using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

​

/* ListA:

 Denom:20

 NumberItems:1

​

ListB:

 Denom:20

 NumberItems:4,

 Denom:50

 NumberItems:2*/

        var listA = new List<Bills>()

        {

            new Bills() { Denom = 20, NumberItems = 1}

        };

        var listB = new List<Bills>()

        {

            new Bills() { Denom = 20, NumberItems = 4},

            new Bills() { Denom = 50, NumberItems = 2}

        };

        var listC = from b in listB join a in listA on b.Denom equals a.Denom into temp

            from t in temp.DefaultIfEmpty()

            select new Bills() { Denom = b.Denom, NumberItems = t != null ? b.NumberItems - t.NumberItems : b.NumberItems };

        listC.Dump();

    }

​

}

​

public class Bills

{

    public int Denom;

    public int NumberItems;

}