The group Clause

The group clause can be used to group results based on a key.

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 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;
		}
	}

	class Program
	{
		static void Main()
		{
			var query = from p in Classroom.GetAttendance()
						group p by p.Country into g
						orderby g.Count() descending
						where g.Count() > 1
						select new
						{
							Country = g.Key,
							Count = g.Count()
						};
			foreach (var obj in query)
			{
				Console.WriteLine("{0, -15} {1}", obj.Country, obj.Count);
			}
		}
	}
}
United States	 3
France			2
Germany		 2

Nested Objects

When the grouped object contains nested sequences, just modify the select clause.

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 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;
		}
	}

	class Program
	{
		static void Main()
		{
			var query = from p in Classroom.GetAttendance()
						group p by p.Country into g
						orderby g.Count() descending
						where g.Count() > 1
						select new
						{
							Country = g.Key,
							Count = g.Count(),
							Students = from p2 in g
										 orderby p2.LastName
										 select p2.LastName + ", " + p2.FirstName
						};
			foreach (var obj in query)
			{
				Console.WriteLine("{0, -15} {1}", obj.Country, obj.Count);
				foreach (string s in obj.Students)
				{
					Console.WriteLine("\t" + s);
				}
			}
		}
	}
}
United States	 3
		Ansell, Freida
		Barreiro, Jessia
		Kellerman, Calandra
France			2
		Chairez, Judy
		Crabtree, Glenn
Germany		 2
		Barnaby, Emelda
		Santore, Cari
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