Unit Testing with NUnit

public async Task<IEnumerable<ApplicationUser>> GetAllUsersAsync() {

    List<ApplicationUser> users = await _userManager.Users.ToListAsync();
    return users;
}

[TestFixture]
public class UserServiceTests {
    Mock<IUserService>? _userService;

    [SetUp]
    public void Setup() {
        _userService = new Mock<IUserService>();
    }

    [Test]
    public async Task Test_UserService() {
        Guid id = Guid.NewGuid();
        ;
        IEnumerable<ApplicationUser> app = new List<ApplicationUser>()
        {
            new ApplicationUser { Id = Guid.NewGuid(), UserName = "TestUser1", Name = "User1", Email = "user1@mail.com"},
            new ApplicationUser { Id = Guid.NewGuid(), UserName = "TestUser2", Name = "User2", Email = "user2@mail.com"},
            new ApplicationUser { Id = Guid.NewGuid(), UserName = "TestUser3", Name = "User2", Email = "user2@mail.com"},
        };

        _userService = new Mock<IUserService>(MockBehavior.Strict);
        _userService.Setup(x => x.GetAllUsersAsync()).ReturnsAsync(app);
        var users = await _userService.Object.GetAllUsersAsync();
        Assert.That(users.Count(), Is.EqualTo(3));

    }
}