Idea for a double-checked lock helper object.

Latch.cs

// Latch definition
public class DoubleCheckLockLatch
{
	private readonly Object _lockSync = new Object();
	private readonly Func<bool> _isLatchClosed;

	public DoubleCheckLockLatch(Func<bool> isLatchClosed)
	{
		_isLatchClosed = isLatchClosed;
	}

	public virtual bool Latch(Action latchedAction)
	{
		var latchWasClosed = false;

		if (_isLatchClosed())
		{
			lock (_lockSync)
			{
				if (_isLatchClosed())
				{
					latchedAction();
				}
				else
				{
					latchWasClosed = true;
				}
			}
		}
		else
		{
			latchWasClosed = true;
		}

		return latchWasClosed;
	}
}

// Usage
public class UseLatch
{
	private readonly ILog _log;

	private readonly DoubleCheckLockLatch _latch;
	private bool _running = false;

	public UseLatch(ILog log)
	{
		_log = log;

		_latch = new DoubleCheckLockLatch(() => _running);
	}

	public void ReEntrant()
	{
		if (_latch.Latch(() => this.DoWork()))
		{
			Log("Attempted to restart work while already in progress.");
		}
	}

	private void DoWork()
	{
		_running = true;
		// Do the work.
		_running = false;
	}
}