Книга: C# 2008 Programmer

Aborting a Thread

Aborting a Thread

You can use the Abort() method of the Thread class to abort a thread after it has started executing. Here's an example:

class Program {
 static void Main(string[] args) {
  Thread t = new Thread(new ThreadStart(DoSomething));
  t.Start();
  Console.WriteLine("Continuing with the execution...");
  while (!t.IsAlive);
  Thread.Sleep(1);
  t.Abort();
  Console.ReadLine();
 }
 static void DoSomething() {
  try {
   while (true) {
    Console.WriteLine("Doing something...");
   }
  } catch (ThreadAbortException ex) {
   Console.WriteLine(ex.Message);
  }
 }
}

When the thread is started, you continue with the next statement and print out the message "Continuing with the execution...". You then use the IsAlive property of the Thread class to find out the execution status of the thread and block the execution of the Main() function (with the while statement) until the thread has a chance to start. The Sleep() method of the Thread class blocks the current thread (Main()) for a specified number of milliseconds. Using this statement, you are essentially giving the DoSomething() function a chance to execute. Finally, you kill the thread by using the Abort() method of the Thread class. 

The ThreadAbortException exception is fired on any thread that you kill. Ideally, you should clean up the resources in this exception handler (via the finally statement):

static void DoSomething() {
 try {
  while (true) {
   Console.WriteLine("Doing something...");
  }
 } catch (ThreadAbortException ex) {
  Console.WriteLine(ex.Message);
 } finally {
  //---clean up your resources here---
 }
}

The output of the preceding program may look like this:

Continuing with the execution...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Thread was being aborted.

Notice that I say the program may look like this. When you have multiple threads running in your application, you don't have control over which threads are executed first. The OS determines the actual execution sequence and that is dependent on several factors such as CPU utilization, memory usage, and so on. It is possible, then, that the output may look like this:

Doing something...
Continuing with the execution...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Thread was being aborted.

While you can use the Abort() method to kill a thread, it is always better to exit it gracefully whenever possible.

Here's a rewrite of the previous program:

class Program {
 private static volatile bool _stopThread = false;
 static void Main(string[] args) {
  Thread t = new Thread(new ThreadStart(DoSomething));
  t.Start();
  Console.WriteLine("Continuing with the execution...");
  while (!t.IsAlive);
  Thread.Sleep(1);
  _stopThread = true;
  Console.WriteLine("Thread ended.");
  Console.ReadLine();
 }
 static void DoSomething() {
  try {
   while (!_stopThread) {
    Console.WriteLine("Doing something...");
   }
  } catch (ThreadAbortException ex) {
   Console.WriteLine(ex.Message);
  } finally {
   //---clean up your resources here---
  }
 }
}

First, you declare a static Boolean variable call _stopThread:

private static volatile bool _stopThread = false;

Notice that you prefix the declaration with the volatile keyword, which is used as a hint to the compiler that this variable will be accessed by multiple threads. The variable will then not be subjected to compiler optimization and will always have the most up-to-date value.

To use the _stopThread variable to stop the thread, you modify the DoSomething() function, like this:

while (!_stopThread) {
 Console.WriteLine("Doing something...");
}

Finally, to stop the thread in the Main() function, you just need to set the _stopThread variable to true:

_stopThread = true;
Console.WriteLine("Thread ended.");

The output of this program may look like this:

Continuing with the execution.

Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Thread ended.
Doing something...

The DoSomething() function may print another message after the "Thread ended." message. That's because the thread might not end immediately. To ensure that the "Thread ended." message is printed only after the DoSomething() function ends, you can use the Join() method of the Thread class to join the two threads:

static void Main(string[] args) {
 Thread t = new Thread(new ThreadStart(DoSomething));
 t.Start();
 Console.WriteLine("Continuing with the execution...");
 while (!t.IsAlive);
 Thread.Sleep(1);
 _stopThread = true;
 //---joins the current thread (Main()) to t---
 t.Join();
 Console.WriteLine("Thread ended.");
 Console.ReadLine();
}

The Join() method essentially blocks the calling thread until the thread terminates. In this case, the Thread ended message will be blocked until the thread (t) terminates.

The output of the program now looks like this:

Continuing with the execution.

Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Doing something...
Thread ended.

Figure 10-4 shows graphically the two different threads of execution.

Figure 10-4

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