Asynchronous population of collection initializer

Question

I would like to populate a collection using collection initializer that will call async methods:

public class Diagnostics
{
    public async Task<IEnumerable<DiagnosticsInfo>> Get() => new List<DiagnosticsInfo>
    {            
        await GetDiagnosticsInfo1(),
        await GetDiagnosticsInfo2(),
        await GetDiagnosticsInfo3()
    };
}

The aim is for all the GetDiagnosticsInfoX() methods to execute in parallel. I have a suspicion that the generated code will invoke GetDiagnosticsInfoX() methods synchronously - it will await the first call before invoking the second.

Is my gut feel correct?

If so I assume I would need to await all the tasks like this:

public class Diagnostics
{
    public async Task<IEnumerable<DiagnosticsInfo>> Get()
    {
        var task1 = GetDiagnosticsInfo1();
        var task2 = GetDiagnosticsInfo2();
        var task3 = GetDiagnosticsInfo3();

        await Task.WhenAll(task1, task2, task3);

        return new List<DiagnosticsInfo> {task1.Result, task2.Result, task3.Result};
    }
}

Is my gut feel correct? yes. ;)

M.kazem Akhgary
– M.kazem Akhgary

2015-11-26 12:18:16 +00:00
Commented Nov 26, 2015 at 12:18 — M.kazem Akhgary
– M.kazem Akhgary, Commented Nov 26, 2015 at 12:18

Yuval Itzchakov · Accepted Answer · 2015-11-26 12:23:35Z

2

Is my gut feel correct?

Your gut feeling is right. All collection initializer does is invoke the Add method for the collection. This means your code is translated to:

public async Task<IEnumerable<DiagnosticsInfo>> Get()
{
    var list = new List<DiagnosticsInfo>();         
    list.Add(await GetDiagnosticsInfo1());
    list.Add(await GetDiagnosticsInfo2());
    list.Add(await GetDiagnosticsInfo3());
    return list;
}

Using Task.WhenAll will wait for all async methods to complete. You can then return the array you receive:

public async Task<IEnumerable<DiagnosticsInfo>> Get()
{
    var task1 = GetDiagnosticsInfo1();
    var task2 = GetDiagnosticsInfo2();
    var task3 = GetDiagnosticsInfo3();

    return await Task.WhenAll(task1, task2, task3);
}

edited Nov 26, 2015 at 12:23

answered Nov 26, 2015 at 12:18

Yuval Itzchakov

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5 Comments

Jakub Konecki Over a year ago

return await Task.WhenAll() - nice

M.kazem Akhgary Over a year ago

i think removing return is enough.

Yuval Itzchakov Over a year ago

@M.kazemAkhgary What do you mean?

M.kazem Akhgary Over a year ago

return await not works. compile time error. cannot convert void to Collection.Generics blah blah

Yuval Itzchakov Over a year ago

@M.kazemAkhgary It would only be void if the return value from GetDiagnosticsInfo is a Task, which it isn't. It's a Task<DiagnosticsInfo>.

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