Lock Manager for .NET

/// <disclaimer>
/// This software is protected by your own conscience (c).
/// You can use it however you want and wherever you want.
/// You are allowed to change this code, copy this code, or delete this code.
/// You can buy this code; sell this code; present this code to your mom on her birthday.
/// You can replace author�s name with your own. You also can replace all the code with your own leaving
/// only author�s name.
/// The only thing you cannot do, is to violate this license agreement. You simply are not able to.
/// </disclaimer>
/// <author>
/// Sergei Zotin 
/// szotin@shaw.ca
/// Burnaby, BC, Canada
/// Feel free to contact me for bug reports, feature requests and contract offers.
/// </author>
/// <version>1.6</version>

using System;
using System.Threading;
using ZEN.Threading;
using System.IO;

class A
{
	private B b;
	private int x;
	private int y;

	public B TheB
	{
		get
		{
			LockManager.Lock ( this );
			try
			{
				return b;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
		set
		{
			LockManager.Lock ( this );
			try
			{
				b = value;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
	}

	public int X
	{
		get
		{
			LockManager.Lock ( this );
			try
			{
				return x;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}

		// NEW: 1. Now we undo the change on DeadlockException. When DeadlockException is thrown we assigned
		// NEW:    value to x, but failed to assign it to b.X. The objects are in inconsistent state. Meanwhile
		// NEW:    all locks are released to before retrying the operation. That means, other threads
		// NEW:    may notice this inconsistence. To prevent it, we undo the assignment before releasing the lock.
		// NEW: 2. External components may lock A themselves before calling A.X. If they do it, we are in
		// NEW:    trouble. We catch DeadlockException release all our locks and retry. But the external lock
		// NEW:    is still there. The deadlock is still there as well. To prevent it, we check 
		// NEW:    LockManager.LockCount. If it is 1: we own all locks and can retry the operation. If not:
		// NEW:    we simply rethrow the exception allowing external components to release their locks and retry.
		set
		{
			bool retry;
			do
			{
				int oldX = x;
				retry = false;
				LockManager.Lock ( this );
				try
				{
					Thread.Sleep ( 10 ); // just to increase the odds of a deadlock
					x = value;
					b.X = value;
				}
				catch ( DeadlockException exc )
				{
					// undo the change
					x = oldX;
					
					// if this is the last lock - retry; else - rethrow - it should be retried on an upper level
					if ( LockManager.LockCount == 1 ) 
						retry = true;
					else
						throw exc;
				}
				finally
				{
					LockManager.Unlock ( this );
				}
			} while ( retry );
		}
	}

	public int Y
	{
		get
		{
			LockManager.Lock ( this );
			try
			{
				return y;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
		set
		{
			LockManager.Lock ( this );
			try
			{
				Thread.Sleep ( 10 ); // just to increase the odds of a deadlock
				y = value;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
	}
}

class B
{
	private A a;
	private int x;
	private int y;

	public A TheA
	{
		get
		{
			LockManager.Lock ( this );
			try
			{
				return a;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
		set
		{
			LockManager.Lock ( this );
			try
			{
				a = value;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
	}

	public int X
	{
		get
		{
			LockManager.Lock ( this );
			try
			{
				return x;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
		set
		{
			LockManager.Lock ( this );
			try
			{
				Thread.Sleep ( 10 ); // just to increase the odds of a deadlock
				x = value;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}
	}

	public int Y
	{
		get
		{
			LockManager.Lock ( this );
			try
			{
				return y;
			}
			finally
			{
				LockManager.Unlock ( this );
			}
		}

		// NEW: 1. Now we undo the change on DeadlockException. When DeadlockException is thrown we assigned
		// NEW:    value to y, but failed to assign it to a.Y. The objects are in inconsistent state. Meanwhile
		// NEW:    all locks are released to before retrying the operation. That means, other threads
		// NEW:    may notice this inconsistence. To prevent it, we undo the assignment before releasing the lock.
		// NEW: 2. External components may lock B themselves before calling B.Y. If they do it, we are in
		// NEW:    trouble. We catch DeadlockException release all our locks and retry. But the external lock
		// NEW:    is still there. The deadlock is still there as well. To prevent it, we check 
		// NEW:    LockManager.LockCount. If it is 1: we own all locks and can retry the operation. If not:
		// NEW:    we simply rethrow the exception allowing external components to release their locks and retry.
		set
		{
			bool retry;
			do
			{
				int oldY = y;
				retry = false;
				LockManager.Lock ( this );
				try
				{
					Thread.Sleep ( 10 ); // just to increase the odds of a deadlock
					y = value;
					a.Y = value;
				}
				catch ( DeadlockException exc )
				{
					// undo the change
					y = oldY;
					
					// if this is the last lock - retry; else - rethrow - it should be retried on an upper level
					if ( LockManager.LockCount == 1 ) 
						retry = true;
					else
						throw exc;
				}
				finally
				{
					LockManager.Unlock ( this );
				}
			} while ( retry );
		}
	}
}

class Step4
{
	private A a;
	private B b;

	private Step4 ( A _a, B _b )
	{
		a = _a;
		b = _b;
		a.TheB = b;
		b.TheA = a;
	}

	private void RunX ()
	{
		for ( int i = 0; i < 20; i++ )
		{
			a.X = i;
			System.Console.WriteLine ( "Thread X: a.X={0}; b.X={1}", a.X, b.X );
		}
	}

	private void RunY ()
	{
		for ( int i = 0; i < 20; i++ )
		{
			b.Y = i;
			System.Console.WriteLine ( "Thread Y: a.Y={0}; b.Y={1}", a.Y, b.Y );
		}
	}

	[STAThread]
	static void Main ( string[] args )
	{
		A a = new A();
		B b = new B();
		Step4 step4 = new Step4 ( a, b );

		TextWriter log = new StreamWriter ( "deadlock.log", false );
		LockManager.Log = log;

		Thread threadX = new Thread ( new ThreadStart ( step4.RunX ) );
		Thread threadY = new Thread ( new ThreadStart ( step4.RunY ) );

		threadX.Name = "X";
		threadY.Name = "Y";

		threadX.Start ();
		threadY.Start ();

		threadX.Join ();
		threadY.Join ();

		// closing the log
		LockManager.Log = null;
		log.Close ();

		System.Console.WriteLine ( "Press Enter..." );
		System.Console.In.Read();
		System.Environment.Exit ( 0 );
	}
}