ReaderWriterLock::RestoreLock Method (LockCookie%)
Restores the lock status of the thread to what it was before calling ReleaseLock.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- lockCookie
-
Type:
System.Threading::LockCookie%
A LockCookie returned by ReleaseLock.
| Exception | Condition |
|---|---|
| NullReferenceException | The address of lockCookie is a null pointer. |
The state restored by RestoreLock includes the recursive lock count.
A thread blocks if it tries to restore a reader lock after another thread has acquired the writer lock, or if it tries to restore the writer lock after another thread has acquired a reader lock or writer lock. Because RestoreLock does not accept a time-out, you should take care to avoid possible deadlocks.
 Caution |
|---|
Before calling RestoreLock, make sure you have released all locks acquired since the call to ReleaseLock. For example, a thread deadlocks if it acquires a reader lock, and then attempts to restore an earlier writer lock. Use IsReaderLockHeld and IsWriterLockHeld to detect such additional locks. |
Do not use a LockCookie returned from UpgradeToWriterLock.
The following code example shows how to use the ReleaseLock method to release the lock, regardless of how many times it has been acquired by the thread, and how to restore the state of the lock later.
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 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 ); } }
Available since 1.1