Understanding the C# Linq SelectMany Operator

The Linq operator SelectMany has always confused me a bit, and yesterday I came across it in some code so had to actually try to understand it.

Simple put, SelectMany selects out all children from a collection of parents in a one to many parent child relation ship. You can add an additional parameter which provides access to the parent and child. Theres a good explanation on this stackoverflow answer. I wrote out the code in a console app (below) and it made a lot more sense:

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Running Select Many Sample...");

            IEnumerable<Person> people = new List<Person>()
            {
                new Person()
                {
                    Name = "Wilma", PhoneNumbers = new List<PhoneNumber>()
                    {
                        new PhoneNumber() { Number = "777"},
                        new PhoneNumber() { Number = "888"},
                        new PhoneNumber() { Number = "999"}
                    }
                },
                new Person()
                {
                    Name = "Bill", PhoneNumbers = new List<PhoneNumber>()
                    {
                        new PhoneNumber() { Number = "0191111111"}
                    }
                },
                new Person() 
                { 
                    Name = "Fred", PhoneNumbers = new List<PhoneNumber>()
                    {
                        new PhoneNumber() { Number = "0131762475"},
                        new PhoneNumber() { Number = "0789123456"}
                    }
                
                }
            };

            Console.WriteLine("Select Many PhoneNumbers:");
            var allPhoneNumbers = people.SelectMany(p => p.PhoneNumbers);
            foreach (var phoneNumber in allPhoneNumbers)
            {
                Console.WriteLine(phoneNumber.Number);
            }

            Console.WriteLine("Select Many With parent:");
            var directory = people.SelectMany(p => p.PhoneNumbers,
                               (parent, child) => new { parent.Name, child.Number });

            foreach( var item in directory)
            {
                Console.WriteLine($"{item.Name}:{item.Number}");
            }
            Console.WriteLine("Done!");
            Console.ReadLine();
        }
    }
    public class PhoneNumber
    {
        public string Number { get; set; }
    }

    public class Person
    {
        public IEnumerable<PhoneNumber> PhoneNumbers { get; set; }
        public string Name { get; set; }
    }
}