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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 In