ReaderWriterLock.WriterSeqNum Eigenschaft
Definition
Wichtig
Einige Informationen beziehen sich auf Vorabversionen, die vor dem Release ggf. grundlegend überarbeitet werden. Microsoft übernimmt hinsichtlich der hier bereitgestellten Informationen keine Gewährleistungen, seien sie ausdrücklich oder konkludent.
Ruft die aktuelle Sequenznummer ab.
public:
property int WriterSeqNum { int get(); };public int WriterSeqNum { get; }member this.WriterSeqNum : intPublic ReadOnly Property WriterSeqNum As IntegerEigenschaftswert
Die aktuelle Sequenznummer.
Beispiele
Im folgenden Codebeispiel wird gezeigt, wie Sie die WriterSeqNum Eigenschaft und die AnyWritersSince Methode verwenden, um zu bestimmen, ob ein anderer Thread die Writer-Sperre auf der geschützten Ressource erworben hat, seit der aktuelle Thread die Writer-Sperre zuletzt gehalten hat.
Dieser Code ist Teil eines größeren Beispiels, das für die ReaderWriterLock Klasse bereitgestellt wird.
// 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;
// The complete code is located in the ReaderWriterLock class topic.
using System;
using System.Threading;
public class Example
{
static ReaderWriterLock rwl = new ReaderWriterLock();
// Define the shared resource protected by the ReaderWriterLock.
static int resource = 0;
' The complete code is located in the ReaderWriterLock class topic.
Imports System.Threading
Public Module Example
Private rwl As New ReaderWriterLock()
' Define the shared resource protected by the ReaderWriterLock.
Private resource As Integer = 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( Random^ rnd, 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 );
}
}
// Release all locks and later restores the lock state.
// Uses sequence numbers to determine whether another thread has
// obtained a writer lock since this thread last accessed the resource.
static void ReleaseRestore(Random rnd, int timeOut)
{
int lastWriter;
try {
rwl.AcquireReaderLock(timeOut);
try {
// It's safe for this thread to read from the shared resource,
// so read and cache the resource value.
int resourceValue = resource; // Cache the resource value.
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 and then restore the previous state of the lock.
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);
}
}
' Release all locks and later restores the lock state.
' Uses sequence numbers to determine whether another thread has
' obtained a writer lock since this thread last accessed the resource.
Sub ReleaseRestore(rnd As Random ,timeOut As Integer)
Dim lastWriter As Integer
Try
rwl.AcquireReaderLock(timeOut)
Try
' It's safe for this thread to read from the shared resource,
' so read and cache the resource value.
Dim resourceValue As Integer = resource
Display("reads resource value " & 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.
Dim lc As LockCookie = rwl.ReleaseLock()
' Wait for a random interval and then restore the previous state of the lock.
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) Then
resourceValue = resource
Interlocked.Increment(reads)
Display("resource has changed " & resourceValue)
Else
Display("resource has not changed " & resourceValue)
End If
Finally
' Ensure that the lock is released.
rwl.ReleaseReaderLock()
End Try
Catch ex As ApplicationException
' The reader lock request timed out.
Interlocked.Increment(readerTimeouts)
End Try
End Sub
};
}
End Module
Hinweise
Die Sequenznummer erhöht sich, wenn ein Thread die Writer-Sperre erhält. Sie können die Sequenznummer speichern und zu einem späteren Zeitpunkt übergeben AnyWritersSince , wenn Sie ermitteln möchten, ob andere Threads die Writer-Sperre in der Zwischenzeit erworben haben.
Sie können die WriterSeqNum Anwendungsleistung verbessern. Beispielsweise kann ein Thread die informationen zwischenspeichern, die er erhält, während eine Reader-Sperre gehalten wird. Nach dem Freigeben und späteren Erneuten Abrufen der Sperre kann der Thread bestimmen, ob andere Threads durch Aufrufen AnyWritersSinceder Ressource in die Ressource geschrieben wurden. Wenn nicht, können die zwischengespeicherten Informationen verwendet werden. Diese Technik ist nützlich, wenn die durch die Sperre geschützten Informationen gelesen werden; Beispiel: Ausführen einer Datenbankabfrage.
Der Anrufer muss eine Lesesperre oder eine Schreibsperre halten, damit die Sequenznummer nützlich sein kann.