Interlocked Class
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Interlocked Class

 

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

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

System.Object
  System.Threading.Interlocked

public static class Interlocked

NameDescription
System_CAPS_pubmethodSystem_CAPS_staticAdd(Int32, Int32)

Adds two 32-bit integers and replaces the first integer with the sum, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticAdd(Int64, Int64)

Adds two 64-bit integers and replaces the first integer with the sum, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange(Double, Double, Double)

Compares two double-precision floating point numbers for equality and, if they are equal, replaces the first value.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange(Int32, Int32, Int32)

Compares two 32-bit signed integers for equality and, if they are equal, replaces the first value.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange(Int64, Int64, Int64)

Compares two 64-bit signed integers for equality and, if they are equal, replaces the first value.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange(IntPtr, IntPtr, IntPtr)

Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange(Object, Object, Object)

Compares two objects for reference equality and, if they are equal, replaces the first object.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange(Single, Single, Single)

Compares two single-precision floating point numbers for equality and, if they are equal, replaces the first value.

System_CAPS_pubmethodSystem_CAPS_staticCompareExchange<T>(T, T, T)

Compares two instances of the specified reference type T for equality and, if they are equal, replaces the first one.

System_CAPS_pubmethodSystem_CAPS_staticDecrement(Int32)

Decrements a specified variable and stores the result, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticDecrement(Int64)

Decrements the specified variable and stores the result, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange(Double, Double)

Sets a double-precision floating point number to a specified value and returns the original value, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange(Int32, Int32)

Sets a 32-bit signed integer to a specified value and returns the original value, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange(Int64, Int64)

Sets a 64-bit signed integer to a specified value and returns the original value, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange(IntPtr, IntPtr)

Sets a platform-specific handle or pointer to a specified value and returns the original value, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange(Object, Object)

Sets an object to a specified value and returns a reference to the original object, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange(Single, Single)

Sets a single-precision floating point number to a specified value and returns the original value, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticExchange<T>(T, T)

Sets a variable of the specified type T to a specified value and returns the original value, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticIncrement(Int32)

Increments a specified variable and stores the result, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticIncrement(Int64)

Increments a specified variable and stores the result, as an atomic operation.

System_CAPS_pubmethodSystem_CAPS_staticMemoryBarrier()

Synchronizes memory access as follows: The processor that executes the current thread cannot reorder instructions in such a way that memory accesses before the call to MemoryBarrier execute after memory accesses that follow the call to MemoryBarrier.

System_CAPS_pubmethodSystem_CAPS_staticRead(Int64)

Returns a 64-bit value, loaded as an atomic operation.

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, requiring 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: comparing two values and storing a third value in one of the variables, based on the outcome of the comparison. The compare and exchange operations are performed as an atomic operation.

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

using System;
using System.Threading;

namespace InterlockedExchange_Example
{
    class MyInterlockedExchangeExampleClass
    {
        //0 for false, 1 for true.
        private static int usingResource = 0;

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

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

            for(int i = 0; i < numThreads; i++)
            {
                myThread = new Thread(new ThreadStart(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()
        {
            for(int i = 0; i < numThreadIterations; i++)
            {
                UseResource();

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

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

                //Code to access a resource that is not thread safe would go here.

                //Simulate some work
                Thread.Sleep(500);

                Console.WriteLine("{0} exiting lock", Thread.CurrentThread.Name);

                //Release the lock
                Interlocked.Exchange(ref usingResource, 0);
                return true;
            }
            else
            {
                Console.WriteLine("   {0} was denied the lock", Thread.CurrentThread.Name);
                return false;
            }
        }

    }
}  

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1

This type is thread safe.

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