using System;

using System.Collections.Generic;

using System.Linq;

​

namespace ConsoleApplication1

{

    class Parent

    {

        public string ParentName { get; set; }

        public IEnumerable<string> ChildNames { get; set; }

    }

​

    class Child

    {

        public string ParentName { get; set; }

        public string ChildName { get; set; }

    }

​

    public class Program

    {

        public static void Main(string[] args)

        {

            var parents = new List<Parent>()

            {

                new Parent() {ParentName = "Lee"},

                new Parent() {ParentName = "Bob"},

                new Parent() {ParentName = "Tom"}

            };

​

            var children = new List<Child>()

            {

                new Child() {ParentName = "Lee", ChildName = "A"},

                new Child() {ParentName = "Tom", ChildName = "B"},

                new Child() {ParentName = "Tom", ChildName = "C"}

            };

​

            ///////////////////////////////////////////////

            // Technique #1:

            foreach (var parent in parents)

            {

                var p = parent; // to avoid foreach closure side-effects

                p.ChildNames = children.Where(c => c.ParentName == p.ParentName).Select(c => c.ChildName);

            }

            Display("Technique #1:", parents);

            ///////////////////////////////////////////////

​

​

            ///////////////////////////////////////////////

            // Technique #2:            

            var parentChildPairs =

                from p in parents

                join c in children on p.ParentName equals c.ParentName into childList

                select new { Parent = p, ChildrenNames = childList.Select(c => c.ChildName) };

​

            foreach (var dzp in parentChildPairs)

            {

                var p = dzp.Parent;

                var c = dzp.ChildrenNames; // to avoid foreach closure side-effects

                p.ChildNames = c;

            }

            Display("Technique #2:", parentChildPairs.Select(p => p.Parent));

            ///////////////////////////////////////////////

            ///////////////////////////////////////////////

            // Technique #3:

            parents.ForEach(p => p.ChildNames = children.Where(c => c.ParentName == p.ParentName).Select(c => c.ChildName).ToList());

            Display("Technique #3:", parents);

            ///////////////////////////////////////////////

​

            ///////////////////////////////////////////////

            // Technique #4:

            var clu = children.ToLookup(c => c.ParentName, c => c.ChildName);

            parents.ForEach(p => p.ChildNames = clu[p.ParentName]);

            Display("Technique #4:", parents);

            ///////////////////////////////////////////////

        }

​

        static void Display(string message, IEnumerable<Parent> parents)

        {

            Console.WriteLine(message);

            foreach (var p in parents)

            {

                var childNames = string.Join(",", p.ChildNames);

                Console.WriteLine("Parent = {0}, Children = {1}", p.ParentName, childNames);

            }

            Console.WriteLine("");

        }

    }

}