c# - How to use await in a loop

Question

I'm trying to create an asynchronous console app that does a some work on a collection. I have one version which uses parallel for loop another version that uses async/await. I expected the async/await version to work similar to parallel version but it executes synchronously. What am I doing wrong?

class Program
{
    static void Main(string[] args)
    {
        var worker = new Worker();
        worker.ParallelInit();
        var t = worker.Init();
        t.Wait();
        Console.ReadKey();
    }
}

public class Worker
{
    public async Task<bool> Init()
    {
        var series = Enumerable.Range(1, 5).ToList();
        foreach (var i in series)
        {
            Console.WriteLine("Starting Process {0}", i);
            var result = await DoWorkAsync(i);
            if (result)
            {
                Console.WriteLine("Ending Process {0}", i);
            }
        }

        return true;
    }

    public async Task<bool> DoWorkAsync(int i)
    {
        Console.WriteLine("working..{0}", i);
        await Task.Delay(1000);
        return true;
    }

    public bool ParallelInit()
    {
        var series = Enumerable.Range(1, 5).ToList();
        Parallel.ForEach(series, i =>
        {
            Console.WriteLine("Starting Process {0}", i);
            DoWorkAsync(i);
            Console.WriteLine("Ending Process {0}", i);
        });
        return true;
    }
}

Answer 1

The way you're using the await keyword tells C# that you want to wait each time you pass through the loop, which isn't parallel. You can rewrite your method like this to do what you want, by storing a list of Tasks and then awaiting them all with Task.WhenAll.

public async Task<bool> Init()
{
    var series = Enumerable.Range(1, 5).ToList();
    var tasks = new List<Task<Tuple<int, bool>>>();
    foreach (var i in series)
    {
        Console.WriteLine("Starting Process {0}", i);
        tasks.Add(DoWorkAsync(i));
    }
    foreach (var task in await Task.WhenAll(tasks))
    {
        if (task.Item2)
        {
            Console.WriteLine("Ending Process {0}", task.Item1);
        }
    }
    return true;
}

public async Task<Tuple<int, bool>> DoWorkAsync(int i)
{
    Console.WriteLine("working..{0}", i);
    await Task.Delay(1000);
    return Tuple.Create(i, true);
}