Jesse Ezell Blog

Making the Possible More Impossible

Jared Parsons thought it would be nice if we could have a ReadOnlyList object in our generic template instead of IEnumerable. I think that's an excellent idea, so a few minor modifications and we have an even better solution, not requiring the generic constraint TWrite : TRead:

    public interface ILockedObject<TRead, TWrite> 
    {
        void Read(Action<TRead> action);
        void Write(Action<TWrite> action);
        void Replace(ReplaceAction<TWrite> action);
    }
        
    public class ReaderWriterLockedObject<TRead, TWrite> : ILockedObject<TRead,TWrite>
    {
        public ReaderWriterLockedObject(TWrite value, Converter<TWrite, TRead> makeReadOnly)
        {
            MakeReadOnly = makeReadOnly;
            _value = value;
        }

        public Converter<TWrite, TRead> MakeReadOnly
        {
            get;
            private set;
        }

        TWrite _value;
        ReaderWriterLockSlim _rwLock = new ReaderWriterLockSlim();
        
        #region ILockedObject<T> Members

        public void Read(Action<TRead> action)

        {
            _rwLock.EnterReadLock();
            try
            {
                action(MakeReadOnly(_value));
            }
            finally
            {
                _rwLock.ExitReadLock();
            }
        }

        public void Write(Action<TWrite> action)
        {
            _rwLock.EnterWriteLock();
            try
            {
                action(_value);
            }
            finally
            {
                _rwLock.ExitWriteLock();
            }
        }

        public void Replace(ReplaceAction<TWrite> action)
        {
            _rwLock.EnterWriteLock();
            try
            {
                action(ref _value);
            }
            finally
            {
                _rwLock.EnterWriteLock();
            }
        }

        #endregion
    }

    public delegate void ReplaceAction<T>(ref T value);

Oh, and here is a ReadOnlyList<T> class to go along:

        public class ReadOnlyList<T> : IEnumerable<T>
        {
            IList<T> _list;
            public ReadOnlyList(IList<T> list)
            {
                _list = list;
            }

            public T this[int index]
            {
                get
                {
                    return _list[index];
                }
            }

            public int Count
            {
                get
                {
                    return _list.Count;
                }
            }
            
            #region IEnumerable<T> Members

            public IEnumerator<T> GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            #endregion

            #region IEnumerable Members

            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            #endregion
        }

So now we can do this:

ReaderWriterLockedObject<ReadOnlyList<Customer>, List<Customer>> lockedObject = new ReaderWriterLockedObject<ReadOnlyList<Customer>, List<Customer>>(l, c =>  new ReadOnlyList<Customer> (c));            

If we really want to be slick, we could take it a step further... protecting the elements inside our list:

ReaderWriterLockedObject<ReadOnlyList<ReadOnlyCustomer>, List<Customer>> lockedObject = new ReaderWriterLockedObject<ReadOnlyList<ReadOnlyCustomer>, List<Customer>>(l, wl =>  new ReadOnlyList<ReadOnlyCustomer> (wl, wc => (ReadOnlyCustomer)wc));