C# - Question about events (also textbox clearing and starting/stopping a timer)

You can do locking to prevent threads from stepping on each other

class ThreadSafe
{
static readonly object _locker = new object();
static int _val1, _val2;
static void Go()
{
lock (_locker)
{
if (_val2 != 0) Console.WriteLine (_val1 / _val2);
_val2 = 0;
}
}
}

Here is a way to communicate between threads.

class TwoWaySignaling
    {
        static EventWaitHandle _ready = new AutoResetEvent(false);
        static EventWaitHandle _go = new AutoResetEvent(false);
        static readonly object _locker = new object();
        static string _message;
        static void Main()
        {
            new Thread(Work).Start();
            _ready.WaitOne(); // First wait until worker is ready
            lock (_locker) _message = "ooo";
            _go.Set(); // Tell worker to go
            _ready.WaitOne();
            lock (_locker) _message = "ahhh"; // Give the worker another message
            _go.Set();
            _ready.WaitOne();
            lock (_locker) _message = null; // Signal the worker to exit
            _go.Set();
        }
        static void Work()
        {
            while (true)
            {
                _ready.Set(); // Indicate that we're ready
                _go.WaitOne(); // Wait to be kicked off...
                lock (_locker)
                {
                    if (_message == null) return; // Gracefully exit
                    Console.WriteLine(_message);
                }
            }
        }
    }