AsParallel.linq.cs

async Task Main()
{
	Action<WorkItem> actionItem = (WorkItem w) => w.DoWork();
	var jobs = (from x in Enumerable.Range(1, 64)
				let workItem = new WorkItem() { Id = x }
				select new ActionItem<WorkItem>()
				{
					item = workItem,
					action = (t) => actionItem(t)
				}).ToList();
	await Job<WorkItem>.Run(jobs, (done) =>
	{
		Console.WriteLine($"Task Id: {done.Id} is done on thread: {done.ThreadId} with result: {done.Result}.");
	});
	var rpt = from x in jobs.Select(x => x.item.ThreadId)
			  group x by x into grp
			  orderby grp.Key
			  select new { Thread = grp.Key, Count = grp.Count() };
	rpt.Dump();
}

public class ActionItem<T>
{
	public T item;
	public Action<T> action;
}

public static class Job<T>
{
	public static async Task Run(IEnumerable<ActionItem<T>> jobs, Action<T> finished)
	{
		var tasks = jobs.AsParallel().Select(job =>
		{
			return Task.Run(() =>
			{
				job.action(job.item);
				return job.item;
			});
		}).ToList();
		while (tasks.Any())
		{
			var done = await Task.WhenAny(tasks);
			tasks.Remove(done);
			finished(done.Result);
		}
	}
}
public class WorkItem
{
	public int Id;
	public int Result;
	public int ThreadId;
	public void DoWork()
	{
		ThreadId = Thread.CurrentThread.ManagedThreadId;
		Result = Id * 100;
		Thread.Sleep(125);
	}
}