using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading;

using System.Threading.Tasks;

using Nito.AsyncEx;

​

public class Program

{

    public static void Main()

    {

        AsyncContext.Run(() =>

        {

            ParallelOptions options = new()

            {

                MaxDegreeOfParallelism = 2,

                TaskScheduler = new SyncCtxScheduler(SynchronizationContext.Current)

            };

​

            textBox1.AppendText(@$"MaxDegreeOfParallelism: {options

                .MaxDegreeOfParallelism}, MaximumConcurrencyLevel: {options

                .TaskScheduler.MaximumConcurrencyLevel}" + "\r\n\r\n");

​

            Parallel.ForEach(Enumerable.Range(1, 5), options, x =>

            {

                textBox1.AppendText($"Processing #{x}\r\n");

                Thread.Sleep(200); // Simulate a blocking I/O-bound operation.

                    textBox1.AppendText($"Completed #{x}\r\n");

            });

        });

    }

​

    internal sealed class SyncCtxScheduler : TaskScheduler

    {

        private readonly SynchronizationContext _ctx;

        public SyncCtxScheduler(SynchronizationContext ctx) => _ctx = ctx;

        protected override void QueueTask(Task task) => _ctx.Post(s => TryExecuteTask((Task)s), task);

        protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => SynchronizationContext.Current == _ctx ? TryExecuteTask(task) : false;

        protected override IEnumerable<Task> GetScheduledTasks() => null;

    }

​

    private static class textBox1 // Simulate updating the UI of a WinForms app.

    {

        public static void AppendText(string text) => Console.Write(text);

    }

}

​