using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Customer> CustomerList = new List<Customer>();

CustomerList.Add( new Customer { ID = 1, Name = "One", GroupID = 1 } );

CustomerList.Add( new Customer { ID = 2, Name = "Two", GroupID = 1 } );

CustomerList.Add( new Customer { ID = 3, Name = "Three", GroupID = 2 } );

CustomerList.Add( new Customer { ID = 4, Name = "Four", GroupID = 1 } );

CustomerList.Add( new Customer { ID = 5, Name = "Five", GroupID = 3 } );

CustomerList.Add( new Customer { ID = 6, Name = "Six", GroupID = 3 } );

​

var groupedCustomerList = CustomerList.GroupBy(u => u.GroupID)

                                      .Select(grp =>new { GroupID =grp.Key, CustomerList = grp.ToList()})

                                      .ToList();

 foreach(var l in groupedCustomerList)

 {

     Console.WriteLine("Group#"+l.GroupID);

     Console.WriteLine();

     foreach(Customer c in l.CustomerList)

     {

         Console.WriteLine(c.GroupID.ToString()+":"+c.Name);

     }

     Console.WriteLine();

 }

​

    }

}

public class Customer

{

    public int ID { get; set; }

    public string Name { get; set; }

    public int GroupID { get; set; }

}