如何从C＃中的另一个线程在主线程中设置变量？

Question

I'm trying to write a C# function to count broken links on a webpage using http requests. Since i want to make this quick, i create a thread for each request, and then simply increase the counter in the threads. My problem is, the counters stays on 0 at the end, although i know there are several broken links on the website. Seems like the threads are not setting the variables in the main thread.

public volatile int found;
public volatile int notfound;

    public void GetBrokenLinks(WebBrowser website)
    {
    HtmlElementCollection links = website.Document.GetElementsByTagName("a");

        foreach (HtmlElement element in links)
        {
            string link = element.GetAttribute("href").ToString();
                Uri urlCheck = new Uri(link);
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(urlCheck);
                request.Timeout = 10000;
                try
                {
                    Thread link_checher_thread = new Thread(delegate ()
                    {
                        HttpWebResponse response;

                        response = (HttpWebResponse)request.GetResponse();
                        if (response.StatusCode == HttpStatusCode.OK)
                        {
                            response.Dispose();
                            found++;
                        }
                        else if (response.StatusCode == HttpStatusCode.NotFound)
                        {
                            response.Dispose();
                            notfound++;
                        }
                    });
                    link_checher_thread.IsBackground = true;
                    link_checher_thread.Start();
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.ToString());
                }
        }
        MessageBox.Show(found.ToString() + ", " + notfound.ToString());
    }

I have searched on the internet for hours, tried volatile variables, but nothing seems to work. How could i force the threads to set the variables in the main thread?

Answer 1

The counter does not stay at 0 - the problems are deeper AND easier.

• ++ is not atomic and
• C# compiler and runtime can optimize our the read access away.

Bes thing: use the Interlocked class method's to increase AND read the class. Finished. They use an atomic API AND are made for multi threaded operations.

Answer 2

在.NET中增加共享计数器的正确方法是通过System.Threading.Interlocked.Increment(ref found)

Answer 3

There are two main issues in your code:

• You are not waiting for all the threads to complete before showing the result.
• ++ is not thread safe, you should use Interlocked.Increment to atomically increment the counter.

You could use Task from the .Net framework to do this easily.

public int found;
public int notfound;

public void GetBrokenLinks(WebBrowser website)
{
   HtmlElementCollection links = website.Document.GetElementsByTagName("a");

   List<Task> tasks = new List<Task>();

   foreach (string element in links)
        {
            string link = element.GetAttribute("href").ToString();

            tasks.Add(Task.Factory.StartNew(() =>
            {
                Uri urlCheck = new Uri(link);
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(urlCheck);
                request.Timeout = 10000;
                HttpWebResponse response;

                response = (HttpWebResponse)request.GetResponse();
                if (response.StatusCode == HttpStatusCode.OK)
                {
                    response.Dispose();
                    Interlocked.Increment(ref found);
                }
                else if (response.StatusCode == HttpStatusCode.NotFound)
                {
                    response.Dispose();
                    Interlocked.Increment(ref notfound);
                }
            }
            ));
        }

        try
        {
            Task.WaitAll(tasks.ToArray());
        }
        catch (AggregateException ae)
        {
            ae.Handle((e) => { MessageBox.Show(e.ToString()); return true; });
        }


        MessageBox.Show(found.ToString() + ", " + notfound.ToString());
}