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Interlocked Class

Provides atomic operations for variables that are shared by multiple threads.

System.Object
  System.Threading.Interlocked

Namespace:  System.Threading
Assembly:  mscorlib (in mscorlib.dll)

public static class Interlocked

  NameDescription
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Add(Int32, Int32)Adds two 32-bit integers and replaces the first integer with the sum, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Add(Int64, Int64)Adds two 64-bit integers and replaces the first integer with the sum, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360CompareExchange(Int32, Int32, Int32)Compares two 32-bit signed integers for equality and, if they are equal, replaces one of the values.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360CompareExchange(Int64, Int64, Int64)Compares two 64-bit signed integers for equality and, if they are equal, replaces one of the values.
Public methodStatic memberSupported by Silverlight for Windows PhoneCompareExchange(Object, Object, Object)Compares two objects for reference equality and, if they are equal, replaces one of the objects.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360CompareExchange<T>(T, T, T)Compares two instances of the specified reference type T for equality and, if they are equal, replaces one of them.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Decrement(Int32)Decrements a specified 32-bit signed integer variable and stores the result, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Decrement(Int64)Decrements the specified 64-bit signed integer variable and stores the result, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Exchange(Int32, Int32)Sets a 32-bit signed integer to a specified value and returns the original value, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Exchange(Int64, Int64)Sets a 64-bit signed integer to a specified value and returns the original value, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Exchange<T>(T, T)Sets a variable of the specified type T to a specified value and returns the original value, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Increment(Int32)Increments a specified 32-bit signed variable and stores the result, as an atomic operation.
Public methodStatic memberSupported by Silverlight for Windows PhoneSupported by Xbox 360Increment(Int64)Increments a specified 64-bit signed integer variable and stores the result, as an atomic operation.
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The methods of this class help protect against errors that can occur when the scheduler switches contexts while a thread is updating a variable that can be accessed by other threads, or when two threads are executing concurrently on separate processors. The members of this class do not throw exceptions.

The Increment and Decrement methods increment or decrement a variable and store the resulting value in a single operation. On most computers, incrementing a variable is not an atomic operation. It requires the following steps:

  1. Load a value from an instance variable into a register.

  2. Increment or decrement the value.

  3. Store the value in the instance variable.

If you do not use Increment and Decrement, a thread can be preempted after executing the first two steps. Another thread can then execute all three steps. When the first thread resumes execution, it overwrites the value in the instance variable, and the effect of the increment or decrement performed by the second thread is lost.

The Exchange method atomically exchanges the values of the specified variables. The CompareExchange method combines two operations: It compares two values and stores a third value in one of the variables, based on the outcome of the comparison. The comparison and the exchange are performed as an atomic operation.

Version Notes

Silverlight for Windows Phone Silverlight for Windows Phone

 64-bit members of the Interlocked class are present but not supported.

The following example shows a thread-safe resource locking mechanism.


using System;
using System.Threading;

class Example
{
    private static System.Windows.Controls.TextBlock outputBlock;

    //0 for false, 1 for true.
    private static int usingResource = 0;

    private const int numThreadIterations = 5;
    private const int numThreads = 10;

    public static void Demo(System.Windows.Controls.TextBlock outputBlock)
    {
        Example.outputBlock = outputBlock;

        // Run the demo on a separate thread because it uses Sleep, which makes
        // the user interface unresponsive.
        Thread ct = new Thread(ControlThread);
        ct.Start();
    }

    private static void ControlThread()
    {
        Thread myThread;
        Random rnd = new Random();

        int i;
        for(i = 0; i < numThreads; i++)
        {
            myThread = new Thread(MyThreadProc);
            myThread.Name = String.Format("Thread{0}", i + 1);

            //Wait a random amount of time before starting next thread.
            Thread.Sleep(rnd.Next(0, 1000));
            myThread.Start();
        }
    }

    private static void MyThreadProc()
    {
        int i;
        for(i = 0; i < numThreadIterations; i++)
        {
            UseResource();

            //Wait 1 second before next attempt.
            Thread.Sleep(1000);
        }
    }

    //A simple method that denies reentrancy.
    public static bool UseResource()
    {
        //0 indicates that the method is not in use.
        if (0 == Interlocked.Exchange(ref usingResource, 1))
        {
            Output(String.Format("{0} acquired the lock\n", Thread.CurrentThread.Name));

            //Code to access a resource that is not thread safe would go here.
            //Simulate some work
            Thread.Sleep(500);

            Output(String.Format("{0} exiting lock\n", Thread.CurrentThread.Name));

            //Release the lock
            Interlocked.Exchange(ref usingResource, 0);
            return true;
        }
        else
        {
            Output(String.Format("   {0} was denied the lock\n", Thread.CurrentThread.Name));
            return false;
        }
    }

    // Helper methods:

    // In order to update the TextBlock object, which is on the UI thread, you must
    // make a cross-thread call by using the Dispatcher object that is associated 
    // with the TextBlock. The DisplayOutput helper method and its delegate, 
    // displayHelper, are used by the BeginInvoke method of the Dispatcher object
    // to append text to the TextBlock. 
    //
    private static void Output(string msg)
    {
        outputBlock.Dispatcher.BeginInvoke(displayHelper, msg);
    }
    private static Action<string> displayHelper = new Action<string>(DisplayOutput);
    private static void DisplayOutput(string msg)
    {
        outputBlock.Text += msg;
    }
}

/* This example produces output similar to the following:

Thread1 acquired the lock
   Thread2 was denied the lock
Thread1 exiting lock 
Thread3 acquired the lock
   Thread4 was denied the lock
   Thread2 was denied the lock
   Thread1 was denied the lock
Thread3 exiting lock 
Thread6 acquired the lock
   Thread7 was denied the lock
   Thread4 was denied the lock
   Thread2 was denied the lock
   Thread5 was denied the lock
Thread6 exiting lock 
...
 */


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

This type is thread safe.

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