Converting a Parallel.ForEach loop into parallel tasks using lambda expression

Question

Need help converting this to a lamdba expression where I can use Task.WhenAll:

public void DoWork(string id)
{            
       var items = GetItems(id);  //takes time

       if (items == null)
       {
            return;
       }

       Parallel.ForEach(items, item =>
       {
            DoSomething(item); //takes time
            DoWork(item.subItemId);                    
       });           
}

I would like to change the Parallel For loop into tasks using a lambda expressions as suggested in Martin's answer in this thread: How to call an async method from within a loop without awaiting?

Not sure how to specify the multiple line after item =>. This doesn't seem to work:

public void DoWork(string id)
{            
       var items = GetItems(id);  //takes time

       if (items == null)
       {
           return;
       } 

       var tasks = items.Select(item => 
       {
           DoSomething(item)
           DoWork(item.subItemId)
       });      

       await Task.WhenAll(tasks);         
}

UPDATE:

Thank you all for your answers. If I have an if condition, would I be wasting tasks? Is there a better to code this? Do I need to be using Where instead?

var tasks = items.Select(async item => 
{ 
    if (item.HasWork)
    {
       await DoSomethingAsync(item);
       await DoWorkAsync(item.subItemId);
    }
});      

await Task.WhenAll(tasks);

Note that you should be somewhat wary of recursive async methods. It's probably worth transforming this method into being iterative, rather than recursive. — Servy
– Servy, Commented Jul 29, 2014 at 15:31

Community · Accepted Answer · 2017-05-23 12:13:59Z

7

It's easiest to use an async lambda:

public async Task DoWorkAsync(string id)
{            
  var items = GetItems(id);  //takes time

  if (items == null)
    return;

  var tasks = items.Select(async item => 
  { 
    await DoSomethingAsync(item);
    await DoWorkAsync(item.subItemId);
  });      

  await Task.WhenAll(tasks);         
}

Martin's original answer assumes you'd write your own async method instead of an async lambda, which may make the code clearer:

public async Task DoWorkAsync(string id)
{            
  var items = GetItems(id);  //takes time

  if (items == null)
    return;

  var tasks = items.Select(item => ProcessAsync(item));

  await Task.WhenAll(tasks);         
}

private async Task ProcessAsync(T item)
{
  await DoSomethingAsync(item);
  await DoWorkAsync(item.subItemId);
}

edited May 23, 2017 at 12:13

CommunityBot

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answered Jul 29, 2014 at 15:28

Stephen Cleary

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8 Comments

Prabhu Over a year ago

thanks. Question is, do I need to make DoSomething and DoWork as Async? Could I keep them as regular sync methods and follow Selman22's or Castro's answer, and still get the full benefit of parallel processing?

Stephen Cleary Over a year ago

@Prabhu: You stated in a comment that you get the "compiler warning saying I am not using the await keyword for DoSomething and DoWork". So I infer they're already asynchronous; I just renamed them to follow the naming convention.

Prabhu Over a year ago

Sure. But DoSomething and DoWork are my methods, so I am able to make then non-async if it will make things simpler. Is there a benefit of keeping them async?

Stephen Cleary Over a year ago

They should be asynchronous if they have asynchronous work to do (usually I/O). They should be synchronous if they have synchronous work to do (usually CPU computation). Let the operation determine how the method looks; don't force the method to be a certain way, imposing a mismatching signature onto the operation.

Prabhu Over a year ago

do you see a potential problem with DoWorkAsync being recursive?

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