using System;
using System.Threading;

namespace PatternExplorer {
   /// <summary>
   /// Provides a default implementation for <see cref="IDisposable"/>.
   /// </summary>
   /// <remarks>
   /// 
   /// Derived classes that need finalizers should implement the following AND be sealed.
   /// 
   ///    /// <summary>
   ///    /// Finalizer.
   ///    /// </summary>
   ///    ~MyClass() {
   ///       Dispose(false);
   ///    }
   /// 
   /// Derived classes that need to protect against access of disposed state can use the following example:
   /// 
   ///    /// <summary>
   ///    /// Member accessing potentially disposed state.
   ///    /// </summary>
   ///    public void MyMethod() {
   ///       ThrowIfDisposed();
   ///       // ... 
   ///    }
   /// 
   /// </remarks>
   public abstract class Disposable : IDisposable {
      /// <summary>
      /// "Not Disposed" state.
      /// </summary>
      private const int NOT_DISPOSED = 0;

      /// <summary>
      /// "Disposing" state.
      /// </summary>
      private const int DISPOSING = 1;

      /// <summary>
      /// "Disposed" state.
      /// </summary>
      private const int DISPOSED = 2;

      /// <summary>
      /// Flag to indicate state if disposal.
      /// </summary>
      private int _State;

      /// <summary>
      /// Constructor.
      /// </summary>
      protected Disposable() { }

      /// <summary>
      /// true if the instance has been disposed; otherwise, false.
      /// </summary>
      public bool IsDisposed {
         get { return Thread.VolatileRead(ref _State) != DISPOSED; }
      }

      /// <summary>
      /// Occurs when the instance is being disposed.
      /// </summary>
      public event EventHandler Disposing;

      /// <summary>
      /// Occurs when the instance has been disposed.
      /// </summary>
      public event EventHandler Disposed;

      /// <summary>
      /// Occurs when the instance is being disposed.
      /// </summary>
      protected virtual void OnDisposing() {
         if( Disposing != null ) {
            Disposing.Invoke(this, EventArgs.Empty);
         }
      }

      /// <summary>
      /// Occurs when the instance has been disposed.
      /// </summary>
      protected virtual void OnDisposed() {
         if( Disposed != null ) {
            Disposed.Invoke(this, EventArgs.Empty);
         }
      }

      /// <summary>
      /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
      /// </summary>
      /// <filterpriority>2</filterpriority>
      public void Dispose() {
         Dispose(true);
      }

      /// <summary>
      /// Disposes the instance.
      /// </summary>
      /// <param name="disposing"><value>true</value> to indicate explicit cleanup (i.e. <see cref="IDisposable.Dispose"/>()), otherwise <value>false</value> (i.e. implicit cleanup from finalizer)</param>
      protected void Dispose( bool disposing ) {
         // if( _State == NOT_DISPOSED ) {
         if( Interlocked.CompareExchange(ref _State, DISPOSING, NOT_DISPOSED) == NOT_DISPOSED ) {
            // _State = DISPOSING;

            if( disposing ) {
               try {
                  OnDisposing();
               } catch {
                  // intentional suppression
               }
               try {
                  DisposeManagedFields();
               } catch {
                  // intentional suppression
               }
               try {
                  RemoveDelegates();
               } catch {
                  // intentional suppression
               }
            }

            try {
               ReleaseUnmanagedResources();
            } catch {
               // intentional suppression
            }

            Interlocked.Increment(ref _State); // _State = DISPOSED;

            if( disposing ) {
               try {
                  GC.SuppressFinalize(this);
               } catch {
                  // intentional suppression
               }
               try {
                  OnDisposed();
               } catch {
                  // intentional suppression
               }
            }
         }
      }

      /// <summary>
      /// When overridden in a derived class, this method should dispose all <see cref="IDisposable"/> fields.
      /// </summary>
      protected virtual void DisposeManagedFields() { }

      /// <summary>
      /// When overridden in a derived class, this method should remove all delegate invocations and references.
      /// </summary>
      protected virtual void RemoveDelegates() { }

      /// <summary>
      /// When overridden in a derived class, this method should clean up any unmanaged resources.
      /// </summary>
      protected virtual void ReleaseUnmanagedResources() { }

      /// <summary>
      /// Derived classes may use this method to prevent member access on a disposed instance.
      /// </summary>
      protected void ThrowIfDisposed() {
         if( IsDisposed ) {
            throw new ObjectDisposedException(GetType().FullName, @"Access to a disposed object is not allowed.");
         }
      }
   }
}