Umbraco-CMS/src/Umbraco.Core/ObjectResolution/ResolverBase.cs

using System;
using System.Threading;

namespace Umbraco.Core.ObjectResolution
{
	/// <summary>
	/// The base class for all resolvers.
	/// </summary>
	/// <typeparam name="TResolver">The type of the concrete resolver class.</typeparam>
	/// <remarks>Provides singleton management to all resolvers.</remarks>
	public abstract class ResolverBase<TResolver> 
		where TResolver : class
	{
		static TResolver _resolver;
		
		/// <summary>
		/// The lock for the singleton.
		/// </summary>
		/// <remarks>
		/// Though resharper says this is in error, it is actually correct. We want a different lock object for each generic type.
		/// See this for details: http://confluence.jetbrains.net/display/ReSharper/Static+field+in+generic+type
		/// </remarks>
		static readonly ReaderWriterLockSlim ResolversLock = new ReaderWriterLockSlim();

		/// <summary>
		/// Gets or sets the resolver singleton instance.
		/// </summary>
		/// <remarks>The value can be set only once, and cannot be read before it has been set.</remarks>
		/// <exception cref="InvalidOperationException">value is read before it has been set, or value is set again once it has already been set.</exception>
		/// <exception cref="ArgumentNullException">value is <c>null</c>.</exception>
		public static TResolver Current
		{
			get
			{
				using (new ReadLock(ResolversLock))
				{
					if (_resolver == null)
						throw new InvalidOperationException(string.Format(
							"Current has not been initialized on {0}. You must initialize Current before trying to read it.",
							typeof(TResolver).FullName));
					return _resolver;
				}
			}

			set
			{
				using (new WriteLock(ResolversLock))
				{
					if (value == null)
						throw new ArgumentNullException("value");
					if (_resolver != null)
						throw new InvalidOperationException(string.Format(
							"Current has already been initialized on {0}. It is not possible to re-initialize Current once it has been initialized.",
							typeof(TResolver).FullName));
					_resolver = value;
				}
			}
		}

		/// <summary>
		/// Resets the resolver singleton instance to null.
		/// </summary>
		/// <remarks>To be used in unit tests.</remarks>
		internal static void Reset()
		{
			_resolver = null;
		}
	}
}