Step 8: A C# port, and what you built

using System;
using System.Collections.Generic;

class InsufficientFundsException : Exception
{
    public InsufficientFundsException(string message) : base(message) { }
}

class Account
{
    private long cents;
    private readonly List<string> history = new List<string>();

    public long Balance => cents;

    public void Deposit(long amount)
    {
        if (amount <= 0) throw new ArgumentException("deposit must be positive");
        cents += amount;
        history.Add("deposit " + amount);
    }

    public void Withdraw(long amount)
    {
        if (amount > cents) throw new InsufficientFundsException("needs " + amount + " but has " + cents);
        cents -= amount;
        history.Add("withdraw " + amount);
    }

    public IReadOnlyList<string> History => history;
}

class Program
{
    static void Main()
    {
        var account = new Account();
        account.Deposit(10000);
        account.Withdraw(2550);
        try
        {
            account.Withdraw(90000);
        }
        catch (InsufficientFundsException e)
        {
            Console.WriteLine("refused: " + e.Message);
        }
        foreach (var line in account.History) Console.WriteLine(line);
        Console.WriteLine("balance " + account.Balance);
    }
}

refused: needs 90000 but has 7450
deposit 10000
withdraw 2550
balance 7450