C# Implementation

gistfile1.cs

 public class Class1
    {
        public static void Main(string[] args)
        {
            var buyTransactions = (new int[200000]).Select(b => new BuyTransaction());
            var sellTransactions = (new int[400000]).Select(b => new SellTransaction());

            var transactions = buyTransactions.Concat<ITransaction>(sellTransactions);

            var startTime = DateTime.Now;
            var total = transactions.Sum(x => x.Amount);
            var endTime = DateTime.Now;

            Console.WriteLine(total + " - " + (endTime - startTime).TotalMilliseconds);
            Console.ReadKey();
        }
    }

    public class SellTransaction : ITransaction
    {
        public decimal Amount
        {
            get { return Decimal.Parse("5.2342"); }
        }
    }

    public class BuyTransaction : ITransaction
    {
        public decimal Amount
        {
            get { return Decimal.Parse("-10.2342"); }
        }
    }

    public interface ITransaction
    {
        decimal Amount { get; }
    }