Question

I’m working on a concurrent dictionary implementation for C#, and I’m wondering if this GetEnumerator() implementation is actually (thread) safe.

It’s not doing a actual snapshot, so I’m wondering if it’ll mess up later read/writes to the internal dictionary, or if it’ll expose potential deadlocks because the enumeration of the exposed IEnumerator will actually run inside the lock.

private readonly Dictionary<TKey, TValue> internalDictionary;
private SpinLock spinLock = new SpinLock();

IEnumerator IEnumerable.GetEnumerator()
{
    IEnumerator enumerator;

    bool lockTaken = false;
    try
    {
        spinLock.TryEnter(ref lockTaken);
        enumerator = (this.internalDictionary as IEnumerable).GetEnumerator();
    }
    finally
    {
        if (lockTaken)
        {
            spinLock.Exit(false);
        }
    }

    return enumerator;
}

Answer 1

Your method is not thread-safe with respect to concurrent writers because

1. Your enumerator is not snapshotting anything. It is referring back the original dictionary. Call ToList on it or something to actually snapshot.
2. Concurrent writers do not use your lock so they execute concurrently. This is unsafe.
3. Don’t use a spinlock if the lock body is big.
4. What if TryEnter fails? It is called try after all.

Here is a fixed version with all the cleverness removed:

IEnumerator IEnumerable.GetEnumerator()
{
    lock (internalDictionary) return internalDictionary.ToList();
}

Concurrent writers have to take the lock, too.