Coin Changer kata in C# using LINQ and helper class to make the aggregate more immutable.

coin_changer_aggregate.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace CoinChanger
{
    public class Changer
    {
        public ICollection<int> Coins { get; set; }
 
        public Changer()
        {
            Coins = new List<int>();
        }
 
        public ICollection<int> For(int amount)
        {
            return Coins.Aggregate(
                new State(new List<int>(), amount), (working, coin) =>
                    {
                        working.Result.Add(working.Amount/coin);
                        return new State(working.Result, working.Amount%coin);
                    }).Result;
        }
 
        class State
        {
            public ICollection<int> Result { get; set; }
            public int Amount { get; set; }
 
            public State(ICollection<int> result, int amount)
            {
                Result = result;
                Amount = amount;
            }
        }
    }
}

See also my blog post which goes with this gist.
http://comp-phil.blogspot.com/2014/05/the-form-of-katas.html