Interlocked.Exchange Method (Int32%, Int32)

Parameters

• location1
  Type: System.Int32%
  The variable to set to the specified value.

• value
  Type: System.Int32
  The value to which the location1 parameter is set.

Return Value

Type: System.Int32
The original value of location1.

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 
'...

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 
...
 */