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probably not consistent； and that is why you need to synchronize access to state。
Let’s consider a simple collection class example。 The following source code executes
in the calling thread； instantiates a list collection； and then adds two numbers to the collection。
Dim elements As List（Of Integer）= New List（Of Integer）（）
elements。Add（10）
elements。Add（20）
In the next step； we define a method that iterates the elements collection （which is declared at
a higher scope）。
Sub Task1（）
Thread。Sleep（1000）
Dim item As Integer
For Each item In elements
Console。WriteLine（〃Item （〃 & item & 〃）〃）
Thread。Sleep（1000）
Next
End Sub
This thread iterates the data； and the two Thread。Sleep（） calls put the threads to sleep for
1000 milliseconds； or 1 second。 By putting the thread to sleep； we can artificially construct a
situation where another thread adds to the collection while the collection is being iterated。
The method that adds an element to the collection is defined as follows：
Sub Task2（）
Thread。Sleep（1500）
elements。Add（30）
End Sub
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350 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
The Task1（） and Task2（） methods are started in separate threads as follows：：
elements。Add（10）
elements。Add（20）
Dim thread1 As New Thread（AddressOf Task1）
Dim thread2 As New Thread（AddressOf Task2）
thread1。Start（）
thread2。Start（）
Running the threaded code will generate an exception； but not immediately。 First； the caller
thread creates and starts thread1 and thread2。 thread1 goes to sleep for 1 second； and thread2
goes to sleep for 1。5 seconds。 After thread1 awakens； one item in the collection is iterated； and
thread1 goes to sleep for another second。 But before thread1 reawakens； thread2 awakens and
adds an element to the collection。 When thread1 reawakens and attempts to iterate another
element in the collection； an exception is generated； as shown in Figure 13…4。
Figure 13…4。 Exception thrown after adding element to the collection； as shown in the Visual Basic IDE
The InvalidOperationException is thrown to indicate that you can’t add elements to a
collection while iterating a collection。 The collection classes think it is a bad idea to modify and
iterate a collection at the same time。 I agree with the collection classes； because doing so could
give unpredictable results。
The problem in the source code is how to use the collection classes in a multithreaded
context。 In the example； items are added to the collection while the collection is being iterated。
A monly suggested approach is to use the type System。Collections。ObjectModel。
ReadOnlyCollection； as in the following example。
Imports System。Collections。ObjectModel
Imports System。Threading
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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 351
Module ThreadProblem
Dim elements As List（Of Integer） = New List（Of Integer）（）
Sub Task1（）
Thread。Sleep（1000）
Dim item As Integer
Dim listElements As New ReadOnlyCollection（Of Integer）（elements）
For Each item In listElements
Console。WriteLine（〃Item （〃 & item & 〃）〃）
Thread。Sleep（1000）
Next
End Sub
Sub Task2（）
Thread。Sleep（1500）
elements。Add（30）
End Sub
Sub Main（）
elements。Add（10）
elements。Add（20）
Dim thread1 As New Thread（AddressOf Task1）
Dim thread2 As New Thread（AddressOf Task2）
thread1。Start（）
thread2。Start（）
End Sub
End Module
The change is the bolded code； which instantiates the type System。Collections。
ReadOnlyCollection； to which we pass the elements list。 The ReadOnlyCollection provides the
base class for a generic read…only collection。 The For Each iterator then iterates a collection
that is read…only； but based on the original collection。 However； running the code will result in
the same exception。 This demonstrates that ReadOnlyCollection does not take a snapshot； but
masks the collection。 The mask disables the addition of items to the collection； but because the
other thread is taking a shortcut and editing the original collection； the read…only collection is
modified as well。
Let’s say that converting the collection into a read…only collection had worked。 It would
not have solved anything。 A read…only collection means that the second thread would generate
an exception because you can’t add elements to a collection that is read…only。 The point is that
when writing multithreaded code that shares variables； you don’t have an easy solution； because
you are trying to solve the problem of how to keep multiple cooks productive in a single kitchen。
We are trying to solve a classic reader/writer problem； where some threads are interested
only in reading the data； and other threads are interested only in modifying the data。 One way
to synchronize the readers and writers is to use an exclusive lock； so that only one thread may
read or write。
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352 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
Using Exclusive Locks
When using exclusive locks in ； you are saying； “Only one thread may execute this piece
of code。” If two threads want to execute a particular piece of code； one will be granted access；
while the other thread waits until the granted thread has exited the code block。 It is important
to understand that an exclusive lock grants access to code； not data； but that code could access
data。 And because only one thread is accessing the code； it is implied that only one thread can
access the data。
The following is an example of code that uses exclusive locks。
。 。 。
Module ThreadProblem
Dim elements As List（Of Integer） = New List（Of Integer）（）
Sub Task1（）
Thread。Sleep（1000）
SyncLock elements
Dim item As Integer
For Each item In elements
Console。WriteLine（〃Item （〃 & item & 〃）〃）
Thread。Sleep（1000）
Next
End SyncLock
End Sub
Sub Task2（）
Thread。Sleep（1500）
SyncLock elements
elements。Add（30）
End SyncLock
End Sub
Sub Main（）
elements。Add（10）
elements。Add（20）
Dim thread1 As New Thread（AddressOf Task1）
Dim thread2 As New Thread（AddressOf Task2）
thread1。Start（）
thread2。Start（）
End Sub
End Module
The bolded lines use the SyncLock and End SyncLock keywords； which represent a code block of
exclusive access。 The thread is granted access to only a single code block in each instance。
Looking at the code within the block； you can see that the collection is accessed in two locations。
Using the exclusi