SingleInstance lock replaced by ReaderWriterLockSlim

gistfile1.cs

/// <summary>
/// Protects shared instances from concurrent access. Other members and the base class are threadsafe.
/// </summary>
readonly ReaderWriterLockSlim _cacheLock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

~LifetimeScope()
{
    if (_cacheLock != null) _cacheLock.Dispose();
}

/// <summary>
/// Try to retrieve an instance based on a GUID key. If the instance
/// does not exist, invoke <paramref name="creator"/> to create it.
/// </summary>
/// <param name="id">Key to look up.</param>
/// <param name="creator">Creation function.</param>
/// <returns>An instance.</returns>
public object GetOrCreateAndShare(Guid id, Func<object> creator)
{
    if (creator == null)
    {
        throw new ArgumentNullException("creator");
    }
    
    object result;
    
    // 1. try to get the shared instance using a read lock which can be acquired by multiple threads
    _cacheLock.EnterReadLock();
    try
    {
        if (_sharedInstances.TryGetValue(id, out result))
        {
            return result;
        }
    }
    finally
    {
        _cacheLock.ExitReadLock();
    }

    // 2. when the instance couldn't be fetched, enter the upgradeablereadlock which can be acquired by only one thread
    _cacheLock.EnterUpgradeableReadLock();
    try
    {
        if (!_sharedInstances.TryGetValue(id, out result))
        {
            // 3. when the instance still couldn't be fetched, enter the write lock which also can be acquired by only one thread
            _cacheLock.EnterWriteLock();
            try
            {
                result = creator();
                _sharedInstances.Add(id, result);
            }
            finally
            {
                _cacheLock.ExitWriteLock();
            }
        }

        return result;
    }
    finally
    {
        _cacheLock.ExitUpgradeableReadLock();
    }
}