Cassia, which was created exclusively for this purpose. Cassia provides a wrapper around the Windows Terminal Services API.

The following code snippet gives you an idea of how to use Cassia (extracted from the Cassia Project Home):

ITerminalServicesManager manager = new TerminalServicesManager();using (ITerminalServer server = manager.GetRemoteServer("your-server-name")){    server.Open();    foreach (ITerminalServicesSession session in server.GetSessions())    {        Console.WriteLine("Session ID: " + session.SessionId);        Console.WriteLine("User: " + session.UserAccount);        Console.WriteLine("State: " + session.ConnectionState);        Console.WriteLine("Logon Time: " + session.LoginTime);    }}