ReaderWriterLock.DowngradeFromWriterLock(LockCookie) Metodo
Definizione
Importante
Alcune informazioni sono relative alla release non definitiva del prodotto, che potrebbe subire modifiche significative prima della release definitiva. Microsoft non riconosce alcuna garanzia, espressa o implicita, in merito alle informazioni qui fornite.
Ripristina lo stato del blocco del thread in vigore prima della chiamata al metodo UpgradeToWriterLock(Int32).
public:
void DowngradeFromWriterLock(System::Threading::LockCookie % lockCookie);public void DowngradeFromWriterLock (ref System.Threading.LockCookie lockCookie);member this.DowngradeFromWriterLock : LockCookie -> unitPublic Sub DowngradeFromWriterLock (ByRef lockCookie As LockCookie)Parametri
- lockCookie
- LockCookie
Oggetto LockCookie restituito dal metodo UpgradeToWriterLock(Int32).
Eccezioni
Il thread non è responsabile del blocco del writer.
L'indirizzo di lockCookie è un puntatore Null.
Esempio
Nell'esempio di codice seguente viene illustrato come richiedere un blocco lettore, aggiornare il blocco del lettore a un blocco writer e effettuare di nuovo il downgrade a un blocco lettore.
Questo codice fa parte di un esempio più ampio fornito per la ReaderWriterLock classe .
// 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 request a reader lock, upgrade the
// reader lock to the writer lock, and downgrade to a
// reader lock again.
static void UpgradeDowngrade( Random^ rnd, 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 );
}
}
// Requests a reader lock, upgrades the reader lock to the writer
// lock, and downgrades it to a reader lock again.
static void UpgradeDowngrade(Random rnd, int timeOut)
{
try {
rwl.AcquireReaderLock(timeOut);
try {
// It's safe for this thread to read from the shared resource.
Display("reads resource value " + resource);
Interlocked.Increment(ref reads);
// To write to the resource, either release the reader lock and
// request the writer lock, or upgrade the reader lock. 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's safe for this thread to read or write from the shared resource.
resource = rnd.Next(500);
Display("writes resource value " + resource);
Interlocked.Increment(ref writes);
}
finally {
// Ensure that the lock is released.
rwl.DowngradeFromWriterLock(ref lc);
}
}
catch (ApplicationException) {
// The upgrade request timed out.
Interlocked.Increment(ref writerTimeouts);
}
// If the lock was downgraded, it's still safe to read from the resource.
Display("reads resource value " + resource);
Interlocked.Increment(ref reads);
}
finally {
// Ensure that the lock is released.
rwl.ReleaseReaderLock();
}
}
catch (ApplicationException) {
// The reader lock request timed out.
Interlocked.Increment(ref readerTimeouts);
}
}
' Requests a reader lock, upgrades the reader lock to the writer
' lock, and downgrades it to a reader lock again.
Sub UpgradeDowngrade(rnd As Random, timeOut As Integer)
Try
rwl.AcquireReaderLock(timeOut)
Try
' It's safe for this thread to read from the shared resource.
Display("reads resource value " & resource)
Interlocked.Increment(reads)
' To write to the resource, either release the reader lock and
' request the writer lock, or upgrade the reader lock. Upgrading
' the reader lock puts the thread in the write queue, behind any
' other threads that might be waiting for the writer lock.
Try
Dim lc As LockCookie = rwl.UpgradeToWriterLock(timeOut)
Try
' It's 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)
End Try
Catch ex As ApplicationException
' The upgrade request timed out.
Interlocked.Increment(writerTimeouts)
End Try
' If the lock was downgraded, it's still safe to read from the resource.
Display("reads resource value " & resource)
Interlocked.Increment(reads)
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
Commenti
DowngradeFromWriterLock rilascia il blocco writer, indipendentemente dal numero di blocchi ricorsivi e ripristina il blocco del lettore mantenuto dal thread prima di eseguire l'aggiornamento al blocco del writer. Viene ripristinato il conteggio dei blocchi nel blocco lettore.
Nota
DowngradeFromWriterLock accetta un LockCookie oggetto ottenuto chiamando UpgradeToWriterLock. Non usare un LockCookie oggetto restituito da ReleaseLock.
Un thread non blocca durante il downgrade dal blocco del writer, anche se altri thread sono in attesa del blocco del writer, perché tutte le richieste di blocco del lettore vengono concesse quando viene rilasciato il blocco del writer.
