lock Statement (C# Reference)

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;
 }
 }
 }

For more information, see Thread Synchronization (C# and Visual Basic).

Remarks

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.

Example

The following sample shows a simple use of threads without locking in C#.

 //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();
 }
 }
 }

C# Language Specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See Also

Reference

C# Keywords

Statement Keywords (C# Reference)

MethodImplAttributes

Mutex

Thread Synchronization (C# and Visual Basic)

Concepts

C# Programming Guide

Monitors

Interlocked Operations

AutoResetEvent

Other Resources

C# Reference

Threading (C# and Visual Basic)