ReaderWriterLock.AnyWritersSince Method (Int32)

 

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 Function AnyWritersSince (
	seqNum As Integer
) As Boolean

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.
Imports System.Threading

Public Module Example
   Private rwl As New ReaderWriterLock()
   ' Define the shared resource protected by the ReaderWriterLock.
   Private resource As Integer = 0
' 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(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

.NET Framework
Available since 1.1
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