Task Parallel Library(TPL) : Adding parallelism and concurrency to program.

TaskParallelLibrary-C#.md

Custom Task parallel library

Method that implement parellelism

public static class TaskParallelLibrary
{
    public static async Task<IEnumerable<TResult>[]> ProcessListWithParallelismAsync<TSource, TResult>(
        this IEnumerable<TSource> source,
        Func<TSource, Task<IEnumerable<TResult>>> workItem, 
        int maxDegreeOfParallelism)
    {
        ConcurrentQueue<Func<Task<IEnumerable<TResult>>>> taskQueue = new ConcurrentQueue<Func<Task<IEnumerable<TResult>>>>
        (source.Select<TSource, Func<Task<IEnumerable<TResult>>>>(element => () => workItem(element)));
        Task<IEnumerable<TResult>>[] processors = new Task<IEnumerable<TResult>>[maxDegreeOfParallelism];

        for (int i = 0; i < maxDegreeOfParallelism; i++)
        {
            processors[i] = RunTasksOnSingleProcessorAsync(taskQueue);
        }

        return await Task.WhenAll(processors).ConfigureAwait(false);
    }

    private static async Task<IEnumerable<TResult>> RunTasksOnSingleProcessorAsync<TResult>(
        ConcurrentQueue<Func<Task<IEnumerable<TResult>>>> queue)
    {
        List<TResult> result = new List<TResult>();

        while (!queue.IsEmpty)
        {
            if (queue.TryDequeue(out Func<Task<IEnumerable<TResult>>> workItem))
            {
                result.AddRange(await workItem().ConfigureAwait(false));
            }
        }

        return result;
    }
}

Use ProcessListWithParallelismAsync method

Call GetStringsFromInt with parallelism

List<int> ints = new List<int> { 1, 3, 5, 7, 9 };
IEnumerable<string> result = (await ints.ProcessListWithParallelismAsync(i => GetStringsFromInt(i), 2).ConfigureAwait(false))
    .SelectMany(_ => _);

private async Task<IEnumerable<string>> GetStringsFromInt(int i)
{
    return await Task.FromResult(new List<string> { $"{i}", $"{i + 1}" }).ConfigureAwait(false);
}