namespace System.Threading
{
	public static class LazyInitializer
	{
		public static T EnsureInitialized<T>(ref T target) where T : class
		{
			return target ?? EnsureInitialized(ref target, GetDefaultCtorValue<T>);
		}

		public static T EnsureInitialized<T>(ref T target, Func<T> valueFactory) where T : class
		{
			if (target == null)
			{
				T value = valueFactory();
				if (value == null)
				{
					throw new InvalidOperationException();
				}
				Interlocked.CompareExchange(ref target, value, null);
			}
			return target;
		}

		public static T EnsureInitialized<T>(ref T target, ref bool initialized, ref object syncLock)
		{
			return EnsureInitialized(ref target, ref initialized, ref syncLock, GetDefaultCtorValue<T>);
		}

		public static T EnsureInitialized<T>(ref T target, ref bool initialized, ref object syncLock, Func<T> valueFactory)
		{
			if (initialized)
			{
				return target;
			}
			if (syncLock == null)
			{
				Interlocked.CompareExchange(ref syncLock, new object(), null);
			}
			lock (syncLock)
			{
				if (initialized)
				{
					return target;
				}
				initialized = true;
				Thread.MemoryBarrier();
				target = valueFactory();
			}
			return target;
		}

		private static T GetDefaultCtorValue<T>()
		{
			try
			{
				return Activator.CreateInstance<T>();
			}
			catch
			{
				throw new MissingMemberException("The type being lazily initialized does not have a public, parameterless constructor.");
			}
		}
	}
}