.NET Language Integrated Query (LINQ)

The join Clause

Use the join clause to join two sources.

using System;
using System.Collections.Generic;
using System.Linq;

namespace LinqExample
{
	[Serializable]
	public class Person : IComparable<Person>, IFormattable
	{
		public string LastName { get; set; }
		public string FirstName { get; set; }
		public string Country { get; set; }
		public string[] Cars { get; private set; }

		public Person(string LastName = null, string FirstName = null, string Country = null)
		{
			this.LastName = LastName;
			this.FirstName = FirstName;
			this.Country = Country;
		}
		public int CompareTo(Person person)
		{
			if (person == null)
				throw new ArgumentNullException("person");
			return LastName.CompareTo(person.LastName);
		}
		public override string ToString()
		{
			return LastName + ", " + FirstName;
		}
		public string ToString(string format)
		{
			return ToString(format, null);
		}
		public string ToString(string format, IFormatProvider FormatProvider)
		{
			switch (format)
			{
				case null:
				case "N": // return full name
					return ToString();
				case "L": // return last name
					return LastName;
				case "F": // return first name
					return FirstName;
				case "A": // return all information
					return ToString() + " - " + Country.ToString();
				default:
					throw new FormatException("Format " + format + " unsupported");
			}
		}
	}

	public static class Classroom
	{
		private static List<Person> students;

		public static IList<Person> GetAttendance()
		{
			if (students == null)
			{
				students = new List<Person>();
				students.Add(new Person("Holman", "Kaylee", "Holland"));
				students.Add(new Person("Crabtree", "Glenn", "France"));
				students.Add(new Person("Kellerman", "Calandra", "United States"));
				students.Add(new Person("Chairez", "Judy", "France"));
				students.Add(new Person("Santore", "Cari", "Germany"));
				students.Add(new Person("Barnaby", "Emelda", "Germany"));
				students.Add(new Person("Beeler", "Rufina", "Britain"));
				students.Add(new Person("Barreiro", "Jessia", "United States"));
				students.Add(new Person("Kummer", "Bebe", "Brazil"));
				students.Add(new Person("Sexson", "Aleida", "Italy"));
				students.Add(new Person("Ansell", "Freida", "United States"));
				students.Add(new Person("Eriksson", "Jae", "Belgium"));
			}
			return students;
		}
	}

	[Serializable]
	public class Country : IComparable<Country>, IFormattable
	{
		public string Name { get; set; }
		public int Population { get; set; }

		public Country(string Name = null, int Population = 0)
		{
			this.Name = Name;
			this.Population = Population;
		}
		public int CompareTo(Country country)
		{
			if (country == null)
				throw new ArgumentNullException("country");
			return Name.CompareTo(country.Name);
		}
		public override string ToString()
		{
			return Name;
		}
		public string ToString(string format)
		{
			return ToString(format, null);
		}
		public string ToString(string format, IFormatProvider FormatProvider)
		{
			switch (format)
			{
				case null:
				case "N": // return name
					return ToString();
				case "A": // return all information
					return ToString() + " - " + Population;
				default:
					throw new FormatException("Format " + format + " unsupported");
			}
		}
	}

	public static class World
	{
		private static List<Country> countries;

		public static IList<Country> GetCountries()
		{
			if (countries == null)
			{
				countries = new List<Country>();
				countries.Add(new Country("United States", 350000000));
				countries.Add(new Country("Germany", 50000000));
				countries.Add(new Country("Holland", 30000000));
				countries.Add(new Country("Britain", 50000000));
				countries.Add(new Country("Ethiopia", 60000000));
			}
			return countries;
		}
	}

	class Program
	{
		static void Main()
		{
			var coQuery = from c in World.GetCountries()
							select c;

			var stQuery = from p in Classroom.GetAttendance()
							group p by p.Country into g
							where g.Count() > 0
							select new
							{
								Country = g.Key,
								Count = g.Count(),
								Students = from p2 in g
										 orderby p2.LastName
										 select p2.LastName + ", " + p2.FirstName
							};
			var queryJoin = from c in coQuery
							join p in stQuery on c.Name equals p.Country
							orderby p.Count descending, c.Name
							select new
							{
								Country = c.Name,
								Population = c.Population,
								Count = p.Count,
								Students = p.Students
							};
			foreach (var obj in queryJoin)
			{
				Console.WriteLine("{0} ({1:0,0}) - {2} student(s)", obj.Country, obj.Population, obj.Count);
				foreach (string s in obj.Students)
				{
					Console.WriteLine("\t" + s);
				}
			}
		}
	}
}

United States (350,000,000) - 3 student(s)
		Ansell, Freida
		Barreiro, Jessia
		Kellerman, Calandra
Germany (50,000,000) - 2 student(s)
		Barnaby, Emelda
		Santore, Cari
Britain (50,000,000) - 1 student(s)
		Beeler, Rufina
Holland (30,000,000) - 1 student(s)
		Holman, Kaylee