Ubuntu – How to correctly sleep/delay in a multi-threaded unit test

I am trying to write some unit tests to make sure my app can handle running multi-threaded.

The main reason is because it will be a web server and many requests could be processed at the same time.

But my tests fail when I introduce Thread.Sleep, which I’m doing to simulate doing database or other background tasks.

I have re-created the essentials down below.

Inside the function PerformTask is a call to Thread.Sleep.
If I comment it out, the tests pass and if I put it back, the tests fail.

Am I doing something wrong?
Also, with the sleep, the tests fail without showing why. Is there a way to catch any exceptions?

using NUnit.Framework;

namespace MultiThreadedUnitTest
{
    [TestFixture]
    [Parallelizable(ParallelScope.None)]
    public class Tests
    {
        private readonly int NumberOfForLoops = 100;
        private int Counter { get; set; } = 0;
        private object CounterLock = new object();

        private void addToCounter(int value)
        {
            lock (CounterLock)
            {
                Counter += value;
            }
        }

        [Test, Timeout(5000)] // Must complete within 5 seconds
        [Parallelizable(ParallelScope.None)]
        public void RunTest()
        {
            var threads = new Thread[NumberOfForLoops];
            for (int i = 0; i < threads.Length; i++)
            {
                threads[i] = new Thread(() => PerformTask());
            }

            foreach (var thread in threads)
            {
                thread.Start();
            }

            foreach (var thread in threads)
            {
                thread.Join();
            }

            Assert.AreEqual(NumberOfForLoops, Counter);

            var task = AsyncTest();
            task.Wait();

            Assert.AreEqual(NumberOfForLoops, task.Result);

            Assert.AreEqual(NumberOfForLoops * 3, Counter);

            // The "performTask" method was called 30 times but this test completed in under 5 seconds.
            // That means some of the Thread.Sleep calls had to have been made at the same time.
        }

        private async Task<int> AsyncTest()
        {
            var tasks = new List<Task<int>>();
            for (var i = 0; i < NumberOfForLoops; i++)
            {
                tasks.Add(Task.Run(() => Test()));
                /* 
                 * tasks.Add(Test());  --> This makes it run in sequence, one after the other
                 * */
            }
            await Task.WhenAll(tasks);

            await Task.Run(() => Parallel.For(0, NumberOfForLoops, s =>
            {
                var res = Test();
            }));

            return tasks.Sum(x => x.Result);
        }

        private async Task<int> Test()
        {
            PerformTask();
            return 1;
        }

        private object Lock = new object();
        private void PerformTask()
        {
            //Thread.Sleep(1000); // Problem Number 2
            try
            {
                lock (Lock)
                {
                    //TODO: Simulate DB lookup
                    var rnd = new Random();
                    var number = rnd.Next(1, 50) * 100;
                    
                    //Task.Delay(number).Wait();
                    var x = "Thread id = " + Thread.CurrentThread.ManagedThreadId;
                    Console.WriteLine(x);

                    Console.WriteLine("Sleeping for : " + number);

                    /* THE FOLLOWING LINE BREAKS MY UNIT TESTS */
                    //Thread.Sleep(number);

                    addToCounter(1);
                    
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }
    }
}

Another weird thing is when the number of for loops was low, i.e. 10, and I had another sleep (indicated as problem number 2), then the unit tests all passed, but only on windows.

If I ran those unit tests on Ubuntu, they always failed.
That took me ages to figure out as the unit tests would fail without showing any line numbers or reasons and I had to add a bunch of console.writeline’s.

If anyone knows why there is this difference between Ubuntu and .NET that would be useful to know too.