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nothing。 You； as a user； are not aware of the time slices； because a time slice operates on the
order of microseconds。 Because time slicing is so fast； you think your program is running
continuously。 To see the time slicing in action； open the Windows Task Manager and select the
Processes tab。
Look closely at the Task Manager window shown in Figure 13…1； and you’ll see that
explorer。exe has 6％ of the CPU。 You can say that the explorer。exe application is using the
appropriate amount of time slices that equals 6％ of the CPU time。 How the CPU slices the 6％
is a detail of the operating system。
Say you have a program that runs two tasks： task 1 and task 2。 The microprocessor is a single
core； and thus when running two separate tasks； there will be two time slices； as illustrated in
Figure 13…2。 In the figure； the entire processing cycle is represented as a pie； and each time slice
is a slice of the pie。
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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 343
Figure 13…1。 Windows time slicing in action
Figure 13…2。 Single…core microprocessor running two tasks
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344 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
Notice how task 1 and task 2 run in a serial manner； rather than concurrently。 This is
because the microprocessor is a single…task device made to look like a multitask device。 You
would run multiple tasks on a single…core microprocessor when you want to have application
background tasks that should not affect the foreground task of the application。 For example；
when running a word processor； you don’t want the spell checker slowing down your typing。
The operating system will still allocate time slices in this case and use preemptive multitasking。
Figure 13…3 illustrates how the same application executes on a multiple…core micropro
cessor。 Did you notice what happened？ The operating system； in a bid to make more efficient
use of the microprocessor； has put one task on one core and another task on the other core。
Now both tasks are running in parallel。 And now it’s possible that both tasks would want to
manipulate the same piece of data at the same time。 In a single…core microprocessor； that is
not physically possible。 However； with a single processor； it is still possible for one task to be
interrupted mid…flight while accessing some data； and for the other task to step in and use
inconsistent data。
Figure 13…3。 Multiple…core microprocessor running two tasks
If you have a multitasking application； you must be careful how you manipulate global
state in the application。 If you are not careful； you could corrupt the state of your program。
■Note As panies like Intel and AMD introduce microprocessors with more and more cores； it is your
job as a software developer to write software that can make use of those cores。 This is not always easy and
requires some forethought； since you need to work out the logic。 For example； if you are processing data that
is sent to a file； you can’t read the file before the data has been processed。
Using Threads
Tasks can run in two ways on the Windows operating system： via threads and via processes。
A process is when you start a program； causing the operating system to instantiate resources
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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 345
and time slices for the program。 A thread can be seen as a lightweight process that executes in
the context of a process。
All processes will start a single thread。 The thread that is executed as part of the process is
the main thread； and when it exits； so does your application。 If the main thread creates other
threads； when the main thread exits； so do the created background threads。 From an architec
tural perspective； when multitasking； the main thread will execute and coordinate the threads
that run your code。 Two processes cannot reference each other。 This is so that if one process
crashes； it does not cause the other process to crash。 Threads executing in the context of a
process have the ability to bring down an entire process。
■Note You could run multiple processes and then use interprocess munication mechanisms to muni
cate。 However； I can’t remend that solution； since you can get the same effect of a process by using a
application domain （AppDomain）。 For the most part； you don’t need to concern yourself with AppDomains。 You
will use threads； since they are lightweight （pared to a process）； easy to manage； and easy to program。
Creating a New Thread
You can create a new thread that will run independently of its originating thread。 In the following
example； the main thread creates two other threads。 Each of the threads outputs some text to
the console。 Note that the type Thread and other related types used in this chapter’s examples
are in the System。Threading namespace。
Imports System。Threading
Module Module1
Sub Main（）
SimpleThread（）
End Sub
Sub SimpleThreadTask1（）
Console。WriteLine（〃hello there〃）
End Sub
Sub SimpleThreadTask2（）
Console。WriteLine（〃Well then goodbye〃）
End Sub
Private Sub SimpleThread（）
Dim thread1 As New Thread（AddressOf SimpleThreadTask1）
Dim thread2 As New Thread（AddressOf SimpleThreadTask2）
thread1。Start（）
thread2。Start（）
End Sub
End Module
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346 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
To create a thread； you instantiate the Thread type and call the Start（） method。 The Thread
type is a class that contains all of the functionality necessary to start and control a multitasking
thread。
When running a thread； the Thread type needs code to execute。 The solution used by Thread is
a delegate； which is passed to Thread via the constructor。 The name of the delegate type is
ThreadStart。 The example here does not use the ThreadStart type explicitly； because both threads
use programming constructs that do not need a delegate declaration。 Here’s the explicit syntax
for pleteness：
Dim thread1 As New Thread（New ThreadStart（AddressOf SimpleThreadTask1））
Dim thread2 As New Thread（New ThreadStart（AddressOf SimpleThreadTask2））
Running the thread example； you may see this output：
well then goodbye
hello there
Notice how hello there is after well then goodbye。 The output implies that the second
thread （thread2） starts before the first thread （thread1）。 However； your output might be the
opposite； which demonstrates the true nature of threading； concurrency； and why threading is
so difficult。
Imagine for a moment that the sample thread code were not multithreaded； but executed
in a serial manner。 Calling thread1。Start（） and then thread2。Start（） results in the text hello
there being first。 The serial be