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ReaderWriterLock.AnyWritersSince Method

Indicates whether the writer lock has been granted to any thread since the sequence number was obtained.

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

public bool AnyWritersSince(
	int seqNum
)

Parameters

seqNum
Type: System.Int32

The sequence number.

Return Value

Type: System.Boolean
true if the writer lock has been granted to any thread since the sequence number was obtained; otherwise, false.

You can use WriterSeqNum and AnyWritersSince to improve application performance. For example, a thread might cache the information it obtains while holding a reader lock. After releasing and later reacquiring the lock, the thread can use AnyWritersSince to determine whether other threads have written to the resource in the interim; if not, the cached information can be used. This technique is useful where reading the information protected by the lock is expensive; for example, running a database query.

The caller must be holding a reader lock or a writer lock in order for the sequence number to be useful.

The following code example shows how to use the AnyWritersSince method and the WriterSeqNum property to determine whether another thread acquired the writer lock on the protected resource since the current thread last held the writer lock.

This code is part of a larger example provided for the ReaderWriterLock class.

// The complete code is located in the ReaderWriterLock 
// class topic. 
using System;
using System.Threading;

public class Test
{
    // Declaring the ReaderWriterLock at the class level 
    // makes it visible to all threads. 
    static ReaderWriterLock rwl = new ReaderWriterLock();
    // For this example, the shared resource protected by the 
    // ReaderWriterLock is just an integer. 
    static int resource = 0;


...


// 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(ref 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 time-out on the Restore method.
            Thread.Sleep(rnd.Next(250));
            rwl.RestoreLock(ref 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(ref reads);
                Display("resource has changed " + resourceValue);
            }
            else
            {
                Display("resource has not changed " + resourceValue);
            }
        }        
        finally
        {
            // Ensure that the lock is released.
            rwl.ReleaseReaderLock();
        }
    }
    catch (ApplicationException)
    {
        // The reader lock request timed out.
        Interlocked.Increment(ref readerTimeouts);
    }
}


...


}

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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