using System;

using System.Collections.Generic;

using System.Linq;

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 g

                       let firstNullElement = g.FirstOrDefault(x => x.Condition == null)

                       select new

                      {

                         ParentID = p.ID,

                         PrimaryChildID = firstNullElement != null ? firstNullElement.ID : 0,

                         Description = p.Description,

                         SubDescription = firstNullElement!= null ? firstNullElement.Description

                                                                  : String.Empty,

                         ConditionalCount = g.Count(x => x.Condition != null)

                      };

​

            foreach (var item in query)

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

                    item.ParentID,

                    item.PrimaryChildID,

                    item.Description,

                    item.SubDescription,

                    item.ConditionalCount);

    }

}

​

public class Parent

    {

        public int ID { get; set; }

        public string Description { get; set; }

    }

​

public class Child

    {

        public int ID { get; set; }

        public int ParentID { get; set; }

        public string Description { get; set; }

        public string Condition { get; set; }

    }