Async task inside parallel.for loop

Question

I have an async method being called from with Parallel.For code below. Now looking at the code it is pretty straight forward with the exception JsonParse class has an static method all it does is calls the web service to download json string and converts that to an PairResults object and returns.

The issue I am having is the Parallel.For loop never exits, I can clearly see the data is coming from the webcall "item.part1=data.value" all working fine, but updateAllResults never finishes. What am I doing wrong?

public void updateAllResults()
{ 
    Parallel.For(0, PairList.Count(), (i) =>
    {            
         var item = PairList[i];
         var data = (Parse.JsonParse<PairResults>
                             .getJsonString("http://localhost:22354/" 
                                                        + item.Original)).Result;
         item.part1 = data.value;
    }); 
}

It should finish, but it could take a long time depending on the number of items in PairList. That being said, consider making this truly async by using async (i) => and await without the .Result call. — Kris Vandermotten
– Kris Vandermotten, Commented Mar 14, 2014 at 11:41
Ah that worked like magic. changing the method to async (i) => was the answer. — Sanj
– Sanj, Commented Mar 14, 2014 at 11:43

noseratio · Accepted Answer · 2014-03-14 12:33:49Z

This is an anti-patern for Parallel.For, because you're using it only to start async operations and block on them. This way, it will block a limited number of pool threads, and the actual level of parallelism may be much lower than if you were using tasks instead:

public void updateAllResults()
{
    var tasks = PairList.Select(async (item) => 
    {
        var data = await Parse.JsonParse<PairResults>
            .getJsonString("http://localhost:22354/" + item.Original).
            .ConfigureAwait(false);

        item.part1 = data.value;
    });

    Task.WaitAll(tasks.ToArray());
}

Also, your question is tagged with winforms tag. Both Parallel.For and the above code will block your UI thread, if you use it in a WinForms app. The right solution would be to use Task.WhenAll:

public async Task updateAllResults()
{
    var tasks = PairList.Select(async (item) => 
    {
        var data = await Parse.JsonParse<PairResults>
            .getJsonString("http://localhost:22354/" + item.Original)
            .ConfigureAwait(false);

        item.part1 = data.value;
    });

    await Task.WhenAll(tasks.ToArray());
}

// button click handler
async void button_Click(object s, EventArgs e)
{
    this.button.Enabled = false;
    try
    {
        await updateAllResults()
    }
    finally
    {
        this.button.Enabled = true;
    }
}

Emond · Accepted Answer · 2014-03-14 11:38:33Z

2

Parallel.For blocks until all iterations have been executed or there is a break from the loop just like a regular for statement. Make sure that all the data has been downloaded. After that the Parallel.For should finish and consequentially the method too.

answered Mar 14, 2014 at 11:38

Emond

50.8k11 gold badges86 silver badges118 bronze badges

Collectives™ on Stack Overflow

Async task inside parallel.for loop

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related