public async void AddGuess_GuessWithIdsNotInEmployeeData_ReturnsNull() { //Arrange var seedService = new Mock <ISeedService <Employee> >(); var employees = new List <Employee>(); var employee = new Employee() { Id = "abc" }; var employee2 = new Employee() { Id = "def" }; employees.Add(employee); employees.Add(employee2); var game = new Game() { Data = employees, Id = 0 }; var gameService = new GameService(seedService.Object); gameService.CachedGames.Add(game); var guess = new Dictionary <string, string> { { "abc", "bob" }, { "ddd", "billy" } }; //Act var result = await gameService.AddGuess(0, guess); //Assert Assert.Null(result); }