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)

'Declaration
Public NotInheritable 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(Of 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(Of 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.
Top

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.


Imports System.Threading

Class Example

    Private Shared outputBlock As System.Windows.Controls.TextBlock

    '0 for false, 1 for true.
    Private Shared usingResource As Integer = 0

    Private Const numThreadIterations As Integer = 5
    Private Const numThreads As Integer = 10

    Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

        Example.outputBlock = outputBlock

        ' Run the demo on a separate thread because it uses Sleep, which makes
        ' the user interface unresponsive.
        Dim ct As New Thread(AddressOf ControlThread)
        ct.Start()

    End Sub

    Private Shared Sub ControlThread()

        Dim myThread As Thread
        Dim rnd As New Random()

        Dim i As Integer
        For i = 1 To numThreads 
            myThread = New Thread(AddressOf MyThreadProc)
            myThread.Name = [String].Format("Thread{0}", i)

            'Wait a random amount of time before starting next thread.
            Thread.Sleep(rnd.Next(0, 1000))
            myThread.Start()
        Next i
    End Sub 

    Private Shared Sub MyThreadProc()
        Dim i As Integer
        For i = 0 To numThreadIterations - 1
            UseResource()

            'Wait 1 second before next attempt.
            Thread.Sleep(1000)
        Next i
    End Sub 

    'A simple method that denies reentrancy.
    Shared Function UseResource() As Boolean
        '0 indicates that the method is not in use.
        If 0 = Interlocked.Exchange(usingResource, 1) Then
            Output(String.Format("{0} acquired the lock" & vbLf, 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" & vbLf, Thread.CurrentThread.Name))

            'Release the lock
            Interlocked.Exchange(usingResource, 0)
            Return True
        Else
            Output(String.Format("   {0} was denied the lock" & vbLf, Thread.CurrentThread.Name))
            Return False
        End If
    End Function 

    ' 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 Shared Sub Output(ByVal msg As String)
        outputBlock.Dispatcher.BeginInvoke(displayHelper, msg)
    End Sub
    Private Shared displayHelper As New Action(Of String)(AddressOf DisplayOutput)
    Private Shared Sub DisplayOutput(ByVal msg As String)
        outputBlock.Text &= msg 
    End Sub

End Class 

' 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.

Community Additions

ADD
Show: