Question

Here is the scenario:

I have an object called a Transaction that needs to make sure that only one entity has permission to edit it at any given time.

In order to facilitate a long-lived lock, I have the class generating a token object that can be used to make the edits.

You would use it like this:

var transaction = new Transaction();

using (var tlock = transaction.Lock())
{
    transaction.Update(data, tlock);
}

Now, I want the TransactionLock class to implement IDisposable so that its usage can be clear. But, I don’t have any unmanaged resources to dispose. however, the TransctionLock object itself is a sort of “unmanaged resource” in the sense that the CLR doesn’t know how to properly finalize it.

All of this would be fine and dandy, I would just use IDisposable and be done with it.

However, my issue comes when I try to do this in the finalizer:

~TransactionLock()
{
    this.Dispose(false);
}

I want the finalizer to release the transaction from the lock, if possible. How, in the finalizer, do I detect if the parent transaction (this.transaction) has already been finalized?

Is there a better pattern I should be using?

Also, the Transaction class itself needn’t be disposable, because it doesn’t maintain a reference to the lock, and doesn’t care whether or not it is unlocked when it goes to the grave.

---

The Transaction class looks something like this:

public sealed class Transaction
{
    private readonly object lockMutex = new object();

    private TransactionLock currentLock;

    public TransactionLock Lock()
    {
        lock (this.lockMutex)
        {
            if (this.currentLock != null)
                throw new InvalidOperationException(/* ... */);

            this.currentLock = new TransactionLock(this);
            return this.currentLock;
        }
    }

    public void Update(object data, TransactionLock tlock)
    {
        lock (this.lockMutex)
        {
            this.ValidateLock(tlock);

            // ...
        }
    }

    internal void ValidateLock(TransactionLock tlock)
    {
        if (this.currentLock == null)
            throw new InvalidOperationException(/* ... */);

        if (this.currentLock != tlock)
            throw new InvalidOperationException(/* ... */);
    }

    internal void Unlock(TransactionLock tlock)
    {
        lock (this.lockMutex)
        {
            this.ValidateLock(tlock);

            this.currentLock = null;
        }
    }
}

And the Dispose(bool) code for the TransactionLock:

private void Dispose(bool disposing)
{
    if (disposing)
    {
        if (this.Transaction != null)
        {
            this.Transaction.Unlock(this);
            this.Transaction = null;
        }
    }
}

Answer 1

This was discussed before. Your case is much easier though, you are also implementing the finalizer. That’s fundamentally wrong, you are hiding a bug in the client code. Beware that finalizers run on a separate thread. Debugging a consistent deadlock is much easier than dealing with locks that disappear randomly and asynchronously.

Recommendation: follow the .NET framework lead: don’t help too much. Microsoft abandoned the Synchronized method for the same reason.