Test that shows that all handlers are invoked before unsubscribing

EventTest.cs

namespace EventUnsubscriptionTest
{
    using System;

    using NUnit.Framework;

    public class EventSource
    {
        public event Action Event;

        public void TriggerEvent()
        {
            this.Event?.Invoke();
        }
    }

    public class EventSourceTests
    {
        [Test]
        public void When_Unsubscribing_Should_InvokeAllBeforeUnsubscribing()
        {
            // Arrange
            var invokeCounter = 0;
            var listener1InvokedOrder = 0;
            var listener2InvokedOrder = 0;
            var listener3InvokedOrder = 0;

            var sut = new EventSource();

            Action listener1 = null;
            Action listener2 = null;
            Action listener3 = null;

            listener1 = () => listener1InvokedOrder = ++invokeCounter;
            listener2 = () =>
            {
                listener2InvokedOrder = ++invokeCounter;
                sut.Event -= listener1;
                sut.Event -= listener2;
                sut.Event -= listener3;
            };

            listener3 = () => listener3InvokedOrder = ++invokeCounter;

            sut.Event += listener1;
            sut.Event += listener2;
            sut.Event += listener3;

            // Act
            sut.TriggerEvent();

            // Assert
            Assert.That(listener1InvokedOrder, Is.EqualTo(1));
            Assert.That(listener2InvokedOrder, Is.EqualTo(2));
            Assert.That(listener3InvokedOrder, Is.EqualTo(3));

            // Ensure that we actually unsubscribed
            sut.TriggerEvent();
            Assert.That(invokeCounter, Is.EqualTo(3));
        }
    }
}