lock Statement
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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. 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).

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.

You can't use the await keyword in the body of a lock statement.

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

Example

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();
        }
    }
}

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

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