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; }
	}
	}

view raw select-many-sample.cs hosted with ❤ by GitHub