Lock only on an Id

Question

I have a method which needs to run exclusivley run a block of code, but I want to add this restriction only if it is really required. Depending on an Id value (an Int32) I w

Answer 1

If you want to use the ID itself and do not allow collisions, caused by hash-code, you can you the next approach. Maintain the Dictionary of objects and store info about the number of the threads, that want to use ID:

class ThreadLockerByID<T>
{
    Dictionary<T, lockerObject<T>> lockers = new Dictionary<T, lockerObject<T>>();

    public IDisposable AcquireLock(T ID)
    {
        lockerObject<T> locker;
        lock (lockers)
        {
            if (lockers.ContainsKey(ID))
            {
                locker = lockers[ID];
            }
            else
            {
                locker = new lockerObject<T>(this, ID);
                lockers.Add(ID, locker);
            }
            locker.counter++;
        }
        Monitor.Enter(locker);
        return locker;
    }
    protected void ReleaseLock(T ID)
    {
        lock (lockers)
        {
            if (!lockers.ContainsKey(ID))
                return;

            var locker = lockers[ID];

            locker.counter--;

            if (Monitor.IsEntered(locker))
                Monitor.Exit(locker);

            if (locker.counter == 0)
                lockers.Remove(locker.id);
        }
    }

    class lockerObject<T> : IDisposable
    {
        readonly ThreadLockerByID<T> parent;
        internal readonly T id;
        internal int counter = 0;
        public lockerObject(ThreadLockerByID<T> Parent, T ID)
        {
            parent = Parent;
            id = ID;
        }
        public void Dispose()
        {
            parent.ReleaseLock(id);
        }
    }
}

Usage:

partial class Program
{
    static ThreadLockerByID<int> locker = new ThreadLockerByID<int>();
    static void Main(string[] args)
    {
        var id = 10;
        using(locker.AcquireLock(id))
        {

        }
    }
}

Answer 2

I use a similar approach to lock resources for related items rather than a blanket resource lock... It works perfectly.

Your almost there but you really don't need to remove the object from the dictionary; just let the next object with that id get the lock on the object.

Surely there is a limit to the number of unique ids in your application? What is that limit?

Answer 3

I used the following approach. Do not check the original ID, but get small hash-code of int type to get the existing object for lock. The count of lockers depends on your situation - the more locker counter, the less the probability of collision.

class ThreadLocker
{
    const int DEFAULT_LOCKERS_COUNTER = 997;
    int lockersCount;
    object[] lockers;

    public ThreadLocker(int MaxLockersCount)
    {
        if (MaxLockersCount < 1) throw new ArgumentOutOfRangeException("MaxLockersCount", MaxLockersCount, "Counter cannot be less, that 1");
        lockersCount = MaxLockersCount;
        lockers = Enumerable.Range(0, lockersCount).Select(_ => new object()).ToArray();
    }
    public ThreadLocker() : this(DEFAULT_LOCKERS_COUNTER) { }

    public object GetLocker(int ObjectID)
    {
        var idx = (ObjectID % lockersCount + lockersCount) % lockersCount;
        return lockers[idx];
    }
    public object GetLocker(string ObjectID)
    {
        var hash = ObjectID.GetHashCode();
        return GetLocker(hash);
    }
    public object GetLocker(Guid ObjectID)
    {
        var hash = ObjectID.GetHashCode();
        return GetLocker(hash);
    }
}

Usage:

partial class Program
{
    static ThreadLocker locker = new ThreadLocker();
    static void Main(string[] args)
    {
        var id = 10;
        lock(locker.GetLocker(id))
        {

        }
    }
}

Of cource, you can use any hash-code functions to get the corresponded array index.

Answer 4

The main semantic issue I see is that an object can be locked without being listed in the collection because the the last line in the lock removes it and a waiting thread can pick it up and lock it.

Change the collection to be a collection of objects that should guard a lock. Do not name it LockedObjects and do not remove the objects from the collection unless you no longer expect the object to be needed.

I always think of this type of objects as a key instead of a lock or blocked object; the object is not locked, it is a key to locked sequences of code.