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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。
…………………………………………………………Page 374……………………………………………………………
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 exclusive SyncLock argument where a single thread can access only a single code
block； one thread will write to the collection； and another thread will read from the collection。
…………………………………………………………Page 375……………………………………………………………
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 353
The SyncLock statement has a parameter that is a reference to lock against。 In both threads；
the reference is elements。 The mon reference synchronizes access to code。 At any given
point in time； the code contained within the SyncLock block will have only a single thread
executing。 This implements the desired feature； where only one thread is accessing code that
reads or writes to the collection。 The flow of the program is as follows：
1。 Both threads wait。
2。 After a 1 second； thread 1 acquires a lock because no other thread has done so。
3。 Thread 1 executes its code。
4。 Once thread 1 has started executing the synchronized code； no other code can acquire
the lock that is associated with the variable elements。
5。 When thread 2 wakes up after a sleep of 1。5 seconds； it will attempt to acquire the lock；
but it can’t because thread 1 is still holding the lock。 So the second thread must wait。
6。 Thread 1 eventually exits the synchronized code block； allowing the second thread to
add an element to the collection。 This time； no exception is thrown。
The reference to lock against does not need to be the reference that is manipulated within
the code block。 The reference is just that： an arbitrary reference。 You could use a different
object instance； and even instantiate an object like this：
Dim _syncRoot As Object = New Object（）
。 。 。
SyncLock _syncRoot
。