What is the meant by 'thread safe' object?

Question

I have used generic queue in C# collection and everyone says that it is better to use the object of System.Collection.Generic.Queue because of thread safety.

Please advise on the right decision to use Queue object, and how it is thread safe?

Answer 1

"Thread safe" is a bit of an unfortunate term because it doesn't really have a solid definition. Basically it means that certain operations on the object are guaranteed to behave sensibly when the object is being operated on via multiple threads.

Consider the simplest example: a counter. Suppose you have two threads that are incrementing a counter. If the sequence of events goes:

• Thread one reads from counter, gets zero.
• Thread two reads from counter, gets zero.
• Thread one increments zero, writes one to counter.
• Thread two increments zero, writes one to counter.

Then notice how the counter has "lost" one of the increments. Simple increment operations on counters are not threadsafe; to make them threadsafe you can use locks, or InterlockedIncrement.

Similarly with queues. Not-threadsafe-queues can "lose" enqueues the same way that not-threadsafe counters can lose increments. Worse, not threadsafe queues can even crash or produce crazy results if you use them in a multi-threaded scenario improperly.

The difficulty with "thread safe" is that it is not clearly defined. Does it simply mean "will not crash"? Does it mean that sensible results will be produced? For example, suppose you have a "threadsafe" collection. Is this code correct?

if (!collection.IsEmpty) Console.WriteLine(collection[0]);

No. Even if the collection is "threadsafe", that doesn't mean that this code is correct; another thread could have made the collection empty after the check but before the writeline and therefore this code could crash, even if the object is allegedly "threadsafe". Actually determining that every relevant combination of operations is threadsafe is an extremely difficult problem.

Now to come to your actual situation: anyone who is telling you "you should use the Queue class, it is better because it is threadsafe" probably does not have a clear idea of what they're talking about. First off, Queue is not threadsafe. Second, whether Queue is threadsafe or not is completely irrelevant if you are only using the object on a single thread! If you have a collection that is going to be accessed on multiple threads, then, as I indicated in my example above, you have an extremely difficult problem to solve, regardless of whether the collection itself is "threadsafe". You have to determine that every combination of operations you perform on the collection is also threadsafe. This is a very difficult problem, and if it is one you face, then you should use the services of an expert on this difficult topic.