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Lazy Alternatives - LazyAndWeak and BackgroundLoader

, 1 Dec 2011 CPOL
This article will present two alternatives to Lazy.
LazyAlternatives.zip
LazyAlternatives
LazyAlternatives.suo
LazyAlternativesSample
Properties
Settings.settings
Pfz
Caching
Collections
DataTypes
DynamicObjects
Internal
Extensions
Factoring
Pfz.csproj.user
Pfz.Phone.csproj.user
Pfz.ruleset
Pfz.Silverlight.csproj.user
Pfz.snk
Pfz.suo
PhoneSpecific
Properties
Remoting
Instructions
Internal
Serializers
Udp
Serialization
BinaryBuiltIn
Threading
Contexts
Disposers
Unsafe
using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using Pfz.Threading;

namespace Pfz.Caching
{
	/// <summary>
	/// A hashset that allows items to be collected.
	/// </summary>
	public sealed class WeakHashSet<T>:
		ReaderWriterSafeDisposable,
		IEnumerable<T>,
		IGarbageCollectionAware
	where
		T: class
	{
		private HashSet<ThreadUnsafeReferenceSlim<T>> _hashset = new HashSet<ThreadUnsafeReferenceSlim<T>>();

		internal WeakHashSet(bool value)
		{
		}

		/// <summary>
		/// Creates a new weak-hashset.
		/// </summary>
		public WeakHashSet()
		{
			GCUtils.RegisterForCollectedNotification(this);
		}

		/// <summary>
		/// Releases all resources used by this hashset.
		/// </summary>
		protected override void Dispose(bool disposing)
		{
			if (disposing)
			{
				GCUtils.UnregisterFromCollectedNotification(this);

				foreach(var item in _hashset)
					item.Dispose();
			}

			base.Dispose(disposing);
		}

		/// <summary>
		/// Adds an item.
		/// </summary>
		public bool Add(T item)
		{
			var reference = new ThreadUnsafeReferenceSlim<T>(item, GCHandleType.Weak);
			try
			{
				using(DisposeLock.WriteLock())
				{
					CheckUndisposed();

					if (!_hashset.Add(reference))
					{
						reference.Dispose();
						return false;
					}

					return true;
				}
			}
			catch
			{
				reference.Dispose();
				throw;
			}
		}

		/// <summary>
		/// Removes an item.
		/// </summary>
		public bool Remove(T item)
		{
			using(DisposeLock.WriteLock())
			{
				CheckUndisposed();

				using(var reference = new ThreadUnsafeReferenceSlim<T>(item, GCHandleType.Weak))
					return _hashset.Remove(reference);
			}
		}

		/// <summary>
		/// Enumerates all valid (non-collected) items in this hashset.
		/// </summary>
		public IEnumerator<T> GetEnumerator()
		{
			return ToList().GetEnumerator();
		}

		/// <summary>
		/// Creates a strong-list with all items still available in this hashset.
		/// </summary>
		public List<T> ToList()
		{
			List<T> result;

			using(DisposeLock.ReadLock())
			{
				CheckUndisposed();

				result = new List<T>(_hashset.Count);

				foreach(var reference in _hashset)
				{
					T item = reference.Value;
					if (item != null)
						result.Add(item);
				}
			}

			return result;
		}

		IEnumerator IEnumerable.GetEnumerator()
		{
			return GetEnumerator();
		}

		void IGarbageCollectionAware.OnCollected()
		{
			using(var upgradeableLock = DisposeLock.UpgradeableLock())
			{
				if (WasDisposed)
					return;

				List<ThreadUnsafeReferenceSlim<T>> toRemove = null;
				HashSet<ThreadUnsafeReferenceSlim<T>> newset;
				
				try
				{
					newset = new HashSet<ThreadUnsafeReferenceSlim<T>>();
					foreach(var reference in _hashset)
					{
						if (reference.Value != null)
							newset.Add(reference);
						else
						{
							if (toRemove == null)
								toRemove = new List<ThreadUnsafeReferenceSlim<T>>();

							toRemove.Add(reference);
						}
					}
				}
				catch(OutOfMemoryException)
				{
					return;
				}

				upgradeableLock.Upgrade();

				_hashset = newset;

				if (toRemove != null)
				{
					int count = toRemove.Count;
					for(int i=0; i<count; i++)
						toRemove[i].Dispose();
				}
			}
		}
	}
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Paulo Zemek
Engineer Microsoft Corporation
United States United States
I started to program computers when I was 11 years old, as a hobbist, programming in AMOS Basic and Blitz Basic for Amiga.
At 12 I had my first try with assembler, but it was too difficult at the time. Then, in the same year, I learned C and, after learning C, I was finally able to learn assembler (for Motorola 680x0).
Not sure, but probably between 12 and 13, I started to learn C++. I always programmed "in an object oriented way", but using function pointers instead of virtual methods.
 
At 15 I started to learn Pascal at school and to use Delphi. At 16 I started my first internship (using Delphi). At 18 I started to work professionally using C++ and since then I've developed my programming skills as a professional developer in C++ and C#, generally creating libraries that help other developers do they work easier, faster and with less errors.
 
Now I just started working as a Senior Software Engineer at Microsoft.
 
Want more info or simply want to contact me?
Take a look at: http://paulozemek.azurewebsites.net/
Or e-mail me at: paulozemek@outlook.com
 
Codeproject MVP 2012
Microsoft MVP 2013-2014

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