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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. This statement takes the following form:

Object thisLock = new Object();
lock (thisLock)
{
    // Critical code section
}

For more information, see Thread Synchronization (C# Programming Guide).

lock ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread attempts to enter a locked code, it will wait, block, until the object is released.

The section Threading (C# Programming Guide) discusses threading.

lock calls Enter at the beginning of the block and Exit at the end of the block.

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 since 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.

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");
    }

    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.

// statements_lock2.cs
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 will never be 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();
        }
    }
}

For more information, see the following sections in the C# Language Specification:

  • 5.3.3.18 Lock statements

  • 8.12 The lock statement

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