Difference between Abort and Interrupt in Threads in .NET

Question

What is the difference between Thraed.Abort() and Thread.Interrupt(). How can I call them in a Thread Safe Manner.It would be helpful,if simple example is provided.

Answer 1

First of all, neither of these are good thread synchronization constructs.

First, Thread.Abort says "I don't care what you're doing, just stop doing it, and leave everything as it is right now". It's basically the programming way of saying "Hey, beat it". If your thread is having files open, those files will be left open until garbage collection gets around to finalizing your objects.

Thread.Abort should only ever be used, and even then probably not, in the case when the app domain that the thread is running inside is being torn down, preferably only when the process is being terminated.

Secondly, Thread.Interrupt is a rather strange beast. It basically says "I don't care what you're waiting for, stop waiting for it". The strange thing here is that if the thread isn't currently waiting for anything, it's instead "I don't care what you're going to wait for next, but when you do, stop waiting for it immediately".

Both of these are signs that you're imposing your will on a thread that wasn't designed to be told such things.

To abort a thread properly, the thread should periodically check some kind of flag, be it a simple volatile Boolean variable, or an event object. If the flag says "You should now terminate", the thread should terminate itself by returning from the methods in an orderly fashion.

To properly wake a thread, a thread should, in places where it has to wait for synchronization objects, include a "please stop waiting" object that it also waits on. So basically it would for either the object it needs becomes signaled, or the "please stop waiting" object becomes signaled, determine which one that did, and do the right thing.

So instead of Thread.Abort and Thread.Interrupt, you should write your threads using normal synchronization objects, like events, mutexes, semaphores, etc.

For the same reason, Thread.Suspend and Thread.Resume should be left alone, and they have also been obsoleted in the later versions of .NET.

Answer 2

Thread.Abort() raises a ThreadAbortException on the target thread. It's intent to generally to force the thread to terminate. It is not a recommended practice for stopping a thread's processing.

Thread.Interrupt() interrupts a thread that is in WaitSleepJoin state - essentially blocking on a resource like a WaitHandle. This allows the caller to unblock the thread.

Neither is really "thread-safe" - in the sense that they are specifically intended to affect the behavior of threads in a way that is hard to predict.

It's generally recommended to use synchronization objects (like WaitHandles or Semaphores) to allows threads to safely synchronize with one another.

Answer 3

The difference between Abort and Interrupt is that while they will both throw an exception (ThreadAbortException and ThreadInterruptException), calling Abort will rethrow the exception at the end of the catch block and will make sure to end your running thread.

Answer 4

Unless you're calling Abort or Interrupt on the currently executing thread (as ASP.NET does to terminate a request abruptly, for example) you basically can't call them in a thread-safe manner.

Use a WaitHandle or Monitor.Wait/Pulse to wait in a wakeable way. You should only abort other threads if you're tearing down the application, basically - as otherwise you can end up in an unknown state.

See my article on graceful thread termination for an example of how to do this nicely.