Best way to mark Client Offline using Web-Service
[WebMethod]
public string Connect(Size desktopsize)
{
  Client clienti = new Client();
  clienti.ID = "Client_" + Counter.ToString();
  clienti.Desktopsize = desktopsize;
  clienti.Lastupdate = DateTime.Now;
  Counter++;
  clients.Add(clienti);
  return clienti.ID;
}

public bool SingleClick
{
  get
  {
    bool tmpBolean = singleclick;
    singleclick = false;
    return tmpBolean;
  }
  set
  {
    this.lastupdate = DateTime.Now;
    singleclick = value;
  }
}

private void CheckOnlinestate()
{
  while (isRunning)
  {
    TimeSpan ts = DateTime.Now - lastupdate;
    if (ts.TotalMinutes >= 1)
    {
      isRunning = false;
      this.Dispose();
    }
  }
}

public void CheckClients()
{
  while (true)
  {
    foreach (Client c in clients)
    {
      if (c.ShouldDispose)
      {
        clients.Remove(c);
      }
    }
    Thread.Sleep(200);
  }
}