static void Main(string[] args)
        {
            var harry = new Person {
                Name = "Harry"
            };
            var mary = new Person {
                Name = "Mary"
            };
            var jill = new Person {
                Name = "Jill"
            };

            // Implementing functionality using methods and operators

            // call instance method
            var baby1 = mary.ProcreateWith(harry);

            // call static method
            var baby2 = Person.Procreate(harry, jill);

            // call an operator
            var baby3 = harry * mary;

            WriteLine($"{harry.Name} has {harry.Children.Count} children.");
            WriteLine($"{mary.Name} has {mary.Children.Count} children.");
            WriteLine($"{jill.Name} has {jill.Children.Count} children.");

            WriteLine(
                format: "{0}'s first child is named \"{1}\".",
                arg0: harry.Name,
                arg1: harry.Children[0].Name);

            // Implementing functionality using local functions

            WriteLine($"5! is {Person.Factorial(5)}");

            // Raising and handling events

            harry.Shout += Harry_Shout;

            harry.Poke();
            harry.Poke();
            harry.Poke();
            harry.Poke();

            // Comparing objects when sorting

            Person[] people =
            {
                new Person {
                    Name = "Simon"
                },
                new Person {
                    Name = "Jenny"
                },
                new Person {
                    Name = "Adam"
                },
                new Person {
                    Name = "Richard"
                }
            };

            WriteLine("Initial list of people:");
            foreach (var person in people)
            {
                WriteLine($"{person.Name}");
            }

            WriteLine("Use Person's IComparable implementation to sort:");
            Array.Sort(people);
            foreach (var person in people)
            {
                WriteLine($"{person.Name}");
            }

            // Comparing objects using a separate class

            WriteLine("Use PersonComparer's IComparer implementation to sort:");
            Array.Sort(people, new PersonComparer());
            foreach (var person in people)
            {
                WriteLine($"{person.Name}");
            }

            // Working with generic types

            var t1 = new Thing();

            t1.Data = 42;
            WriteLine($"Thing with an integer: {t1.Process(42)}");

            var t2 = new Thing();

            t2.Data = "apple";
            WriteLine($"Thing with a string: {t2.Process("apple")}");

            var gt1 = new GenericThing <int>();

            gt1.Data = 42;
            WriteLine($"GenericThing with an integer: {gt1.Process(42)}");

            var gt2 = new GenericThing <string>();

            gt2.Data = "apple";
            WriteLine($"GenericThing with a string: {gt2.Process("apple")}");

            // Working with generic methods

            string number1 = "4";

            WriteLine("{0} squared is {1}",
                      arg0: number1,
                      arg1: Squarer.Square <string>(number1));

            byte number2 = 3;

            WriteLine("{0} squared is {1}",
                      arg0: number2,
                      arg1: Squarer.Square(number2));

            // Working with struct types

            var dv1 = new DisplacementVector(3, 5);
            var dv2 = new DisplacementVector(-2, 7);
            var dv3 = dv1 + dv2;

            WriteLine($"({dv1.X}, {dv1.Y}) + ({dv2.X}, {dv2.Y}) = ({dv3.X}, {dv3.Y})");

            // Inheriting from classes

            Employee john = new Employee
            {
                Name        = "John Jones",
                DateOfBirth = new DateTime(1990, 7, 28)
            };

            john.WriteToConsole();

            // Extending classes

            john.EmployeeCode = "JJ001";
            john.HireDate     = new DateTime(2014, 11, 23);
            WriteLine($"{john.Name} was hired on {john.HireDate:dd/MM/yy}");

            // Overriding  members

            WriteLine(john.ToString());

            // Understanding polymorphism

            Employee aliceInEmployee = new Employee
            {
                Name = "Alice", EmployeeCode = "AA123"
            };

            Person aliceInPerson = aliceInEmployee;

            aliceInEmployee.WriteToConsole();

            aliceInPerson.WriteToConsole();

            WriteLine(aliceInEmployee.ToString());

            WriteLine(aliceInPerson.ToString());

            // Explicit casting

            if (aliceInPerson is Employee)
            {
                WriteLine($"{nameof(aliceInPerson)} IS an Employee");

                Employee explicitAlice = (Employee)aliceInPerson;
                // safely do something with explicitAlice
            }

            Employee aliceAsEmployee = aliceInPerson as Employee;

            if (aliceAsEmployee != null)
            {
                WriteLine($"{nameof(aliceInPerson)} AS an Employee");
                // do something with aliceInEmployee
            }

            // Inheriting exceptions

            try
            {
                john.TimeTravel(new DateTime(1999, 12, 31));
                john.TimeTravel(new DateTime(1950, 12, 25));
            }
            catch (PersonException ex)
            {
                WriteLine(ex.Message);
            }

            // Using static methods to reuse functionality

            string email1 = "*****@*****.**";
            string email2 = "ian&test.com";

            WriteLine(
                "{0} is a valid e-mail address: {1}",
                arg0: email1,
                arg1: StringExtensions.IsValidEmail(email1));

            WriteLine(
                "{0} is a valid e-mail address: {1}",
                arg0: email2,
                arg1: StringExtensions.IsValidEmail(email2));

            WriteLine(
                "{0} is a valid e-mail address: {1}",
                arg0: email1,
                arg1: email1.IsValidEmail());

            WriteLine(
                "{0} is a valid e-mail address: {1}",
                arg0: email2,
                arg1: email2.IsValidEmail());
        }
        static void Main(string[] args)
        {
            var harry = new Person {
                Name = "Harry"
            };
            var mary = new Person {
                Name = "Mary"
            };
            var jill = new Person {
                Name = "Jill"
            };

            var baby1 = mary.ProcreateWith(harry);

            baby1.Name = "Gary";
            var baby2 = Person.Procreate(harry, jill);
            var baby3 = harry * mary;

            WriteLine($"{harry.Name} has {harry.Children.Count} children.");
            WriteLine($"{mary.Name} has {mary.Children.Count} children.");
            WriteLine($"{jill.Name} has {jill.Children.Count} children.");
            WriteLine(
                format: "{0}'s first child is named \"{1}\".",
                arg0: harry.Name,
                arg1: harry.Children[0].Name);

            WriteLine($"5! is {Person.Factorial(5)}");

            harry.Shout += Harry_Shout;
            harry.Poke();
            harry.Poke();
            harry.Poke();
            harry.Poke();

            Person[] people =
            {
                new Person {
                    Name = "Simon"
                },
                new Person {
                    Name = "Jenny"
                },
                new Person {
                    Name = "Adam"
                },
                new Person {
                    Name = "Richard"
                }
            };
            WriteLine("Initial list of people:");
            foreach (var person in people)
            {
                WriteLine($"  {person.Name}");
            }

            WriteLine("Use Person's IComparable implementation to sort:");
            Array.Sort(people);
            foreach (var person in people)
            {
                WriteLine($"  {person.Name}");
            }

            var t1 = new Thing();

            t1.Data = 42;
            WriteLine($"Thing with an integer: {t1.Process(42)}");

            var t2 = new Thing();

            t2.Data = "apple";
            WriteLine($"Thing with a string: {t2.Process("apple")}");

            var gt1 = new GenericThing <int>();

            gt1.Data = 42;
            WriteLine($"GenericThing with an integer: {gt1.Process(42)}");

            var gt2 = new GenericThing <string>();

            gt2.Data = "apple";
            WriteLine($"GenericThing with a string: {gt2.Process("apple")}");

            string number1 = "4";

            WriteLine("{0} squared is {1}", arg0: number1, arg1: Squarer.Square <string>(number1));

            byte number2 = 3;

            WriteLine("{0} squared is {1}", arg0: number2, arg1: Squarer.Square <byte>(number2));

            var dv1 = new DisplacementVector(3, 5);
            var dv2 = new DisplacementVector(-2, 7);
            var dv3 = dv1 + dv2;

            WriteLine($"({dv1.X}, {dv1.Y}) + ({dv2.X}, {dv2.Y}) = ({dv3.X}, {dv3.Y})");

            Employee john = new Employee {
                Name        = "John Jones",
                DateOfBirth = new DateTime(1990, 7, 28)
            };

            john.WriteToConsole();
            john.EmployeeCode = "JJ001";
            john.HireDate     = new DateTime(2014, 11, 23);
            WriteLine($"{john.Name} was hired on {john.HireDate:dd/MM/yy}");

            WriteLine(john.ToString());

            Employee aliceInEmployee = new Employee
            {
                Name = "Alice", EmployeeCode = "AA123"
            };
            Person aliceInPerson = aliceInEmployee;

            aliceInEmployee.WriteToConsole();
            aliceInPerson.WriteToConsole();
            WriteLine(aliceInEmployee.ToString());
            WriteLine(aliceInPerson.ToString());

            if (aliceInPerson is Employee)
            {
                WriteLine($"{nameof(aliceInPerson)} IS an Employee");
                Employee explicitAlice = (Employee)aliceInPerson;
            }
            Employee aliceAsEmployee = aliceInPerson as Employee;

            if (aliceAsEmployee != null)
            {
                WriteLine($"{nameof(aliceInPerson)} AS an Employee");
            }

            try {
                john.TimeTravel(new DateTime(1999, 12, 31));
                john.TimeTravel(new DateTime(1950, 12, 25));
            }
            catch (PersonException ex) {
                WriteLine(ex.Message);
            }

            string email1 = "*****@*****.**";
            string email2 = "ian&test.com";

            WriteLine("{0} is a valid e-mail address: {1}",
                      arg0: email1,
                      arg1: StringExtensions.IsValidEmail(email1));

            WriteLine("{0} is a valid e-mail address: {1}",
                      arg0: email2,
                      arg1: StringExtensions.IsValidEmail(email2));
        }