Problem with Multi - thread Parameters using delegate in compact framework

Question

My issue is that I have a List of strings, and I want to create one thread for one string, pass the string into the thread. This is my code:

public void getImageSource(List<string> UrlLinks)
          foreach (string urlLink in UrlLinks)

            {
                ThreadStart myThread = delegate { Fetch(urlLink); };
                Thread t = new Thread(myThread);
                t.Priority = ThreadPriority.Lowest;
                t.IsBackground = true;
                t.Start();

            }

public void Fetch(string Link)
    {
        MessageBox.Show(Link);
    }

But all my messagebox return the same results, the first element in the List. How can I fix it? Thanks in advance.

Answer 1

You're running into the captured variable issue... you're capturing the urlLink variable, not the value at the time that the delegate is created. The fix for this is to create a temporary variable inside the loop:

public void getImageSource(List<string> UrlLinks)
{
    foreach (string urlLink in UrlLinks)
    {
        string urlCopy = urlLink;
        ThreadStart myThread = delegate { Fetch(urlCopy); };
        Thread t = new Thread(myThread);
        t.Priority = ThreadPriority.Lowest;
        t.IsBackground = true;
        t.Start();
    }
}

I wouldn't expect the first URL to be fetched multiple times though - it would be more likely that the last URL would be fetched (or some variety of URLs).

See Eric Lippert's blog post on this topic for more information.