lock-statement.md

title	lock Statement (C# Reference)
ms.date	07/20/2015
ms.prod	.net
ms.technology	devlang-csharp
ms.topic	article
f1_keywords	lock_CSharpKeyword lock
helpviewer_keywords	lock keyword [C#]
ms.assetid	656da1a4-707e-4ef6-9c6e-6d13b646af42
caps.latest.revision	43
author	BillWagner
ms.author	wiwagn

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.

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 discusses threading.

The lock keyword calls xref:System.Threading.Monitor.Enter%2A at the start of the block and xref:System.Threading.Monitor.Exit%2A at the end of the block. A xref:System.Threading.ThreadInterruptedException is thrown if xref:System.Threading.Thread.Interrupt%2A 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#.

[!code-csharpcsrefKeywordsFixedLock#5]

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.

[!code-csharpcsrefKeywordsFixedLock#6]

C# Language Specification

[!INCLUDECSharplangspec]

See Also

xref:System.Reflection.MethodImplAttributes
xref:System.Threading.Mutex
C# Reference
C# Programming Guide
Threading
C# Keywords
Statement Keywords
xref:System.Threading.Monitor
Interlocked Operations
AutoResetEvent
Thread Synchronization