C＃中的线程问题

Question

I am trying to generate a random fruit and display it on GUI in a label. I am using this code to do it.

 partial class Form1 : Form
 {
    int MagicNumber = 0;
    List<string> NameList = new List<string>();
    Random r = new Random();

    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        NameList.Add("Apples");
        NameList.Add("Pears");
        NameList.Add("Oranges");
        NameList.Add("Bananas");
        NameList.Add("Kiwi");

        for (int i = 0; i < 8; i++)
        {
            Thread t = new Thread(new ThreadStart(Display));
            t.Start();

            label1.Text = NameList[MagicNumber];
            Thread.Sleep(1000);
        }
   }

    private void Display()
    {
        MagicNumber = r.Next(5);
    }
}

The problem is the fact that in GUI i see only the last result of fruits choice and not how they are skipped from an iteration to other. I thought that this code will give me the possibility to see how fruits changes until the last was chosen , when i is 8.

Please if you have an idea why this code is not displaying how the fruits are chosen in label give me a hand !

Thanks.

Answer 1

You seem to be confusing timers and threads. In this case, I think what you want is a timer; specifically, System.Windows.Forms.Timer . You might do something like this:

partial class Form1 : Form
{
   Timer timer = new Timer();

   private void button1_Click(object sender, EventArgs e)
   {
      int i = 0;

      timer.Tick += (s, e) =>
      {
         if (i < 8)
         {
               label1.Text = nameList[r.Next(5)];
               i++;
         }
         else
            timer.Stop();
      };

      timer.Interval = 1000;
      timer.Start();
   }
}

The idea is that you set a timer to tick once a second, and then each time it ticks, you change the label and increment the counter until it reaches 8 -- at which point it stops. You always want to make sure you call Start() after you've set Tick and Interval ; otherwise, under some strange circumstances, the timer might tick before you have a chance to change the settings.

Alternatively, you could use threading and Sleep(), in which case it might look like this:

private void button1_Click(object sender, EventArgs e)
{
   Thread t = new Thread(new ThreadStart(Display));
   t.Start();
}

private void Display()
{
   for(int i = 0; i < 8; i++)
   {
      label1.Text = NameList[r.Next(5)];
      Thread.Sleep(1000);
   }
}

Thread.Sleep() always sleeps the thread that it's called from -- so maybe this is what you meant to do.

However, this might throw a thread synchronization exception -- Forms prevents you from accessing UI controls from another thread, since it might be in an invalid state (ie in the middle of rendering or doing something else that's volatile). System.Windows.Forms.Timer actually runs on the UI thread, so it's easier to manage.

Answer 2

Your approach is flawed, but you may want to understand what is going on in your code, as it may help you find a better approach:

   for (int i = 0; i < 8; i++)
    {
        Thread t = new Thread(new ThreadStart(Display));
        t.Start();

        label1.Text = NameList[MagicNumber];
        Thread.Sleep(1000);
    }

You are looking through, creating eight threads every time the button is clicked. Do you have a reason to create eight threads? If so, you may want to create them once, inside your init function and reuse them.

Then there is a race here in that your threads may not have had time to change MagicNumber before it is used, as the loop starts the threads then immediately changes the text, before going to sleep.

The sleep is another problem, as you haven't gotten off of the main (event) thread, so the text isn't changed until you exit that event handler.

If you want to see the text changing, then you will need to get off of the main thread, and in a second thread go through and do the loop of eight.

Then, you can put that thread to sleep, and since the main thread was free to make the change you will see it.

Here is an article from MS that is a bit dated, but the basic idea should help you:

http://msdn.microsoft.com/en-us/magazine/cc188732.aspx

Now you can use lambda expressions for your threads, as shown here:

http://www.rvenables.com/2009/01/threading-tips-and-tricks/

Answer 3

Your observed problem of the form not refreshing is due to your function blocking the GUI thread and preventing a redraw of the window while its running. And it's continuously running for 8 seconds. The GUI thread needs to handle messages to allow a window to be redrawn.

But apart from what you observed it has has at least two theoretical problems related to threading:

1. The read of MagicNumber isn't volatile, so the compiler may read it only once and cache the result. It probably won't do that in practice since the code between each reading of the variable is so complicated that it can't guarantee that they won't affect the variable.
2. r.Next isn't threadsafe. So calling it from two different threads at the same time can corrupt the Random instance. Won't happen in practice either since the delay is so long that one thread will most likely have finished before the next one starts.

Answer 4

The problem is that when you execute an event handler or a function called from it, the changes are rendered at the end. Try changhing the label text inside the thread where you get the random number. You also have to set the CheckForIllegalCrossThreadCalls property to false in the form constructor.

Answer 5

Just call Application.DoEvents(); after assigning text to label - that will refresh UI.

BTW I don't understand why you are using threads to generate random numbers

Answer 6

There is a much better way to choose a random item:

label1.Text = NameList.OrderBy(f => Guid.NewGuid()).First();

Randomizing on different threads is a bad idea in of itself.