using System;

using System.Linq;

​

public class Program

{

    public class CustomKey : IEquatable<CustomKey>

    {

        public string A { get; set; }

        public string B { get; set; }

        public bool Equals(CustomKey other)

        {

            return other.A == A && other.B == B;

        }

        public override int GetHashCode()

        {

            return string.Format("{0}{1}", A, B).GetHashCode();

        }

    }

    public class Custom

    {

        public string A { get; set; }

        public string B { get; set; }

        public string C { get; set; }

    }

    public static void Main()

    {

        var c = new Custom[]

        {

            new Custom {A = "hello", B = "frog" },

            new Custom {A = "jim", B = "jog" },

            new Custom {A = "hello", B = "frog" },

        };

​

        var groups = c.GroupBy(x => new CustomKey { A = x.A, B = x.B } );

        Console.WriteLine(groups.Count() == 2);  

    }

}