using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

         List<Contact> contacs = new List<Contact>

            {

                new Contact { id =1, Number = 1234, ContactID = 5},

                new Contact { id =2, Number = 9873, ContactID = 6},

                new Contact { id =3, Number = 1234, ContactID = 7},

                new Contact { id =4, Number = 9873, ContactID = 6},

                new Contact { id =5, Number = 1234, ContactID = 8},

            };

​

           var result = from c in contacs

                     group c by c.Number into g

                     let count = g.GroupBy(x => x.ContactID).Where(x => x.Count() == 1).Count()

                     where count != 0

                     select new

                         {

                             Number = g.Key,

                             Count = count

                         };

​

            foreach (var item in result)

            {

                Console.WriteLine("Number: {0}, Count: {1}", item.Number, item.Count);

            }

    }

}

​

   public class Contact

        {

            public int id { get; set; }

            public int Number { get; set; }

            public int ContactID { get; set; }

        }