Question

i am trying to make webrequests with multiple threads but if i try with more than 2 i get error

Index was outside the bonds of the array

on this line:

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

Here’s the code:

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

This code is not thread safe.

You need the code for performing an HttpWebRequest to be atomic and outside the context of looping through the collection.

For example

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 { }


        }
    }
}

Assuming ScrapeBox.Lines implements IEnumerable, I would reccomend using Parallel.ForEach and passing the ScrapeBox.Lines as the IEnumerable over which to iterate.

Now, there is one additional problem, the code for reading the response from the HttpWebRequest still needs to write its output to a shared location. To accomplish that in a thread safe manner. A common way to do this is with a semaphore. You need a object accessible to each thread instance. A class level private variable private object sharedMutex = new object(); would work. Then the code ExtractEmails(strippedResponse); should be changed to
lock(sharedMutex)
{
ExtractEmails(strippedResponse);
}

Without having the code for the ExtractEmails(<string>) method, I can’t provide a thread safe implementation for that, so that part of the solution may still cause a problem.