c# asp.net selective locking between multiple instances

Question

Assuming there is the following C# POCO:

public class Gift
{
   public int Id { get; set; }
   public string Owner { get; set; }
}

And the following Service:

public class GiftClaimService
{
    private ISomeRepository _someRepository; // Going to be injected or whatever

    public bool ClaimGift(int giftId, string myName)
    {
        var gift = _someRepository.Get(giftId);
        if (gift != null && gift.Owner == null)
        {
            gift.Owner = myName;
            _someRepository.Save(gift);
            return true;
        }
        return false;
    }
}

I hope this is easy to unterstand. Any user calling "ClaimGift" with a correct ID is able to claim the gift, in this case, his name is put as the Owner of the Gift. The repository used is out of scope, it doesn't matter if a Database etc.

Now this scenario obviously suffers some concurrency issues. Two or more users, having own instances of the GiftClaimService and the ISomeRepository, might attempt to claim the gift at virtually the same time, causing some issues with the method (e.g. overriding the Owner multiple times, returing the wrong result or whatever you can imagine).

I'd like to lock that up, by creating a "global" lock object which will prevent others (other threads/instances etc.) from aquiring the same lock, limited to the type (Gift) and it's Id. Of couse this might run "simulataneously" (though not in the same instance) for different Ids.

The lock is released by it's owner once the changes are committed (by calling Save on the repository). For others it behaves like usual locks: The processing is on hold at this command. The new method would look like this:

public bool ClaimGift(int giftId, string myName)
{
    var result = false;

    // Lock it up (Pseudo-command)... I have no idea how this could work?
    lock(typeof(Gift), giftId)
    {
        var gift = _someRepository.Get(giftId);
        if (gift != null && gift.Owner == null)
        {
            gift.Owner = myName;
            _someRepository.Save(gift);
            result = true;
        }
    }

    return result;
}

This is how i wish it could look like, is there any common technique/pattern how to solve this?

Answer 1

void Main()
{
    var s = new GiftClaimService ();
    s.ClaimGift(1,"Me");
}

public class Gift
{
   public int Id { get; set; }
   public string Owner { get; set; }
}
public interface ISomeRepository
{
    Gift Get(int giftId);
    void Save(Gift gift);
}
public class GiftClaimService
{
    private ISomeRepository _someRepository; // Going to be injected or whatever
   public readonly static Dictionary<int, string> _claimsInProgress = new Dictionary<int, string>();
   private static readonly object _locker = new object();

    public bool ClaimGift(int giftId, string myName)
    {
        lock (_locker)
        {
        if ( _claimsInProgress.ContainsKey(giftId))
            return false;
        }

        var gift = _someRepository.Get(giftId);
        if (gift == null)
            return false; // no such gift

        lock (_locker)
        {
            if ( _claimsInProgress.ContainsKey(giftId))
                return false;// someone claimed it just now

            _claimsInProgress.Add(giftId, myName);
        }
        bool retValue;
        if (gift.Owner == null)
        {
            gift.Owner = myName;
            _someRepository.Save(gift);
            retValue = true;
        }
        else
            retValue = false;

        lock (_locker)
        {
            _claimsInProgress.Remove(giftId);
        }           

        return retValue;
    }
}

Answer 2

If you want to handle a web-farm scenario, this would have to be handled at the database level. Otherwise you could try the code below:

public class GiftClaimService
{
    private static readonly object Lock = new object();
    private ISomeRepository _someRepository; // Going to be injected or whatever

    public bool ClaimGift(int giftId, string myName)
    {
        lock (Lock)
        {
            var gift = _someRepository.Get(giftId);
            if (gift != null && gift.Owner == null)
            {
                gift.Owner = myName;
                _someRepository.Save(gift);
                return true;
            }
            return false;
        }
    }
}