Let’s look at a collection example that has four threads： three readers and one writer。 The example uses Thread。Sleep（） strategically； so that you can see how a reader thread and writer thread interact with each other。 Imports System。Threading Friend Module TestMoreTopics Private rwlock As New ReaderWriterLock（） Private elements As New List（Of Integer）（） …………………………………………………………Page 382…………………………………………………………… 360 CH AP T E R 1 3 ■ L E A R N I N G A B OU T M U L T I TH R E A DI N G Sub Task1（） Thread。Sleep（1000） Console。WriteLine（〃Thread 1 waiting for read lock〃） rwlock。AcquireReaderLock（…1） Console。WriteLine（〃Thread 1 has read lock〃） Dim item As Integer For Each item In elements Console。WriteLine（〃Thread 1 Item （〃 & item & 〃）〃） Thread。Sleep（1000） Next Console。WriteLine（〃Thread 1 releasing read lock〃） rwlock。ReleaseLock（） End Sub Sub Task2（） Thread。Sleep（1250） Console。WriteLine（〃Thread 2 waiting for read lock〃） rwlock。AcquireReaderLock（…1） Console。WriteLine（〃Thread 2 has read lock〃） Dim item As Integer For Each item In elements Console。WriteLine（〃Thread 2 Item （〃 & item & 〃）〃） Thread。Sleep（1000） Next Console。WriteLine（〃Thread 2 releasing read lock〃） rwlock。ReleaseLock（） End Sub Sub Task3（） Thread。Sleep（1750） Console。WriteLine（〃Thread 3 waiting for read lock〃） rwlock。AcquireReaderLock（…1） Console。WriteLine（〃Thread 3 has read lock〃） Dim item As Integer For Each item In elements Console。WriteLine（〃Thread 3 Item （〃 & item & 〃）〃） Thread。Sleep（1000） Next Console。WriteLine（〃Thread 3 releasing read lock〃） rwlock。ReleaseLock（） End Sub Sub Task4（） Thread。Sleep（1500） Console。WriteLine（〃Thread 4 waiting for write lock〃） rwlock。AcquireWriterLock（…1） Console。WriteLine（〃Thread 4 has write Lock〃） …………………………………………………………Page 383…………………………………………………………… C HA P TE R 1 3 ■ L E AR N IN G AB O U T M U L T IT HR E AD IN G 361 elements。Add（30） Console。WriteLine（〃Thread 4 releasing write lock〃） rwlock。ReleaseLock（） End Sub Private Sub ReaderWriter（） elements。Add（10） elements。Add（20） Dim thread1 As New Thread（AddressOf Task1） Dim thread2 As New Thread（AddressOf Task2） Dim thread3 As New Thread（AddressOf Task3） Dim thread4 As New Thread（AddressOf Task4） thread1。Start（） thread2。Start（） thread3。Start（） thread4。Start（） End Sub Public Sub RunAll（） TestMoreTopics。ReaderWriter（） End Sub End Module The bolded code contains all of the references to the reader/writer class imple mentation。 Unlike the keyword SyncLock or the type Monitor； the ReaderWriterLock type is instantiated and the instance is shared between threads。 The code to acquire a reader lock or writer lock has a parameter of value …1； which means to wait until the lock is acquired。 A positive value means to wait for a number of milliseconds； and if the lock has not been acquired； then return from the method call。 If you do use a timeout； before you attempt to manipulate shared code； you need to reference the property IsReaderLockHeld or IsWriterLockHeld to ensure that you have acquired the lock。 In the reader threads； after having acquired the reader locks； the items are iterated。 ■Note The example seems to break the rule regarding keeping locks for as short a time possible； since it holds onto the lock while iterating。 In the case of a reader/writer implementation； you have a unique situation in that you should be manipulating data that is mostly to be read； which implies that most of the time； you will be treating the shared data as read…only。 For those times when you are writing to the shared data； it is fine if the thread must wait a moment or two。 A reader/writer lock does not make sense if you do not have data that is essen tially read…only。 In other situations； you should use the Monitor approach； as described in the previous section。 The example demonstrates handling data that is mostly to be read； since it has three threads reading and one thread writing。 It is important to make sure that you don’t end up writing while holding a read…only lock。 …………………………………………………………Page 384…………………………………………………………… 362 CH AP T E R 1 3 ■ L E A R N I N G A B OU T M U L T I TH R E A DI N G Running the code results in the following output。 Thread 1 waiting for read lock Thread 1 has read lock Thread 1 Item （10） Thread 2 waiting for read lock Thread 2 has read lock Thread 2 Item （10） Thread 4 waiting for write lock Thread 3 waiting for read lock Thread 1 Item （20） Thread 2 Item （20） Thread 1 releasing read lock Thread 2 releasing read lock Thread 4 has write Lock Thread 4 releasing write lock Thread 3 has re