private readonly IEmployeeService _employeeService;

    public EmployeeServiceTests()
    {
        var mockData = new List<Employee>
        {
            new Employee { Id = 0, FirstName = "Homer", LastName = "Simpson" },
            new Employee { Id = 1, FirstName = "Carl", LastName = "Carlson" },
            new Employee { Id = 2, FirstName = "Lenny", LastName = "Leonard" },
        };

        _employeeService = MockSetup.SetupEmployeeService(mockData);
    }

    [Fact]
    public void UpdateEmployee_EmployeeExists_EmployeeGetsUpdated()
    {
        var homer = _employeeService.GetEmployee(0);
        homer.FirstName = "Homer Jay";

        _employeeService.UpdateEmployee(homer);

        var actual = homer.FirstName;
        var expected = _employeeService.GetEmployee(0).FirstName;

        Assert.Equal(expected, actual);
    }

public static Mock<MockDbSet<Employee>> CreateEmployeeMockSet(List<Employee> data)
    {
        var mockSet = new Mock<MockDbSet<Employee>>();
        mockSet.As<IQueryable<Employee>>().Setup(m => m.Provider).Returns(data.AsQueryable().Provider);
        mockSet.As<IQueryable<Employee>>().Setup(m => m.Expression).Returns(data.AsQueryable().Expression);
        mockSet.As<IQueryable<Employee>>().Setup(m => m.ElementType).Returns(data.AsQueryable().ElementType);
        mockSet.As<IQueryable<Employee>>().Setup(m => m.GetEnumerator()).Returns(data.AsQueryable().GetEnumerator());
        mockSet.Setup(d => d.Add(It.IsAny<Employee>())).Callback<Employee>(e => data.Add(e));
        mockSet.Setup(d => d.AddOrUpdate(It.IsAny<Employee>())).Callback<Employee>(e => UpdateList(e, data));
        mockSet.Setup(d => d.Remove(It.IsAny<Employee>())).Callback<Employee>(e => data.Remove(e));

        return mockSet;
    }

    private static void UpdateList(Employee employee, List<Employee> data)
    {
        var index = data.FindIndex(e => e.Id == employee.Id);

        data[index] = employee;
    }

Message: Assert.Equal() Failure
               ↓ (pos 5)
Expected: Homer
Actual:   Homer Jay
               ↑ (pos 5)