Is this algorithm thread safe?

Question

I want to know if the following class is thread safe and works perfectly in multithread context?

public class ThreadSafeClass
{
    private List<int> commandList = new List<int>();
    public void AddCommand(int newCommand)
    {
        lock(syncObj) {
          commandList.Add(newCommand);
        }
    }
    public List<int> Split()
    {
        List<int> oldList = commandList;
        commandList = new List<int>();
        return oldList;
    }
}

ThreadA periodically call split method, many thread call AddCommand, commandList in the split method is pointing to some list in memory and when the new list is assigned all of its content is in oldList

The purpose of split is that I want to get all of the queued commands and in the next call return rest of them, ... and meanwhile let the application add new items to commandList.

cause I focused on split method I forgot to add lock for add operation cause it is not thread-safe THANKS TO: (can poyrazoğlu) but the question remains for Split

Answer 1

The code is not thread safe, as List.Add isn't guaranteed to be. (just the first thing - there are other problems).

You are not locking on shared data (any access to commandList) either.

Answer 2

Having thread-safe class doesn't basically mean your program is thread-safe. Thread safe class just means that you can work with it from multiple threads and it still will be fine. Thread-safety basically means "being consistent on the multithreaded environment".

The List class is not thread safe, so your code is absolutely not thread safe. But the idea is, even if you use thread-safe collection, it doesn't mean your code is thread-safe.

Answer 3

The issue here boils down to what you expect the behavior to be. For example, take your split method:

public List<int> Split()
{
    List<int> oldList = commandList;
    commandList = new List<int>();
    return oldList;
}

There is a period of time between assigning oldList and reassigning commandList that the AddCommand method could add values to commandList that will appear in oldList:

public List<int> Split()
{
    // Say commandList contains 1 and 2

    List<int> oldList = commandList;

    // Now, on another thread, this happens:
    //
    // AddCommand 3
    // AddCommand 4
    // AddCommand 5
    //
    // The list hasn't been reassigned yet, so at this point oldList and
    // commandList both have 1, 2, 3, 4, and 5.

    commandList = new List<int>();

    // Now, commandList is empty, and oldList contains 1, 2, 3, 4, and 5,
    // even though it only contained 1 and 2 when you first assigned it.

    return oldList;
}

This sequence demonstrates the fact that oldList is not a snapshot containing only the values at the time of its assignment, but can actually be modified between the time it is assigned and the time commandList is reassigned.

One thing that is true of this code is that each command you add will be in either oldList or commandList exactly once. You won't experience any repeats no matter how many threads are calling AddCommand. This sounds like what you are trying to accomplish, so I think your code is correct as-is.

This works because .NET reference assignment is atomic. There is no period of time during the assignment of commandList that a call to AddCommand could cause the value to be added to more than one list or not be added at all.

Please let me know if I misinterpreted your question.

Answer 4

No. Think about the case where a thread calls AddCommand while another thread is calling Split. Your new command could potentially be cleared. You need to synchronize it by using a locking mechansim.

// Bad
List oldList = commandList;
commandList.Add(newCommand); // newCommand would get lost.
commandList = new List();
return oldList; 

Answer 5

I don't think this is thread-safe. You modify shared data, and there's no guard that I can see.

Answer 6

No.

If one thread starts the operation of "add" while the other is doing the split then there's a concurrency issue (hint, although the code for add is only one line it is numerous operations by the computer itself)

Answer 7

No the code is not thread safe.By default the classes are not thread safe.

Here is a link Why is List<T> not thread-safe?

Answer 8

List<T> class is not thread-safe whatsoever. Use SynchronizedCollection<T> where methods like Add, Remove etc. are thread-safe (enumeration still isn't though, so don't use foreach)

Answer 9

You should sync the access to the list, by locking on an object (and it should be the same object for all methods in this case), where it's safe to use the commandList itself as it is not recreated or anything.

    public class ThreadSafeClass
    {
        private object sync = new object();
        private List<int> commandList = new List<int>();
        public void AddCommand(int newCommand)
        {
          lock(sync){
            commandList.Add(newCommand);
          }
        }
        public List<int> Split()
        {
          lock(sync){
            List<int> oldList = commandList;
            commandList = new List<int>();
          }
            return oldList;
        }
    }

Answer 10

The shape of your class here seems to reveal a desire to solve a variant of the classic producer-consumer problem:

http://en.wikipedia.org/wiki/Producer-consumer_problem

I could be misunderstanding your desires. First, and foremost, as many other people have mentioned, there are no locking mechanisms in your code, and, therefore, there is an inherent race condition between the two methods. If you don't immediately understand what the term "race condition" means, you shouldn't be writing multithreaded code, please go hit the books. Not trying to be rude here, but I share Alex's sentiments.

The question really becomes: what do you want to do in a multithreaded scenario with this class? The answer to this question will guide you towards an appropriate synchronization mechanism for your application.

For the following two examples, let's assume a simple mutex lock, like other people have posted already.

1. What if there are 100 threads all trying to call the AddCommand(...) method at the same time? 99 of them will block, especially in a case where the underlying list is large, and is in the middle of a reallocation. Is that okay? Is that what you want? Is that impossible in the application you're working on?

2. In the case of 100 threads, where only one of them would be calling Split(), and the other 99 calling AddCommand(...), the one calling Split() would get a subset of the list of 99 commands, instead of all of them, since a simple locking structure wouldn't make the call to Split() block until all the pending AddCommand(...) calls are completed, which would (from a data processing perspective) be more ideal.

Now, a more sophisticated locking mechanism may solve these problems; however, these problems may not exist in your application, and therefore, don't need to be solved, and a simple locking mechanism will suffice.

Hopefully this helps you get pointed in the right direction.