TcpClient流.Read意外结束线程

我有一个线程检查用户在线

var user = (User)userObj;
if (!IsUserOnline(user)) return;
if (!TryToConnect(user)) return;
var TCP = user.CmdTCP;
Stream stm = TCP.GetStream();

每次都成功完成，但是当我

stm.Write(GetBytes("chk"), 0, 3);

和

var bb = new byte[1];
try {
stm.Read(bb, 0, 1);
} catch (Exception e) { /*error*/ }
if (bb[0] == CHR_SUCCESS) {
//etc etc
}

当第二次检查用户是否在线时，流读取失败且没有警告。没有例外，代码不会走得更远。线程停止，程序运行。

整个线程:

        var user = (User)userObj;
        if (!IsUserOnline(user)) return;
        if (!TryToConnect(user)) return;
        var TCP = user.CmdTCP;
        Stream stm = TCP.GetStream();
        try {
            stm.Write(GB("chk"), 0, 3);
            toLog("refresh::chk sent ;)");
        } catch (Exception e) {
            #region unable to send packet, online
            toLog("'trefresh::" + user.name + " is online (Connected, unable to send packet) " + e.Message);
            setUserStatus(user, userStatus.Online);
            return;
            #endregion
        }
        var bb = new byte[1];
        try {
            stm.Read(bb, 0, 1); //<<<<<<<<< here thread just ends, no toLog("..");, no exception
            toLog("refresh::got " + (char)bb[0]);
        } catch (Exception e) {
            #region unable to get packet, online
            toLog("'trefresh::" + user.name + " is online (Connected, unable to get packet) " + e.Message);
            setUserStatus(user, userStatus.Online);
            return;
            #endregion
        }
        if (bb[0] == CHR_SUCCESS) {
            try {
                stm.Write(GB(toStr(CHR_SSUCCESS)), 0, 1);
                toLog("refresh::CHR_SSUCCESS sent ;)");
            } catch (Exception e) {
                #region unable to send packet 2, online
                toLog("'trefresh::" user.name + " is online (Connected, unable to send packet #2) " + e.Message);
                setUserStatus(user, userStatus.Online);
                return;
                #endregion
            }
            toLog("'trefresh::" + user.name + " is online");
            setUserStatus(user, userStatus.Online);
        } else {
            for (int x = 0; x < 5; x++) {
                toLog("'t'trefresh::got wrong answer");
                System.Media.SystemSounds.Beep.Play();
                Thread.Sleep(500);
            }
            setUserStatus(user, userStatus.Online);
        }