Deadlock.cs

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        private static ManualResetEventSlim _mutex = new ManualResetEventSlim();
        private static Task _sharedTask;

        static void Main(string[] args)
        {
            var tcs = new TaskCompletionSource<bool>();
            _sharedTask = tcs.Task;

            Task.Run(Thread1);
            Thread.Sleep(2000); // Wait for the task to start

            Task.Run(Thread2);
            Thread.Sleep(2000); // Wait for the task to start

            Console.WriteLine("Completing the shared task");
            tcs.SetResult(true);

            Console.WriteLine("Will never get there");
        }

        private static async Task Thread1()
        {
            await _sharedTask;
            _mutex.Wait();

            Console.WriteLine("Will never get there");
        }

        private static async Task Thread2()
        {
            // Task.WhenAll won't be completed because the completion thread is blocked in Thread1()
            Task.WhenAll(_sharedTask).ContinueWith(_ =>
            {
                _mutex.Set();

                Console.WriteLine("Will never get there");
            });
        }
    }
}