Example code illustrating how to unit test if a call is awaited

UnitTestingAsyncAwaitExample.cs

using System.Threading.Tasks;
using Moq;
using NUnit.Framework;

namespace UnitTestingAsyncAwait
{
    public interface IUserRepository
    {
        Task CreateNewUser();
    }

    public interface IMessageDispatcher
    {
        void DispatchEmailVerificationMessage();
    }
    
    [TestFixture]
    public class Demo
    {
        private UserRegistrator _classUnderTest;
        
        private Mock<IUserRepository> _userRepositoryMock;
        private Mock<IMessageDispatcher> _messageDispatcherMock;

        [SetUp]
        public void SetUp()
        {
            _userRepositoryMock = new Mock<IUserRepository>();
            _messageDispatcherMock = new Mock<IMessageDispatcher>();
            
            _classUnderTest = new UserRegistrator(_userRepositoryMock.Object, _messageDispatcherMock.Object);
        }
        
        [Test]
        public void AwaitsAsyncCall()
        {
            // Arrange
            var tcs = new TaskCompletionSource<bool>();
            var task = tcs.Task;
            _userRepositoryMock.Setup(a => a.CreateNewUser()).Returns(task);
            
            // Act
            _classUnderTest.Register();
            
            // Assert
            _userRepositoryMock.Verify(a => a.CreateNewUser(), Times.Once);
            _messageDispatcherMock.Verify(s => s.DispatchEmailVerificationMessage(), Times.Never, "Did not await the async thing.");
            tcs.SetResult(true); //NOTE: The actual value is not relevant for this test but needed since there is no Task<void> in .Net.
            
            _messageDispatcherMock.Verify(s => s.DispatchEmailVerificationMessage(), Times.Once);
        }
    }

    public class UserRegistrator
    {
        private readonly IUserRepository _userRepository;
        private readonly IMessageDispatcher _messageDispatcher;

        public UserRegistrator(IUserRepository userRepository, IMessageDispatcher messageDispatcher)
        {
            _userRepository = userRepository;
            _messageDispatcher = messageDispatcher;
        }

        public async Task Register()
        {
            await _userRepository.CreateNewUser();
            _messageDispatcher.DispatchEmailVerificationMessage();
        }
    }
}