AsyncCollector usage example

AsyncCollectorExample.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace App
{
    class Program
    {
        static async Task Main(string[] args)
        {
            // Instance received from an external API.
            var transport = await FetchFromWebAPIAsync();

            // Integrate the received data with our values.
            await IntegrateDataAsync(transport);
        }

        private static async Task IntegrateDataAsync(Transport transport)
        {
            var collector = new AsyncCollector();

            collector.Register(async () => transport.Vehichle.Name = await GetVehichleNameAsync(transport.Vehichle.Id));

            if (transport.Stops != null)
            {
                foreach (var s in transport.Stops)
                    collector.Register(async () => s.Description = await GetStopDescriptionAsync(s.Id));
            }

            await collector.WhenAll();
        }

        #region Other methods

        private static Task<Transport> FetchFromWebAPIAsync()
        {
            var t = new Transport
            {
                Vehichle = new Vehichle { Id = "1234", Name = null },
                Stops = new List<Stop>
                {
                    new Stop { Id = "a1", Time = DateTime.Now.AddSeconds(1), Description = null },
                    new Stop { Id = "b2", Time = DateTime.Now.AddSeconds(5), Description = null },
                    new Stop { Id = "c3", Time = DateTime.Now.AddSeconds(9), Description = null },
                }
            };

            return Task.FromResult(t);
        }

        private static Task<string> GetVehichleNameAsync(string vehicleId) => Task.FromResult($"A vehichle: {vehicleId}");

        private static Task<string> GetStopDescriptionAsync(string stopId) => Task.FromResult($"A stop: {stopId}");

        #endregion

        #region Models

        class Transport
        {
            public Vehichle Vehichle { get; set; }
            public List<Stop> Stops { get; set; }
        }

        class Vehichle
        {
            public string Id { get; set; }
            public string Name { get; set; }
        }

        class Stop
        {
            public string Id { get; set; }
            public DateTime Time { get; set; }
            public string Description { get; set; }
        }

        #endregion

        #region Async collector

        class AsyncCollector
        {
            public IReadOnlyList<Task> Tasks => _tasks.AsReadOnly();

            private readonly List<Task> _tasks = new List<Task>();

            public void Register(Func<Task> asyncDelegate) =>
                this.Register(asyncDelegate, out _);

            public void Register(Func<Task> asyncDelegate, out Task registeredTask)
            {
                registeredTask = asyncDelegate();
                _tasks.Add(registeredTask);
            }

            public async Task WhenAll(bool clearAfterwards = true)
            {
                try
                {
                    await Task.WhenAll(_tasks);
                }
                finally
                {
                    if (clearAfterwards)
                        _tasks.Clear();
                }
            }

            public void Clear() => _tasks.Clear();
        }

        #endregion
    }
}