ReaderWriterLock::DowngradeFromWriterLock Method (LockCookie%)
Restores the lock status of the thread to what it was before UpgradeToWriterLock was called.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- lockCookie
-
Type:
System.Threading::LockCookie%
A LockCookie returned by UpgradeToWriterLock.
| Exception | Condition |
|---|---|
| ApplicationException | The thread does not have the writer lock. |
| NullReferenceException | The address of lockCookie is a null pointer. |
DowngradeFromWriterLock releases the writer lock, regardless of the recursive lock count, and restores the reader lock that was held by the thread before upgrading to the writer lock. The lock count on the reader lock is restored.
 Note |
|---|
DowngradeFromWriterLock accepts a LockCookie obtained by calling UpgradeToWriterLock. Do not use a LockCookie returned by ReleaseLock. |
A thread does not block when downgrading from the writer lock, even if other threads are waiting for the writer lock, because all reader-lock requests are granted when the writer lock is released.
The following code example shows how to request a reader lock, upgrade the reader lock to a writer lock, and downgrade to a reader lock again.
This code is part of a larger example provided for the ReaderWriterLock class.
// 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;
// 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 ); } }
Available since 1.1