How to force Paralled.Invoke to wait for all tasks to complete?

Question

I am using Parallel.invoke for executing multiple tasks but it returns the response before all the operations are completed. I want to use await keyboard while calling methods as all the methods are async but it doesn't allow me to do so. Also I know I can use Task.WhenAll() but the requirement is to use Parallel.Invoke so is there any way to achieve my goal?

var list = new List<string>();
var myArr = new string[] { "usa", "eng", "fra"  };

Parallel.Invoke(() =>
{
    myArr.Select(i => {
        var data= MyMethod(i);
        list.Add(1, data);
    });
});

and the MyMethod implementation

public async Task<string> GetName(string country)
{
    var data = await _db.GetAll().FindAsync(country).Capital;
    return data;
}

Also I have another question is there any better approach to call the method 3 or 4 times inside Parallel.Invoke?

`Parallel.Invoke()` waits for all the tasks that it spawns to complete. You must have a mistake in your code somewhere. — Matthew Watson, Jul 20 '18 at 10:39
Thanks for the response. let me recheck my code. Also another thing that I forgot to mention in question that I want add the response returned by async method in a list so I can do that without await keyword? – Ask just now — Ask, Jul 20 '18 at 10:42
If those methods return a `Task` then `Parallel.Invoke` will return immediately because it doesn't care if the tasks are running or not. — Enigmativity, Jul 20 '18 at 10:45
method1 is basically an async method that returns a response from http request. I want to add that response in list like {var a = method1(); list.Add(a)} — Ask, Jul 20 '18 at 10:49
Can you show the whole code? The request and everything? I think I have a solution that would encompass the entire thing that would be better than doing just this one bit. — Enigmativity, Jul 20 '18 at 10:52
@Backs, you're right. Actually I was working on that part but I thought first I should fix the first issue than I'll go to the next one — Ask, Jul 20 '18 at 11:10
@Ask - You mentioned a HTTP request, but the code doesn't show it. Your code shows a `_db` but you don't say what it is. There's a `MyMethod` call, but you don't provide details for that. We need a [mcve] please. — Enigmativity, Jul 20 '18 at 13:34

Backs · Answer 1 · 2018-07-20T11:25:32.270

1

var list = new List<string>();
var myArr = new string[] { "usa", "eng", "fra" };

var tasks = myArr.Select(o => GetName(o));

var results = await Task.WhenAll(tasks);

edited Jul 20 '18 at 11:25

answered Jul 20 '18 at 11:12

Backs

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I already had this solution but my friend was convinced to use Paraller.Invoke – Ask Jul 20 '18 at 11:16
@Ask using Task, async, await is more preferable way in modern C# solutions – Backs Jul 20 '18 at 11:17
@Ask your friend is wrong – Liam Jul 20 '18 at 11:18
Thanks, One last question. can I use Paraller.ForEach() here? – Ask Jul 20 '18 at 11:19
@Ask don't mix Parallel and Task - it's bad idea, try to use only one way – Backs Jul 20 '18 at 11:20
Maybe but the [semantics are tricky](https://stackoverflow.com/questions/11564506/nesting-await-in-parallel-foreach) – Liam Jul 20 '18 at 11:20
Why does this have a `Task.Run`? `GetName` already returns a `Task`. There is no benefit in utilising another thread pool thread here. – Liam Jul 20 '18 at 11:24
1

@Liam yes, you are right about Task.Run, but return is needn't – Backs Jul 20 '18 at 11:25

Liam · Accepted Answer · 2018-07-20T12:17:02.150

1

Also I know I can use Task.WhenAll() but the requirement is to use Parallel.Invoke so is there any way to achieve my goal?

Your requirement to use Parallel.Invoke is likely wrong.

Your GetName method is IO bound. There is very little benefit in threading this. All you're doing is spawning more threads that you then have to await the async result of. This is less efficient than just looping.

It would make more sense to run this asynchronously. That way you don't use unnecessary threads but you use your main thread more effectively. You can do this using Task.WhenAll:

var list = new List<string>();
var myArr = new string[] { "usa", "eng", "fra"  };
IEnumerable<Task<string>> myTasks = myArr.Select(i => {
                    GetName(i);
                });

await Task.WhenAll(myTasks);
foreach(Task<string> task in myTasks)
{
     string result = task.Result;
}

edited Jul 20 '18 at 12:17

answered Jul 20 '18 at 11:13

Liam

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thanks, another thing I might need to add the GetName() response in dictionary instead of a list. So is this approach fine for(int i = 0 i <= myTasks.Length -1; i++) { dictionary.Add(myArr(i), myTasks[i]); } Or is there any better approach? – Ask Jul 20 '18 at 11:43
You'd need to process the `Dictionary` after your async calls have returned I think. But it's not clear what your asking in comments. Probably ask a new question – Liam Jul 20 '18 at 12:15

How to force Paralled.Invoke to wait for all tasks to complete?

2 Answers2