using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        // Student collection

        IList<Child> childList = new List<Child>() { 

                new Child() { Id = 100, Name = "John" } ,

                new Child() { Id = 200, Name = "Steve"} ,

                new Child() { Id = 300, Name = "Bill" } ,

                new Child() { Id = 400, Name = "Ram" } ,

                new Child() { Id = 500, Name = "Ron" } 

            };

​

        IList<Parent> parentList = new List<Parent>() { 

                new Parent() { Id = 111, UserId = 1 } ,

                new Parent() { Id = 222, UserId = 2} ,

                new Parent() { Id = 333, UserId = 3 } ,

                new Parent() { Id = 444, UserId = 4 } ,

                new Parent() { Id = 555, UserId = 5 } 

            };

​

        IList<ParentChild> parentChildList = new List<ParentChild>() {

            new ParentChild() {Id = 0, ParentId = 111, ChildId = 100 },

            new ParentChild() {Id = 1, ParentId = 111, ChildId =200 },

            new ParentChild() {Id = 2, ParentId = 111, ChildId =300 },

            new ParentChild() {Id = 3, ParentId = 111, ChildId =400 },

            new ParentChild() {Id = 4, ParentId = 111, ChildId = 500 },

            new ParentChild() {Id = 5, ParentId = 444, ChildId = 200 },

            new ParentChild() {Id = 6, ParentId = 444, ChildId = 300 },

            new ParentChild() {Id = 7, ParentId = 444, ChildId = 400 },

            new ParentChild() {Id = 8, ParentId = 444, ChildId = 500 },

            new ParentChild() {Id = 9, ParentId = 333, ChildId = 100 },

            new ParentChild() {Id = 10, ParentId = 333, ChildId = 200 },

            new ParentChild() {Id = 11, ParentId = 333, ChildId = 300 },

            new ParentChild() {Id = 12, ParentId = 222, ChildId = 200 },

            new ParentChild() {Id = 13, ParentId = 222, ChildId = 400 },

            new ParentChild() {Id = 14, ParentId = 555, ChildId = 500 },

        };

​

var parentChildGroup = from parent in parentList 

                    join parentChild in parentChildList on parent.Id equals parentChild.ParentId

                    join child in childList on parentChild.ChildId equals child.Id

                    group child by child.Id into newGroup 

                    select new { 

                        Children = newGroup, 

                    };

        foreach (var item in parentChildGroup)

        { 

            Console.WriteLine(item);

            foreach(var child in item.Children)

                Console.WriteLine(child.Id + ", " + child.Name);

        }

    }

}

​

public class Parent {

    public int Id { get; set; }

    public int UserId { get; set; }

}

​

public class Child {

    public int Id { get; set; }

    public string Name { get; set; }

}

​

public class ParentChild {

    public int Id { get; set; }

    public int ParentId { get; set; }

    public int ChildId { get; set; }

}

​