Lock Statement And Thread Synchronization
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. The following example includes a lock statement.
class Account
{
decimal balance;
private Object thisLock = new Object();
public void Withdraw(decimal amount)
{
lock (thisLock)
{
if (amount > balance)
{
throw new Exception("Insufficient funds");
}
balance -= amount;
}
}
}
The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread tries to enter a locked code, it will wait, block, until the object is released.
The section Threading (C# and Visual Basic) discusses threading.
The lock keyword calls Enter at the start of the block and Exit at the end of the block. A ThreadInterruptedException is thrown if Interrupt interrupts a thread that is waiting to enter a lock statement.
In general, avoid locking on a public type, or instances beyond your code's control. The common constructs lock (this), lock (typeof (MyType)), and lock ("myLock") violate this guideline:
· lock (this) is a problem if the instance can be accessed publicly.
· lock (typeof (MyType)) is a problem if MyType is publicly accessible.
· lock("myLock") is a problem because any other code in the process using the same string, will share the same lock.
Best practice is to define a private object to lock on, or a private static object variable to protect data common to all instances.
//using System.Threading;
class ThreadTest
{
public void RunMe()
{
Console.WriteLine("RunMe called");
}
static void Main()
{
ThreadTest b = new ThreadTest();
Thread t = new Thread(b.RunMe);
t.Start();
}
}
// Output: RunMe called
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.
// 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));
}
}
}
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();
}
}
}
