在 while() 循环中使用 ManualResetEvent()

Question

i have a question about my simply code:

 public partial class Form1 : Form {

  ManualResetEvent ResetEvt = new ManualResetEvent(false);
  Thread t1;
  public Form1() {
     InitializeComponent();
  }
  //create
  private void button1_Click(object sender, EventArgs e) {
     t1 = new Thread(new ParameterizedThreadStart((reset) => {
        int cont = 0;
        var resett = reset as ManualResetEvent;
        Thread.CurrentThread.IsBackground = true;
        while (resett.WaitOne()) {
           try {
              this.BeginInvoke(new Action(() => this.richTextBox1.AppendText("Ciao " + cont)));
              cont++;
              Thread.Sleep(500);
           }
           catch (Exception) {
              MessageBox.Show("END");
           }
        }
        this.BeginInvoke(new Action(() => this.richTextBox1.AppendText("ok finished " + cont)));
     }));
  }

  private void button2_Click(object sender, EventArgs e) {

     try {
        t1.Start(this.ResetEvt);
     }
     catch (Exception) {

     }
  }

  private void button3_Click(object sender, EventArgs e) {
     this.ResetEvt.Reset();
  }

  private void button4_Click(object sender, EventArgs e) {
     ResetEvt.Set();
  }

  private void button5_Click(object sender, EventArgs e) {
     // ??? ho can stop while? and continue execution?
  }

}

Weel, how to use ManualResetEvent is clear and simple, but why the line this.BeginInvoke(new Action(() => this.richTextBox1.AppendText("ok finished " + cont))); will be never executed? I can stop my thread (.Reset) and restart it (.Set), but when resett.WaitOne() became FALSE why my code don't jump to this.BeginInvoke(new Action(() => this.richTextBox1.AppendText("ok finished " + cont))); ?

thanks a lot.

Answer 1

I' not sure if I can see what you want with your code but you can do something like this:

while (!resett.WaitOne(500))
{
}

and then have a third button to stop the loop as this:

private void button3_Click(object sender, EventArgs e)
{
  ResetEvt.Set();
}

The WaitOne() without argument blocks the current thread until the handler i set, but with an argument in milliseconds it returns after that time, but with false as long as the handler is not set.

Answer 2

ManualResetEvent.WaitOne() will never return false. Read the documentation .

ManualResetEvent is a simple signalling primitive. It only has two logical values - set, and not set. When you WaitOne on an event that isn't set, it will block until the event is set. That's it - WaitOne doesn't return a boolean indicating the state of the event - it will simply return only when the event is set.

The return value is used for the overloads that take timeouts. In that case, WaitOne will return false when the timeout expires, and true if the event was signalled in the meantime.

Multi-threading is hard - you don't want to guess around. I'd strongly recommend going at the very least through an introductory text to multi-threading - http://www.albahari.com/threading/ is a great free resource that gives nice explanations that get you most of the way to at least understanding the issues you'll be facing.

In your case, it seems that what you're really looking for is a Timer . Or better, if you're waiting for some asynchronous event to happen, use asynchronous code - that avoids all the busy-looping, as well as the pointless Thread s.

There's other issues with your code as well. You might want to take your (working) code to CodeReview for a few pointers.