Parallel Programming C# - TPL - Update Shared Variable & Suggestions

Question

I am new to Parallel Programming and infact this is the first time I am trying it. I am currently doing a project in .NET 4 and prefer to do have 4 or 5 parallel executions.

I see some options. There is Task.Factory.StartNew Parallel.For Parallel.ForEach etc.

What I am going to do is post to a web-site and fetch the responses for about 200 URLs.

When I use Parallel.ForEach I didn't find a way to control the number of threads and the application went using 130+ threads and the website went unresponsive :)

I am interested in using Task.Factory.StartNew within a for loop and divide the URLs in to 4 or 5 tasks.

List<Task> tasks = new List<Task>();
for (int i = 0; i < 5; i++)
{
    List<string> UrlForTask = GetUrlsForTask(i,5); //Lets say will return some thing like 1 of 5 of the list of URLs
    int j = i;
    var t = Task.Factory.StartNew(() =>
    {
        List<PageSummary> t = GetSummary(UrlForTask);
        Summary.AddRange(t); //Summary is a public variable
    }
    tasks.Add(t);
}

I believe that these Tasks kind of boil down to threads. So if I make Summary a List<PageSummary> will it be kind of thread safe (I understand there are issues accessing a shared variable by multiple threads)?

Is this where we should use ConcurrentQueue<T> ?

Do you know of a good resource that helps to learn about accessing and updating a shared variable by multiple tasks etc?

What is the best way I could use for this type of task as you may think ?

Answer 1

Parallel.ForEach has overloads that take a ParallelOptions instance. The MaxDegreeOfParallelism property of that class is what you need to use.

List<MyRequest> requests = ...;
BlockingCollection<MyResponse> responses = ...;
Task.Factory.StartNew(() =>
{
    Parallel.ForEach(
        requests,
        new ParallelOptions { MaxDegreeOfParallelism = 4 },
        request => responses.Add(MyDownload(request)));
    responses.CompleteAdding();
});

foreach (var response in responses.GetConsumingEnumerable())
{
    Console.WriteLine(response.MyMessage);
}