The lock keyword marks a statement block as a critical section by obtaining the mutual-exclusion lock for a given object, executing a statement, and then releasing the lock. This statement takes the following form:
Object thisLock = new Object();
lock (thisLock)
{
// Critical code section
}
lock (thisLock)
{
// Critical code section
}
/*********************/
The following sample shows a simple use of threads in C#.
// statements_lock.cs
using System;
using System.Threading;
class ThreadTest
{
public void RunMe()
{
Console.WriteLine("RunMe called");
}
{
public void RunMe()
{
Console.WriteLine("RunMe called");
}
static void Main()
{
ThreadTest b = new ThreadTest();
Thread t = new Thread(b.RunMe);
t.Start();
}
}
{
ThreadTest b = new ThreadTest();
Thread t = new Thread(b.RunMe);
t.Start();
}
}
/*********************/
The following sample uses threads and lock. As long as the lock statement is present, the statement block is a critical section and
balance
will never become a negative number.// statements_lock2.cs
using System;
using System.Threading;
class Account
{
private Object thisLock = new Object();
int balance;
{
private Object thisLock = new Object();
int balance;
Random r = new Random();
public Account(int initial)
{
balance = initial;
}
{
balance = initial;
}
int Withdraw(int amount)
{
{
// This condition will never be true unless the lock statement
// is commented out:
if (balance < 0)
{
throw new Exception("Negative Balance");
}
// 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
}
}
}
// 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));
}
}
}
{
for (int i = 0; i < 100; i++)
{
Withdraw(r.Next(1, 100));
}
}
}
class Test
{
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();
}
}
}
{
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();
}
}
}