using System;
using System.Collections.Generic;
using System.Linq;

public class Test
{
	public static void Main()
	{
            List<Destination> destinations = new List<Destination>() {
                new Destination() { A = 1, B = 1 },
                new Destination() { A = 1, B = 2 },
                new Destination() { A = 2, B = 1 }
            };
            
            List<Source> sources = new List<Source>() {
                new Source() { A = 1, B = 1, C = "a" },
                new Source() { A = 2, B = 1, C = "b" },
                new Source() { A = 2, B = 1, C = "c" },
                new Source() { A = 1, B = 1, C = "d" },
                new Source() { A = 2, B = 2, C = "e" }
            };

            var lookup = sources.ToLookup(s => new { s.A, s.B }, s => s.C);
            foreach (var d in destinations)            
                d.Cs = lookup[new { d.A, d.B }].ToList();

            // results
            Console.WriteLine("A B  Cs");
            foreach(var d in destinations)
            {
                Console.WriteLine("{0}-{1}: {2}", d.A, d.B, String.Join(",", d.Cs.Cast<string>().ToArray()));
            }
	}
}

class Source
{
    public object A { get; set; }
    public object B { get; set; }
    public object C { get; set; }
}

class Destination
{
    public object A { get; set; }
    public object B { get; set; } // (A,B) is a unique key
    public List<object> Cs { get; set; }
    public object D { get; set; }
}