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ReaderWriterLock Class

Defines a lock that supports single writers and multiple readers.

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

[ComVisibleAttribute(true)] 
public ref class ReaderWriterLock sealed : public CriticalFinalizerObject
/** @attribute ComVisibleAttribute(true) */ 
public final class ReaderWriterLock extends CriticalFinalizerObject
ComVisibleAttribute(true) 
public final class ReaderWriterLock extends CriticalFinalizerObject

NoteNote

The HostProtectionAttribute attribute applied to this class has the following Resources property value: Synchronization | ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

ReaderWriterLock is used to synchronize access to a resource. At any given time, it allows either concurrent read access for multiple threads, or write access for a single thread. In a situation where a resource is changed infrequently, a ReaderWriterLock provides better throughput than a simple one-at-a-time lock, such as Monitor.

ReaderWriterLock works best where most accesses are reads, while writes are infrequent and of short duration. Multiple readers alternate with single writers, so that neither readers nor writers are blocked for long periods.

NoteNote

Holding reader locks or writer locks for long periods will starve other threads. For best performance, consider restructuring your application to minimize the duration of writes.

A thread can hold a reader lock or a writer lock, but not both at the same time. Instead of releasing a reader lock in order to acquire the writer lock, you can use UpgradeToWriterLock and DowngradeFromWriterLock.

Recursive lock requests increase the lock count on a lock.

Readers and writers are queued separately. When a thread releases the writer lock, all threads waiting in the reader queue at that instant are granted reader locks; when all of those reader locks have been released, the next thread waiting in the writer queue, if any, is granted the writer lock, and so on. In other words, ReaderWriterLock alternates between a collection of readers, and one writer.

While a thread in the writer queue is waiting for active reader locks to be released, threads requesting new reader locks accumulate in the reader queue. Their requests are not granted, even though they could share concurrent access with existing reader-lock holders; this helps protect writers against indefinite blockage by readers.

Most methods for acquiring locks on a ReaderWriterLock accept time-out values. Use time-outs to avoid deadlocks in your application. For example, a thread might acquire the writer lock on one resource and then request a reader lock on a second resource; in the meantime, another thread might acquire the writer lock on the second resource, and request a reader lock on the first. Unless time-outs are used, the threads deadlock.

If the time-out interval expires and the lock request has not been granted, the method returns control to the calling thread by throwing an ApplicationException. A thread can catch this exception and determine what action to take next.

Time-outs are expressed in milliseconds. If you use a System.TimeSpan to specify the time-out, the value used is the total number of whole milliseconds represented by the TimeSpan. The following table shows the valid time-out values in milliseconds.

Value

Description

-1

Infinite.

0

No time-out.

> 0

The number of milliseconds to wait.

With the exception of -1, negative time-out values are not allowed. If you use a negative integer other than -1 to specify the time-out, zero (no time-out) is used. If you specify a TimeSpan that represents a negative number of milliseconds other than -1, ArgumentOutOfRangeException is thrown.

// This example shows a ReaderWriterLock protecting a shared
// resource that is read concurrently and written exclusively
// by multiple threads.
// The complete code is located in the ReaderWriterLock
// class topic.
using namespace System;
using namespace System::Threading;
public ref class Test
{
public:

   // Declaring the ReaderWriterLock at the class level
   // makes it visible to all threads.
   static ReaderWriterLock^ rwl = gcnew ReaderWriterLock;

   // For this example, the shared resource protected by the
   // ReaderWriterLock is just an integer.
   static int resource = 0;

   literal int numThreads = 26;
   static bool running = true;
   static Random^ rnd = gcnew Random;

   // Statistics.
   static int readerTimeouts = 0;
   static int writerTimeouts = 0;
   static int reads = 0;
   static int writes = 0;
   static void ThreadProc()
   {
      
      // As long as a thread runs, it randomly selects
      // various ways to read and write from the shared 
      // resource. Each of the methods demonstrates one 
      // or more features of ReaderWriterLock.
      while ( running )
      {
         double action = rnd->NextDouble();
         if ( action < .8 )
                  ReadFromResource( 10 );
         else
         if ( action < .81 )
                  ReleaseRestore( 50 );
         else
         if ( action < .90 )
                  UpgradeDowngrade( 100 );
         else
                  WriteToResource( 100 );
      }
   }


   // Shows how to request and release a reader lock, and
   // how to handle time-outs.
   static void ReadFromResource( int timeOut )
   {
      try
      {
         rwl->AcquireReaderLock( timeOut );
         try
         {
            
            // It is safe for this thread to read from
            // the shared resource.
            Display( String::Format( "reads resource value {0}", resource ) );
            Interlocked::Increment( reads );
         }
         finally
         {
            
            // Ensure that the lock is released.
            rwl->ReleaseReaderLock();
         }

      }
      catch ( ApplicationException^ ) 
      {
         
         // The reader lock request timed out.
         Interlocked::Increment( readerTimeouts );
      }

   }


   // Shows how to request and release the writer lock, and
   // how to handle time-outs.
   static void WriteToResource( int timeOut )
   {
      try
      {
         rwl->AcquireWriterLock( timeOut );
         try
         {
            
            // It is safe for this thread to read or write
            // from the shared resource.
            resource = rnd->Next( 500 );
            Display( String::Format( "writes resource value {0}", resource ) );
            Interlocked::Increment( writes );
         }
         finally
         {
            
            // Ensure that the lock is released.
            rwl->ReleaseWriterLock();
         }

      }
      catch ( ApplicationException^ ) 
      {
         
         // The writer lock request timed out.
         Interlocked::Increment( writerTimeouts );
      }

   }


   // Shows how to request a reader lock, upgrade the
   // reader lock to the writer lock, and downgrade to a
   // reader lock again.
   static void UpgradeDowngrade( int timeOut )
   {
      try
      {
         rwl->AcquireReaderLock( timeOut );
         try
         {
            
            // It is safe for this thread to read from
            // the shared resource.
            Display( String::Format( "reads resource value {0}", resource ) );
            Interlocked::Increment( reads );
            
            // If it is necessary to write to the resource,
            // you must either release the reader lock and 
            // then request the writer lock, or upgrade the
            // reader lock. Note that upgrading the reader lock
            // puts the thread in the write queue, behind any
            // other threads that might be waiting for the 
            // writer lock.
            try
            {
               LockCookie lc = rwl->UpgradeToWriterLock( timeOut );
               try
               {
                  
                  // It is safe for this thread to read or write
                  // from the shared resource.
                  resource = rnd->Next( 500 );
                  Display( String::Format( "writes resource value {0}", resource ) );
                  Interlocked::Increment( writes );
               }
               finally
               {
                  
                  // Ensure that the lock is released.
                  rwl->DowngradeFromWriterLock( lc );
               }

            }
            catch ( ApplicationException^ ) 
            {
               
               // The upgrade request timed out.
               Interlocked::Increment( writerTimeouts );
            }

            
            // When the lock has been downgraded, it is 
            // still safe to read from the resource.
            Display( String::Format( "reads resource value {0}", resource ) );
            Interlocked::Increment( reads );
         }
         finally
         {
            
            // Ensure that the lock is released.
            rwl->ReleaseReaderLock();
         }

      }
      catch ( ApplicationException^ ) 
      {
         
         // The reader lock request timed out.
         Interlocked::Increment( readerTimeouts );
      }

   }


   // Shows how to release all locks and later restore
   // the lock state. Shows how to use sequence numbers
   // to determine whether another thread has obtained
   // a writer lock since this thread last accessed the
   // resource.
   static void ReleaseRestore( int timeOut )
   {
      int lastWriter;
      try
      {
         rwl->AcquireReaderLock( timeOut );
         try
         {
            
            // It is safe for this thread to read from
            // the shared resource. Cache the value. (You
            // might do this if reading the resource is
            // an expensive operation.)
            int resourceValue = resource;
            Display( String::Format( "reads resource value {0}", resourceValue ) );
            Interlocked::Increment( reads );
            
            // Save the current writer sequence number.
            lastWriter = rwl->WriterSeqNum;
            
            // Release the lock, and save a cookie so the
            // lock can be restored later.
            LockCookie lc = rwl->ReleaseLock();
            
            // Wait for a random interval (up to a 
            // quarter of a second), and then restore
            // the previous state of the lock. Note that
            // there is no timeout on the Restore method.
            Thread::Sleep( rnd->Next( 250 ) );
            rwl->RestoreLock( lc );
            
            // Check whether other threads obtained the
            // writer lock in the interval. If not, then
            // the cached value of the resource is still
            // valid.
            if ( rwl->AnyWritersSince( lastWriter ) )
            {
               resourceValue = resource;
               Interlocked::Increment( reads );
               Display( String::Format( "resource has changed {0}", resourceValue ) );
            }
            else
            {
               Display( String::Format( "resource has not changed {0}", resourceValue ) );
            }
         }
         finally
         {
            
            // Ensure that the lock is released.
            rwl->ReleaseReaderLock();
         }

      }
      catch ( ApplicationException^ ) 
      {
         
         // The reader lock request timed out.
         Interlocked::Increment( readerTimeouts );
      }

   }


   // Helper method briefly displays the most recent
   // thread action. Comment out calls to Display to 
   // get a better idea of throughput.
   static void Display( String^ msg )
   {
      Console::Write( "Thread {0} {1}.       \r", Thread::CurrentThread->Name, msg );
   }

};


int main()
{
   array<String^>^args = Environment::GetCommandLineArgs();
   
   // Start a series of threads. Each thread randomly
   // performs reads and writes on the shared resource.
   array<Thread^>^t = gcnew array<Thread^>(Test::numThreads);
   for ( int i = 0; i < Test::numThreads; i++ )
   {
      t[ i ] = gcnew Thread( gcnew ThreadStart( Test::ThreadProc ) );
      t[ i ]->Name = gcnew String( Convert::ToChar( i + 65 ),1 );
      t[ i ]->Start();
      if ( i > 10 )
            Thread::Sleep( 300 );

   }
   
   // Tell the threads to shut down, then wait until they all
   // finish.
   Test::running = false;
   for ( int i = 0; i < Test::numThreads; i++ )
   {
      t[ i ]->Join();

   }
   
   // Display statistics.
   Console::WriteLine( "\r\n {0} reads, {1} writes, {2} reader time-outs, {3} writer time-outs.", Test::reads, Test::writes, Test::readerTimeouts, Test::writerTimeouts );
   Console::WriteLine( "Press ENTER to exit." );
   Console::ReadLine();
   return 0;
}


// This example shows a ReaderWriterLock protecting a shared
// resource that is read concurrently and written exclusively
// by multiple threads.

// The complete code is located in the ReaderWriterLock
// class topic.
import System.*;
import System.Threading.*;
import System.Threading.Thread;    

public class Test
{
    // Declaring the ReaderWriterLock at the class level
    // makes it visible to all threads.
    private static ReaderWriterLock rwl = new ReaderWriterLock();

    // For this example, the shared resource protected by the
    // ReaderWriterLock is just an integer.
    private static int resource = 0;
    private static int numThreads = 26;
    private static boolean running = true;
    private static Random rnd = new Random();

    // Statistics.
    private static int readerTimeouts = 0;
    private static int writerTimeouts = 0;
    private static int reads = 0;
    private static int writes = 0;

    public static void main(String[] args)
    {
        // Start a series of threads. Each thread randomly
        // performs reads and writes on the shared resource.
        Thread t[] = new Thread[numThreads];

        for (int i = 0; i < numThreads; i++) {
            t[i] = new Thread(new ThreadStart(ThreadProc));
            t[i].set_Name(new String(Convert.ToChar((i + 65)), 1));
            t[i].Start();
            if (i > 10) {
                Thread.Sleep(300);
        }
    } //main

    // Tell the threads to shut down, then wait until they all
    // finish.
    running = false;
    for (int i = 0; i < numThreads; i++) {
        t[i].Join();
    }

    // Display statistics.
    Console.WriteLine("\r\n{0} reads, {1} writes, {2} reader time-outs," 
        + " {3} writer time-outs.",
        new Object[] { (Int32)(reads), (Int32)(writes), 
        (Int32)(readerTimeouts), (Int32)(writerTimeouts) });
    Console.WriteLine("Press ENTER to exit.");
    Console.ReadLine();
} //Test


    static void ThreadProc()
    {
        // As long as a thread runs, it randomly selects
        // various ways to read and write from the shared 
        // resource. Each of the methods demonstrates one 
        // or more features of ReaderWriterLock.
        while (running) {
            double action = rnd.NextDouble();

            if (action < 0.8) {
                ReadFromResource(10);
            }
            else {
                if (action < 0.81) {
                    ReleaseRestore(50);
                }
                else {
                    if (action < 0.9) {
                        UpgradeDowngrade(100);
                    }
                    else {
                        WriteToResource(100);
                    }
                }
            }
        }
    } //ThreadProc

    // Shows how to request and release a reader lock, and
    // how to handle time-outs.
    static void ReadFromResource(int timeOut)
    {
        try {
            rwl.AcquireReaderLock(timeOut);
            try {
                // It is safe for this thread to read from
                // the shared resource.
                Display(("reads resource value " + resource));
                Interlocked.Increment(reads);
            }
            finally {
                // Ensure that the lock is released.
                rwl.ReleaseReaderLock();
            }
        }
        catch (ApplicationException exp) {
            // The reader lock request timed out.
            Interlocked.Increment(readerTimeouts);
        }
    } //ReadFromResource

    // Shows how to request and release the writer lock, and
    // how to handle time-outs.
    static void WriteToResource(int timeOut)
    {
        try {
            rwl.AcquireWriterLock(timeOut);
            try {
                // It is safe for this thread to read or write
                // from the shared resource.
                resource = rnd.Next(500);
                Display(("writes resource value " + resource));
                Interlocked.Increment(writes);
            }
            finally {
                // Ensure that the lock is released.
                rwl.ReleaseWriterLock();
            }
        }
        catch (ApplicationException exp) {
            // The writer lock request timed out.
            Interlocked.Increment(writerTimeouts);
        }
    } //WriteToResource

    // Shows how to request a reader lock, upgrade the
    // reader lock to the writer lock, and downgrade to a
    // reader lock again.
    static void UpgradeDowngrade(int timeOut)
    {
        try {
            rwl.AcquireReaderLock(timeOut);
            try {
                // It is safe for this thread to read from
                // the shared resource.
                Display(("reads resource value " + resource));
                Interlocked.Increment(reads);

                // If it is necessary to write to the resource,
                // you must either release the reader lock and 
                // then request the writer lock, or upgrade the
                // reader lock. Note that upgrading the reader lock
                // puts the thread in the write queue, behind any
                // other threads that might be waiting for the 
                // writer lock.
                try {
                    LockCookie lc = rwl.UpgradeToWriterLock(timeOut);

                    try {
                        // It is safe for this thread to read or write
                        // from the shared resource.
                        resource = rnd.Next(500);
                        Display(("writes resource value " + resource));
                        Interlocked.Increment(writes);
                    }
                    finally {
                        // Ensure that the lock is released.
                        rwl.DowngradeFromWriterLock(lc);
                    }
                }
                catch (ApplicationException exp) {
                    // The upgrade request timed out.
                    Interlocked.Increment(writerTimeouts);
                }

                // When the lock has been downgraded, it is 
                // still safe to read from the resource.
                Display(("reads resource value " + resource));
                Interlocked.Increment(reads);
            }
            finally {
                // Ensure that the lock is released.
                rwl.ReleaseReaderLock();
            }
        }
        catch (ApplicationException exp) {
            // The reader lock request timed out.
            Interlocked.Increment(readerTimeouts);
        }
    } //UpgradeDowngrade


    // Shows how to release all locks and later restore
    // the lock state. Shows how to use sequence numbers
    // to determine whether another thread has obtained
    // a writer lock since this thread last accessed the
    // resource.
    static void ReleaseRestore(int timeOut)
    {
        int lastWriter;

        try {
            rwl.AcquireReaderLock(timeOut);
            try {
                // It is safe for this thread to read from
                // the shared resource. Cache the value. (You
                // might do this if reading the resource is
                // an expensive operation.)
                int resourceValue = resource;

                Display(("reads resource value " + resourceValue));
                Interlocked.Increment(reads);

                // Save the current writer sequence number.
                lastWriter = rwl.get_WriterSeqNum();

                // Release the lock, and save a cookie so the
                // lock can be restored later.
                LockCookie lc = rwl.ReleaseLock();

                // Wait for a random interval (up to a 
                // quarter of a second), and then restore
                // the previous state of the lock. Note that
                // there is no time-out on the Restore method.
                Thread.Sleep(rnd.Next(250));
                rwl.RestoreLock(lc);

                // Check whether other threads obtained the
                // writer lock in the interval. If not, then
                // the cached value of the resource is still
                // valid.
                if (rwl.AnyWritersSince(lastWriter)) {
                    resourceValue = resource;
                    Interlocked.Increment(reads);
                    Display(("resource has changed " + resourceValue));
                }
                else {
                    Display(("resource has not changed " + resourceValue));
                }
            }
            finally {
                // Ensure that the lock is released.
                rwl.ReleaseReaderLock();
            }
        }
        catch (ApplicationException exp) {
            // The reader lock request timed out.
            Interlocked.Increment(readerTimeouts);
        }
    } //ReleaseRestore

    // Helper method briefly displays the most recent
    // thread action. Comment out calls to Display to 
    // get a better idea of throughput.
    static void Display(String msg)
    {
        Console.Write("Thread {0} {1}.       \r",
            Thread.get_CurrentThread().get_Name(), msg);
    } //Display
}

This type is safe for multithreaded operations.

Windows 98, Windows 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.

.NET Framework

Supported in: 2.0, 1.1, 1.0

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