Simple Data Access in C#

using System;
using System.Collections;
using System.Runtime.CompilerServices;

using Yap.Data.Client.Properties;


namespace Yap.Data.Client
{
	/// <summary>
	/// Represents the scope stack for command execution.
	/// </summary>
	internal class ScopeStack
	{
		[ThreadStatic]
		private static ScopeStack _Instance;

		/// <summary>
		/// Gets the <see cref="Yap.Data.Client.ScopeStack"/> for current thread.
		/// </summary>
		public static ScopeStack Instance
		{
			[MethodImpl(MethodImplOptions.Synchronized)]
			get
			{
				if (_Instance == null)
				{
					_Instance = new ScopeStack();
				}
				return _Instance;
			}
		}

		private readonly Stack _CurrentStack;

		/// <summary>
		/// Registers specified scope making it the active one.
		/// </summary>
		/// <param name="scope">The scope to register.</param>
		public void RegisterScope(Scope scope)
		{
			_CurrentStack.Push(scope);
		}

		/// <summary>
		/// Unregisters specified scope.
		/// </summary>
		/// <remarks>Specified scope should be on top of the current thread's scope stack to be unregistered.</remarks>
		/// <param name="scope">The scope to unregister.</param>
		public void UnregisterScope(Scope scope)
		{
			if (GetActiveScope() != scope)
			{
				throw new InvalidOperationException(
					Resources.ItIsNotAllowedToUnregisterInactiveScopeException);
			}
			_CurrentStack.Pop();
		}

		/// <summary>
		/// Gets the active scope from stack.
		/// </summary>
		/// <returns></returns>
		public Scope GetActiveScope()
		{
			if (_CurrentStack.Count == 0)
			{
				return null;
			}
			return _CurrentStack.Peek() as Scope;
		}

		/// <summary>
		/// Gets the value indicating whether active scope exists.
		/// </summary>
		public Boolean HasActiveScope
		{
			get
			{
				return GetActiveScope() != null;
			}
		}

		private ScopeStack()
		{
			_CurrentStack = new Stack();
		}
	}
}