Mock vs Stub vs Fake vs Spy — когда что использовать?

public sealed class RegisterUserHandler(IUserRepository users, IPasswordHasher hasher, IEmailSender email, TimeProvider clock)
{
    public async Task<Result<Guid>> Handle(RegisterUser cmd, CancellationToken ct)
    {
        if (await users.ExistsAsync(cmd.Email, ct)) return Result<Guid>.Fail("Email занят");
        var user = new User(cmd.Email, hasher.Hash(cmd.Password), clock.GetUtcNow());
        await users.AddAsync(user, ct);
        await email.SendWelcomeAsync(cmd.Email, ct);
        return Result<Guid>.Ok(user.Id);
    }
}

[Fact]
public async Task Fails_when_email_taken()
{
    var users = Substitute.For<IUserRepository>();
    users.ExistsAsync("ann@x.uz", Arg.Any<CancellationToken>()).Returns(true);      // stub: готовый ответ

    var sut = new RegisterUserHandler(users, Substitute.For<IPasswordHasher>(),        // dummy
                                      Substitute.For<IEmailSender>(), TimeProvider.System);

    var result = await sut.Handle(new RegisterUser("ann@x.uz", "P@ssw0rd"), default);

    result.IsSuccess.Should().BeFalse();                                             // проверяем РЕЗУЛЬТАТ
}

public sealed class InMemoryUserRepository : IUserRepository
{
    public List<User> Users { get; } = [];
    public Task<bool> ExistsAsync(string email, CancellationToken ct) => Task.FromResult(Users.Any(u => u.Email == email));
    public Task AddAsync(User user, CancellationToken ct) { Users.Add(user); return Task.CompletedTask; }
}

public sealed class FakeEmailSender : IEmailSender          // fake + spy: запоминает отправленные письма
{
    public List<string> Sent { get; } = [];
    public Task SendWelcomeAsync(string to, CancellationToken ct) { Sent.Add(to); return Task.CompletedTask; }
}

[Fact]
public async Task Registers_user_and_sends_welcome_email()
{
    var users = new InMemoryUserRepository();
    var email = new FakeEmailSender();
    var clock = new FakeTimeProvider(new DateTimeOffset(2025, 9, 30, 10, 0, 0, TimeSpan.Zero));   // Microsoft.Extensions.TimeProvider.Testing
    var sut = new RegisterUserHandler(users, new FakeHasher(), email, clock);

    await sut.Handle(new RegisterUser("ann@x.uz", "P@ssw0rd"), default);

    users.Users.Should().ContainSingle(u => u.Email == "ann@x.uz" && u.CreatedAt == clock.GetUtcNow());
    email.Sent.Should().Equal("ann@x.uz");
}

[Fact]
public async Task Sends_welcome_email()
{
    var email = new Mock<IEmailSender>();
    var sut = new RegisterUserHandler(new InMemoryUserRepository(), new FakeHasher(), email.Object, TimeProvider.System);

    await sut.Handle(new RegisterUser("ann@x.uz", "P@ssw0rd"), default);

    email.Verify(e => e.SendWelcomeAsync("ann@x.uz", It.IsAny<CancellationToken>()), Times.Once);   // проверка ВЗАИМОДЕЙСТВИЯ
}