using System;

using System.Collections.Generic;   

using System.Linq;

public class Program

{

    public static void Main()

    {

        var listA = new List<int> {1, 1, 5, 2, 2 , 7};

        var listB = from g in listA.GroupBy(x => x)  where g.Count() == 1   select g.First();

​

        foreach (var item in listB)

        {

            Console.WriteLine(item);

        }

    }

}