Question

I have an enumerator written in C#, which looks something like this:

try
{
    ReadWriteLock.EnterReadLock();
    yield return foo;
    yield return bar;
    yield return bash;
}
finally
{
    if (ReadWriteLock.IsReadLockHeld)
        ReadWriteLock.ExitReadLock();
}

I believe this may be a dangerous locking pattern, as the ReadWriteLock will only be released if the enumeration is complete, otherwise the lock is left hanging and is never released, am I correct? If so, what’s the best way to combat this?

Answer 1

No, the finally block will always be executed, pretty much unless somebody pulls the plug from the computer (well and a few other exceptions).

public static IEnumerable<int> GetNumbers() {
    try
    {
        Console.WriteLine("Start");
        yield return 1;
        yield return 2;
        yield return 3;
    }
    finally
    {
        Console.WriteLine("Finish");
    }
}

…

foreach(int i in GetNumbers()) {
    Console.WriteLine(i);
    if(i == 2) break;
}

The output of the above will be

Start
1
2
Finish

Note that in C# you write yield return, not just yield. But I guess that was just a typo.