Code for blog article https://honestillusion.com/2021/12/17/TaskCollection.html

TaskCollection.cs

// Copyright (c) 2019-2020 James M. Curran/Novel Theory LLC.  All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

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

namespace NovelTheory.Common
{
	public class TaskCollection
	{
		private bool _frozen = false;
		private readonly object _lockList = new object();
		private List<Task> _tasks = new List<Task>();

		public static TaskCollection operator +(TaskCollection coll, Task task)
		{
			if (coll._frozen)
				throw new InvalidOperationException("Cannot add to collection once Wait is started.");

			lock(coll._lockList)
				coll._tasks.Add(task);

			return coll;
		}

		/// <summary>
		/// Creates a task that will complete when all in Task objects in the collection have completed.
		/// </summary>
		/// <returns></returns>
		public Task WhenAll()
		{
			_frozen = true;
			Task[] tasks;
			lock (_lockList)
				tasks = _tasks.ToArray();

			return Task.WhenAll(tasks)
				.ContinueWith(t => { RemoveCompleted(); _frozen = false; });
		}

		public int Count => _tasks.Count;
		/// <summary>
		/// Returns count of running tasks.
		/// </summary>
		public int Running => _tasks.Count(tk => !tk.IsCompleted);

		/// <summary>
		/// Removes completed tasks from collection.
		/// </summary>
		public void RemoveCompleted()
		{
			_tasks = _tasks.Where(tk => !tk.IsCompleted).ToList();
		}

		/// <summary>
		/// Waits until all tasks in collection have completed.
		/// </summary>
		public void  WaitAll()
		{
			_frozen = true;
			RemoveCompleted();
			Task[] tasks;
			lock (_lockList)
				tasks = _tasks.ToArray();

			Task.WaitAll(tasks);

			RemoveCompleted();
			_frozen = false;
		}
	}
}