How to return another value from method in "third party" lib?

Question

Task<string>[] tableOfWebClientTasks = new Task<string>[taskCount];

for (int i = 0; i < taskCount; i++)
{
    tableOfWebClientTasks[i] = new WebClient().DownloadStringTask(allUrls[count - i - 1]);
}

Task.Factory.ContinueWhenAll(tableOfWebClientTasks, tasks =>
{
    Parallel.ForEach(tasks, task =>
    {
        //Here I have result from each task.
        //But information which url is executed on this task, is lost.
    });
});

I could, for example, create class (with two public property, one for task, and second for url) and return instance. But This method i connected with others methods.

Have you some solution for this issue?

Why are you using ContinueWhenAll() and Paralell.ForEach()? Wouldn't ContinueWith() on each Task be better? — svick
– svick, Commented May 20, 2012 at 11:35
See here: blogs.msdn.com/b/pfxteam/archive/2010/05/04/10007557.aspx — mike00
– mike00, Commented May 20, 2012 at 11:42
That post doesn't explain why are you doing it this way. Why wouldn't ContinueWith() on each Task be suitable for you? — svick
– svick, Commented May 20, 2012 at 12:07

RichardTowers · Accepted Answer · 2012-05-20 11:54:00Z

1

If you want to be able to associate your tasks with the url that created them you could use a dictionary to do the mapping:

Task<string>[] tableOfWebClientTasks = new Task<string>[taskCount];
var taskIdToUrl = new Dictionary<int,string>();

for (int i = 0; i < taskCount; i++)
{
    var url = allUrls[count - i - 1];
    var task = new WebClient().DownloadStringTask(url);
    tableOfWebClientTasks[i] = task;
    taskIdToUrl.Add(task.Id, url);
}

TaskFactory.ContinueWhenAll(tableOfWebClientTasks, tasks =>
{
    Parallel.ForEach(tasks, task =>
    {
        // To get the url just do:
        var url = taskIdToUrl[task.Id];
    });
});

answered May 20, 2012 at 11:54

RichardTowers

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3 Comments

svick Over a year ago

I think doing it think way is not a good idea in general (although it might work in this specific case). For example, some async method could return the same Task for multiple different inputs, which would break your code.

RichardTowers Over a year ago

Yes. Unless there's some concrete reason to do things this way, I'd agree with calling ContinueWith() on each Task.

mike00 Over a year ago

@RichardTowers Thanks, indeed such a thing I wanted.

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