public void CommonCase()
        {
            var source = GenerateStorage;
            var singleAverageGetter = new SingleAverageGetter(new AverageKeeperFactory(), new SelectorByCountryAndAge());
            var age     = 20;
            var country = "USA";
            var average = 4000;

            var selectorByCountryAndAgeMock = new SelectorByCountryAndAgeMock();
            var grossSalaryAverageSetter    = new GrossSalaryAverageSetter(selectorByCountryAndAgeMock);


            var result = grossSalaryAverageSetter.SetAverage(source, age, country, average);

            Assert.Equal(average, singleAverageGetter.Calculate(result.ToList(), age, country));
            Assert.Equal(1, selectorByCountryAndAgeMock.CountOfCalling);
            Assert.True(result.SequenceEqual(selectorByCountryAndAgeMock.CopyOfLastReturnedCollection));
        }
        public void EmptyCase()
        {
            var source  = GenerateStorage;
            var age     = 25;
            var country = "USA";
            var average = 4000;

            var selectorByCountryAndAgeMock = new SelectorByCountryAndAgeMock();
            var grossSalaryAverageSetter    = new GrossSalaryAverageSetter(selectorByCountryAndAgeMock);

            var result       = grossSalaryAverageSetter.SetAverage(source, age, country, average);
            var checkStorage = GenerateStorage;

            Assert.Equal(1, selectorByCountryAndAgeMock.CountOfCalling);
            Assert.False(result.Any());
            Func <Person, string> serializer = p => $"{p.Age} {p.Country} {p.Gender} {p.GrossSalary}";

            Assert.True(checkStorage.Select(serializer).SequenceEqual(source.Select(serializer)));
        }