using System;

using System.Linq;

using System.Collections.Generic;

public class Program {

    public static void Main() {

        IEnumerable<Parent> parents = new Parent[] { 

            new Parent { ID = 1, Description = "Apple" },

            new Parent { ID = 2, Description = "Orange" },

            new Parent { ID = 3, Description = "Pear" },

            new Parent { ID = 4, Description = "Banana" } };

        IEnumerable<Child> children = new Child[] {

            new Child { ID = 1, ParentID = 2, Description = "Mercury", Condition = null },

            new Child { ID = 2, ParentID = 3, Description = "Venus", Condition = null },

            new Child { ID = 3, ParentID = 3, Description = "Earth", Condition = null },

            new Child { ID = 4, ParentID = 4, Description = "Mars", Condition = null },

            new Child { ID = 5, ParentID = 4, Description = "Saturn", Condition = "> 5" } };

​

​

        var query = from p in parents

                    join c in children on p.ID equals c.ParentID into jc                    

                    from subchildren in jc.DefaultIfEmpty()

                    select new Item { 

                        ParentID = p.ID, 

                        Description = p.Description, 

                        PrimaryChildID = subchildren == null ? 0 : subchildren.ID, 

                        SubDescription = subchildren == null ? null : subchildren.Description,

                        ConditionalCount = 0};

        foreach (var item in query) 

            Console.WriteLine("{0} {1} {2} {3} {4}",

                item.ParentID, 

                item.PrimaryChildID,                              

                item.Description,                

                item.SubDescription, 

                item.ConditionalCount);

    }

    class Parent {

        public int ID {get;set;}

        public string Description {get;set;}

    }

    class Child {

        public int ID {get;set;}

        public int ParentID {get;set;}

        public string Description {get;set;}

        public string Condition {get;set;}

    }

    class Item {

        public int ParentID {get;set;}

        public string Description {get;set;}

        public int? PrimaryChildID {get;set;}  // The first one with null condition

        public string SubDescription {get;set;} // The first one with null condition

        public int ConditionalCount {get;set;} // Number of children with condition != null

    }

}