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Working with .NET Threads : Page 2

This article describes the dos and don'ts of the Thread class, and presents a wrapper class that simplifies starting a thread, correctly terminates a thread, and offers a more consistent class interface than that of the raw Thread class.


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Creating Threads
To spawn a new thread, you need to create a new Thread object, and associate it with a method, called the thread method. The new Thread object executes the method on a separate thread. The thread terminates once the thread method returns. The thread method can either be a static or an instance method, a public or a private one, on your object or on another. The only requirement is that the thread method has this exact signature:

void <MethodName>();

That is, no parameters and a void return type. You associate a Thread object with the thread method using a dedicated delegate called ThreadStart, defined as:

public delegate void ThreadStart();

The Thread class constructor accepts as a single construction parameter an instance of the ThreadStart delegate, targeting the thread method:



public sealed class Thread { public Thread(ThreadStart start); //Other methods and properties }

Once you create a new thread object, you must explicitly call its Start() method to have it actually execute the thread method. Listing 1 demonstrates creating and using a new thread.

Calling the Start() method is a non-blocking operation, meaning that control returns immediately to the client that started the thread, even though it may be some time later (depending on the operating system internal threading management) until the new thread actually starts. As a result, do not make any assumptions in your code (after calling Start()) that the thread is actually running.

Although you should only have one thread method as a target for the ThreadStart delegate, you could associate it with multiple targets, in which case the new thread will execute all the methods in order, and the thread will terminate once the last target method returns. However, there is little practical use for such a setting. In general, you should only have one target thread method.

Designing Thread Methods
A thread method can do whatever you want it to, but typically, it will contain a loop of some sort. In each loop iteration, the thread performs a finite amount of work, and then checks some condition, letting it know whether to perform another iteration or to terminate:

public void MyThreadMethod() { while(<some condition>) { <Do some unit of work> } }

The condition is usually the result of some external event telling the thread that its work is done. The condition can be as simple as checking the value of a flag, to waiting on a synchronization event. The condition is usually changed by another thread. As a result, changing and verifying the condition must be done in a thread-safe manner, using threading synchronization objects.



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