Question

我正在尝试使用多个线程进行webrequests，但如果我尝试使用超过2个，我会收到错误

Index was outside the bonds of the array

在这一行：

string username = ScrapeBox1.Lines[NamesCounter].ToString();

以下是代码：

while (working)
{
    while (usernamescount > NamesCounter)
    {
        string username = ScrapeBox1.Lines[NamesCounter].ToString();
        string url = "http://www.someforum.com/members/" + username + ".html";
        var request = (HttpWebRequest)(WebRequest.Create(url));
        var response = request.GetResponse();
        request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; rv:16.0) Gecko/20100101 Firefox/16.0";

        using (var responseStream = response.GetResponseStream())
        {
            using (var responseStreamReader = new StreamReader(responseStream))
            {

                var serverResponse = responseStreamReader.ReadToEnd();
                int startpoint = serverResponse.IndexOf("Contact Info</span>");
                try
                {
                    string strippedResponse = serverResponse.Remove(0, startpoint);
                    ExtractEmails(strippedResponse);

                }
                catch { }


            }
        }
        NamesCounter++;
        textBox1.Text = NamesCounter.ToString();
    }

}

Answer 1

此代码不是线程安全的。

您需要用于执行HttpWebRequest的代码是原子的，并且在循环遍历集合的上下文之外。

例如

public void MakeHttpWebRequest(string userName)
{
    string url = "http://www.someforum.com/members/" + userName + ".html";
    var request = (HttpWebRequest)(WebRequest.Create(url));
    var response = request.GetResponse();
    request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; rv:16.0) Gecko/20100101 Firefox/16.0";

    using (var responseStream = response.GetResponseStream())
    {
        using (var responseStreamReader = new StreamReader(responseStream))
        {

            var serverResponse = responseStreamReader.ReadToEnd();
            int startpoint = serverResponse.IndexOf("Contact Info</span>");
            try
            {
                string strippedResponse = serverResponse.Remove(0, startpoint);
                ExtractEmails(strippedResponse);

            }
            catch { }


        }
    }
}

假设ScrapeBox.Lines实现了IEnumerable，我建议使用Parallel.ForEach并将ScrapeBox.Lines作为IEnumerable传递给它进行迭代。

现在，还有一个问题，用于从HttpWebRequest读取响应的代码仍然需要将其输出写入共享位置。以线程安全的方式实现这一点。执行此操作的常用方法是使用信号量。您需要一个可供每个线程实例访问的对象。类级私有变量private object sharedMutex = new object();可以工作。然后应将代码ExtractEmails(strippedResponse);更改为 lock(sharedMutex) { ExtractEmails(strippedResponse); }

如果没有ExtractEmails(<string>)方法的代码，我就无法为此提供线程安全的实现，因此解决方案的某些部分仍可能导致问题。

多线程webrequests上的C＃错误

1 个答案: