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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 read lock
Thread 3 Item （10）
Thread 3 Item （20）
Thread 3 Item （30）
Thread 3 releasing read lock
In the generated output； the sequence of events is as follows：
1。 Thread 1 wants and acquires a read…only lock。
2。 Thread 1 outputs the first number in the collection。 Thread 1 then sleeps； which lets in
another thread for execution。
3。 Thread 2 wants and acquires another read…only lock。
4。 Thread 2 outputs the first number in the collection。 Thread 2 then sleeps； which lets in
another thread for execution。
5。 Thread 4 wants a writer lock and is kept on hold。
6。 Thread 3 wants a read…only lock； but because thread 4 has asked for a writer lock and is
queued； thread 3 is put on hold。 At this step； threads 3 and 4 are put on hold and are
waiting for the read…only locks of threads 1 and 2 to be released。
7。 Threads 1 and 2 output the remaining numbers in the collection。
8。 Threads 1 and 2 release the read…only locks。
9。 Thread 4 is given a writer lock； and thread 3 is still on hold。
10。 Thread 4 writes to the collection and releases the writer lock。
11。 Thread 3 acquires a read…only lock and iterates the individual numbers； including the
number added by thread 4。
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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 363
Notice that the reader/writer lock makes the sequence of reading and writing events orderly；
so that the shared state is always consistent。 The reader/writer lock does not hinder or stop
deadlocks； which can occur if you are not careful with how you write your code。 The reader/
writer lock is concerned about only the code that is used to manage data。
Implementing a Producer/Consumer Architecture
The producer/consumer technique has never been defined as a type； but it is used throughout
many multithreaded applications。 The idea behind a producer/consumer architecture is to
split the problem into two parts。 One side is the producer of data； information； and tasks。 The
producer wraps up the information into a task to be executed。 The other side is the consumer；
and it is responsible for unwrapping the information and doing something with it。
Using a Hidden Producer/Consumer Implementation
In Windows GUIs； multithreaded applications are not allowed to access UI ponents if they
are not the thread that created the UI element。 To get around that problem； the Windows。Forms
library uses the Invoke（） method。 To demonstrate； we’ll create a GUI application that uses another
thread to periodically increment a counter that is displayed in a text box。
Follow these steps：
1。 Create a new Windows Forms application； and set it as the startup project if it isn’t already
（right…click its name and select Set as StartUp Project）。
2。 Drag a TextBox control onto Form1 in the design window。
3。 Select the TextBox control。 If the Properties window isn’t visible； right…click the control
and select Properties。
4。 Change the TextBox’s Name property to txtMessage。
5。 Right…click the form and select View Code。
6。 Add the following code。
Imports System。Threading
Public Class Form1
Private _counter As Integer
Private Sub IncrementCounter（）
Me。txtMessage。Text = 〃Counter （〃 & _counter & 〃）〃
_counter ＋= 1
End Sub
Delegate Sub DelegateIncrementCounter（）
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364 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
Private Sub PeriodicIncrement（）
Do While True
Invoke（New DelegateIncrementCounter（AddressOf IncrementCounter））
' You can't call the GUI using a thread other than the GUI thread
'IncrementCounter（）
Thread。Sleep（1000）
Loop
End Sub
Private _thread As Thread
End Class
7。 Switch back to the design view and double…click the form； which should take you back
to the code view in the Form1_Load（） method。
8。 Add the following code to the Form1_Load（） method。
Private Sub Form1_Load（ByVal sender As System。Object； _
ByVal e As System。EventArgs） Handles MyBase。Load
_thread = New Thread（AddressOf PeriodicIncrement）
_thread。Start（）
End Sub
When Form1 is loaded； the Form1_Load（） method is executed； which instantiates a new
thread； which then executes the PeriodicIncrement（） method。 Within the implementation of
PeriodicIncrement（） is a never…ending loop； which calls the Form。Invoke（） method； to which
we pass a delegate。 The delegate is the method IncrementCounter（）； which increments a
counter and outputs the result to the text box txtMessage。
From a user perspective； it would seem obvious to call the method IncrementCounter（）
directly from the other thread （_thread）。 How