using System;

using System.Linq;

using System.Collections.Generic;

​

​

public class Program

{

    public static void Main()

    {

        var left= new List<Product>(){

            new Product(){ID=1,Name="A",Qty1=0,Qty2=5},

            new Product(){ID=2,Name="B",Qty1=0,Qty2=52}

        };

        var right= new List<Product>(){

            new Product(){ID=1,Name="A",Qty1=57,Qty2=0},

            new Product(){ID=2,Name="B",Qty1=84,Qty2=0}

        };

        var outer=left.Union(right);

        Console.WriteLine(outer.Count());//will be four which is not expected.

        var query = left.Join(right, d => d.ID, e=> e.ID, (f,g) => new Product(){ID = f.ID, Name = f.Name, Qty1 = f.Qty1+g.Qty1, Qty2 = f.Qty2+g.Qty2});

        foreach(var item in query)

        {

            item.Print();

        }

    }

}

​

public class Product {

    public int ID{get; set;}

    public string Name{get; set;}

    public int Qty1{get; set;}

    public int Qty2{get; set;}

    public void Print()

    {

        Console.WriteLine("ID  : {0}", ID);

        Console.WriteLine("Name: {0}", Name);

        Console.WriteLine("Qty1: {0}", Qty1);

        Console.WriteLine("Qty2: {0}", Qty2);

    }

}