不能理解:使用lock()却从不执行代码

我正在学习c#中的线程，并且我得到了我无法理解的这种行为。

代码模拟I/O操作，如文件或串行端口，每次只有一个线程可以访问它，并且它会阻塞直到完成。

4个线程启动。每个只执行一次计数。它工作得很好，我可以看到表单上的计数在增长。但是有一个按钮可以从表单线程中计数。当我推它时，主线程就冻结了。调试器显示，其他线程继续计数，一个接一个，但表单线程永远无法访问资源。

1)为什么锁(tty)从表单线程永远无法访问它，当其他线程没有问题?2)是否有更好的方法来实现这种类型的同步?

抱歉，代码太大了:

public class MegaAPI
{
    public int SomeStupidBlockingFunction(int c)
    {
        Thread.Sleep(800);
        return ++c;
    }
}
class UIThread
{
    public delegate void dlComandoMaquina();
    public class T0_SyncEvents
    {
        private EventWaitHandle _EventFechar; // Exit thread event
        public T0_SyncEvents()
        {
            _EventFechar = new ManualResetEvent(false);
        }
        public EventWaitHandle EventFecharThread // Exit thread event
        {
            get { return _EventFechar; }
        }
    }
    public class T0_Thread
    {
        private T0_SyncEvents _syncEvents;
        private int _msTimeOut;
        private dlComandoMaquina _ComandoMaquina;
        public T0_Thread(T0_SyncEvents e, dlComandoMaquina ComandoMaquina, int msTimeOut)
        {
            _syncEvents = e;
            _msTimeOut = msTimeOut;
            _ComandoMaquina = ComandoMaquina;
        }
        public void VaiRodar() // thread running code
        {
            while (!_syncEvents.EventFecharThread.WaitOne(_msTimeOut, false))
            {
                _ComandoMaquina();
            }
        }
    }
}
public partial class Form1 : Form
{
    MegaAPI tty;
    UIThread.T0_Thread thr1;
    UIThread.T0_SyncEvents thrE1;
    Thread Thread1;
    int ACount1 = 0;
    void UIUpdate1()
    {
        lock (tty)
        {
            ACount1 = tty.SomeStupidBlockingFunction(ACount1);
        }
        this.BeginInvoke((Action)delegate { txtAuto1.Text = ACount1.ToString(); });
    }
    UIThread.T0_Thread thr2;
    UIThread.T0_SyncEvents thrE2;
    Thread Thread2;
    int ACount2 = 0;
    void UIUpdate2()
    {
        lock (tty)
        {
            ACount2 = tty.SomeStupidBlockingFunction(ACount2);
        }
        this.BeginInvoke((Action)delegate { txtAuto2.Text = ACount2.ToString(); });
    }
    UIThread.T0_Thread thr3;
    UIThread.T0_SyncEvents thrE3;
    Thread Thread3;
    int ACount3 = 0;
    void UIUpdate3()
    {
        lock (tty)
        {
            ACount3 = tty.SomeStupidBlockingFunction(ACount3);
        }
        this.BeginInvoke((Action)delegate { txtAuto3.Text = ACount3.ToString(); });
    }
    UIThread.T0_Thread thr4;
    UIThread.T0_SyncEvents thrE4;
    Thread Thread4;
    int ACount4 = 0;
    void UIUpdate4()
    {
        lock (tty)
        {
            ACount4 = tty.SomeStupidBlockingFunction(ACount4);
        }
        this.BeginInvoke((Action)delegate { txtAuto4.Text = ACount4.ToString(); });
    }

    public Form1()
    {
        InitializeComponent();
        tty = new MegaAPI();
        thrE1 = new UIThread.T0_SyncEvents();
        thr1 = new UIThread.T0_Thread(thrE1, UIUpdate1, 500);
        Thread1 = new Thread(thr1.VaiRodar);
        Thread1.Start();
        thrE2 = new UIThread.T0_SyncEvents();
        thr2 = new UIThread.T0_Thread(thrE2, UIUpdate2, 500);
        Thread2 = new Thread(thr2.VaiRodar);
        Thread2.Start();
        thrE3 = new UIThread.T0_SyncEvents();
        thr3 = new UIThread.T0_Thread(thrE3, UIUpdate3, 500);
        Thread3 = new Thread(thr3.VaiRodar);
        Thread3.Start();
        thrE4 = new UIThread.T0_SyncEvents();
        thr4 = new UIThread.T0_Thread(thrE4, UIUpdate4, 500);
        Thread4 = new Thread(thr4.VaiRodar);
        Thread4.Start();
    }
    private void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
        thrE1.EventFecharThread.Set();
        thrE2.EventFecharThread.Set();
        thrE3.EventFecharThread.Set();
        thrE4.EventFecharThread.Set();
        Thread1.Join();
        Thread2.Join();
        Thread3.Join();
        Thread4.Join();
    }
    int Mcount = 0;
    private void btManual_Click(object sender, EventArgs e)
    {
        Cursor.Current = Cursors.WaitCursor;
        lock (tty)  // locks here ! Never runs inside! But the other threads keep counting..
        {
            Mcount = tty.SomeStupidBlockingFunction(Mcount);
            txtManual.Text = Mcount.ToString();
        }
        Cursor.Current = Cursors.Default;
    }
}