Loop Compare

Data

 public class Data
    {

        public string Name { get; set; }
        public int ID { get; set; }
        public int ID2 { get; set; }
    }

DataHolder

 public class DataHolder
        {
            public Data x;
            public Data y;
            public override int GetHashCode()
            {
                return x.ID + y.ID;
            }
            public override bool Equals(object obj)
            {
                var v = obj as DataHolder;
                return v.x.ID == x.ID && v.y.ID == y.ID;
            }
        }

Linq

 var itemx = (
                        from x in list
                        join y in list2 on x.ID equals y.ID
                        select new DataHolder { x = x, y = y }
                        ).ToList();

Logic

            var list = new List<Data>();
            var list2 = new List<Data>();

            var random = new Random();
            for (var i = 0; i < 1000; i++)
            {
                //var rand = random.Next(2, 500);
                //var rand2 = random.Next(2, 90);
                //list.Add(new Data { ID = rand, ID2 = rand2, Name = "Name" + rand });
                list.Add(new Data { ID = 1, ID2 = 1, Name = "Name" + 1 });
            }


            for (var i = 0; i < 200; i++)
            {
                //var rand = random.Next(2, 500);
                //var rand2 = random.Next(2, 90);
                //list.Add(new Data { ID = rand, ID2 = rand2, Name = "Name" + rand });
                list.Add(new Data { ID = 2, ID2 = 2, Name = "Name" + 2 });
            }

            for (var i = 0; i < 1000; i++)
            {
                //var rand = random.Next(2, 500);
                //var rand2 = random.Next(2, 90);
                //list.Add(new Data { ID = rand, ID2 = rand2, Name = "Name" + rand });
                list2.Add(new Data { ID = 1, ID2 = 1, Name = "Name" + 1 });
            }

Regular Loop

static List<JoinedQuery<Data>> GetList(List<Data> list, List<Data> list2)
        {
            var listCount = list.Count;
            var list2Count = list2.Count;
            var result = new List<JoinedQuery<Data>>();

            for (var i = 0; i < listCount; i++)
            {
                var item = list[i];
                for (var j = 0; j < list2Count; j++)
                {
                    var item2 = list2[j];
                    if (item2.ID == item.ID)
                    {
                        result.Add(new JoinedQuery<Data> { First = item,
                            Others = new Dictionary<string, object>() { {"list2", item2} } });
                        break;
                    }

                }
            }

            return result;
        }

Regular Loop 2

static List<JoinedQuery<Data>> GetList2(List<Data> list, List<Data> list2)
        {
            var listCount = list.Count;
            var list2Count = list2.Count;
            var result = new List<JoinedQuery<Data>>();
            var dictList2 = new Dictionary<int, Data>();
            var key = 0;

            for (var i = 0; i < list2Count; i++)
            {
                var item = list2[i];
                key = int.MaxValue;
                key = (-1521134295 * key) + item.ID;
                dictList2[key] = item;
            }

            for (var i = 0; i < listCount; i++)
            {
                var item = list[i];
                key = int.MaxValue;
                key = (-1521134295 * key) + item.ID;

                if (dictList2.ContainsKey(key))
                {
                    result.Add(new JoinedQuery<Data> {
                        First = item,
                        Others = new Dictionary<string, object>() { { "list2", dictList2[key] } }
                    });
                }
            }

            return result;
        }