ForEachAsyncAnalyzer 1.0.4

This package contains a Roslyn C# code analyzer and fixer for the following pattern where a ForEach statement is combined with an async method. The ForEach method can be a custom extension method like in the code below. ForEach is also a given method in the great MoreLinq nuget package. The MoreLinq package also has methods like Cartesian, Partition and Permutations which are really nice!

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

namespace ForEachAsyncAnalyzerTestCode
{
    public static class IEnumerableExtensions
    {
        public static async Task ForEachAsync<T>(this IEnumerable<T> source, Func<T, Task> action)
        {
            if(source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if(action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            foreach(var element in source)
            {
                await action(element).ConfigureAwait(false);
            }
        }

        public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
        {
            if(source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if(action == null)
            {
                throw new ArgumentNullException(nameof(action));
            }

            foreach(var element in source)
            {
                action(element);
            }
        }

        class Program
        {
            private static readonly int[] _numbers = { 1, 2, 3 };

            static async Task Main(string[] args)
            {
                // Flavour one. This is already detected as compiler warning in CS4014.
                // See https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/cs4014.
                // After fixing CS4014, you can end up in flavour two (more or less).
                // _numbers
                //     .ForEach(i => DoAsyncAction(i));

                // Flavour two.
                _numbers
                    .ForEach(async i => await DoAsyncAction(i).ConfigureAwait(false));

                // Just wait for all the tasks to finish their async tasks.
                await Task.Delay(2400).ConfigureAwait(false);
            }

            private static async Task DoAsyncAction(int i)
            {
                // The first number waits 700 ms, the second number 400 ms and number 3 waits 100 ms.
                var waitingTime = 1000 - i * 300;
                await Task.Delay(waitingTime).ConfigureAwait(false);
                Console.WriteLine($"{i}, {waitingTime}");

                // Throwing an exception at this point can give you a 'weird' call stack when this method is not awaited.
                // throw new Exception("Just a crash.");
            }
        }
    }
}

The code above just prints the numbers 1, 2 and 3 to the console. It depends on the case what you want: A) exactly this order B) any given order. When you want case A you will have to await the async task. I encountered some nasty bugs when this await wasn't done and for example an exception was thrown within the async method. Normally I always want to await the async task so that is why I build this analyzer.

It does detect the case above and advises you to fix it. So this line:

_numbers
    .ForEach(async i => await DoAsyncAction(i).ConfigureAwait(false));

becomes this line

await _numbers
    .ForEachAsync(i => DoAsyncAction(i)).ConfigureAwait(false);