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You need to instantiate a single ThreadedTask and associate it with a single Thread instance；
like this：
Dim task1 As ThreadedTask = New ThreadedTask（〃hello〃）
Dim task2 As ThreadedTask = New ThreadedTask（〃goodbye〃）
Dim thread1 As Thread = New Thread（AddressOf task1。MethodToRun）
Dim thread2 As Thread = New Thread（AddressOf task2。MethodToRun）
thread1。Start（）
thread2。Start（）
If you are running a stateful method； you need to associate a task instance with a thread
instance。
Using a Thread Parameter
Suppose that you want to create a stateful thread。 You can do this by using a thread parameter。 In
other words； the method that you invoke on another thread takes a parameter。 Here’s an example：
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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 349
Sub SimpleThreadWithStateTask（ByVal buffer As Object）
Console。WriteLine（〃You said （〃 & buffer。ToString（） & 〃）〃）
End Sub
Private Sub SimpleThreadWithState（）
Dim thread1 As New Thread（AddressOf SimpleThreadWithStateTask）
thread1。Start（〃my text〃）
End Sub
The major difference with a thread parameter is that you declare the function that is executed
with a parameter； and the Start（） method has a parameter。
Two restrictions apply to using a thread parameter： you can send only a single parameter；
and that parameter must be an object。
Synchronizing Between Threads
When you have two threads and both share state （such as a variable）； a concurrency situation
may arise。 Sharing state between two independent threads is not a problem if both threads
consider the data as read…only。 What does the reading thread read？ Is the state consistent？ It is
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）