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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）（）
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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〃）
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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。
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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