using System;

using System.Threading;

​

class Account

{

    private Object thisLock = new Object();

    int balance;

    Random r = new Random();

    public Account(int initial)

    {

        balance = initial;

    }

​

    int Withdraw(int amount)

    {

        // This condition never is true unless the lock statement

        // is commented out.

        if (balance < 0)

        {

            throw new Exception("Negative Balance");

        }

​

        // Comment out the next line to see the effect of leaving out 

        // the lock keyword.

        lock (thisLock)

        {

            if (balance >= amount)

            {

                Console.WriteLine("Balance before Withdrawal :  " + balance);

                Console.WriteLine("Amount to Withdraw        : -" + amount);

                balance = balance - amount;

                Console.WriteLine("Balance after Withdrawal  :  " + balance);

                return amount;

            }

            else

            {

                return 0; // transaction rejected

            }

        }

    }

​

    public void DoTransactions()

    {

        for (int i = 0; i < 100; i++)

        {

            Withdraw(r.Next(1, 100));

        }

    }

}

​

public class Test

{

    public static void Main()

    {

        Thread[] threads = new Thread[10];

        Account acc = new Account(1000);

        for (int i = 0; i < 10; i++)

        {

            Thread t = new Thread(new ThreadStart(acc.DoTransactions));

            threads[i] = t;

        }

​

        for (int i = 0; i < 10; i++)

        {

            threads[i].Start();

        }

    }

}