Locking on an interned string?

Question

Update: It is acceptable if this method is not thread safe, but I\'m interested in learning how I would make it thread safe. Also, I do not want to lock on

Answer 1

I would go with the pragmatic approach and use the dummy variable.
If this is not possible for whatever reason, I would use a Dictionary with key as the key and a dummy object as the value and lock on that value, because strings are not suitable for locking:

private object _syncRoot = new Object();
private Dictionary _syncRoots = new Dictionary();

public static T CheckCache(string key, Func fn, DateTime expires)
{
    object keySyncRoot;
    lock(_syncRoot)
    {

        if(!_syncRoots.TryGetValue(key, out keySyncRoot))
        {
            keySyncRoot = new object();
            _syncRoots[key] = keySyncRoot;
        }
    }
    lock(keySyncRoot)
    {

        object cache = HttpContext.Current.Cache.Get(key);
        if (cache == null)
        {
            T result = fn();
            HttpContext.Current.Cache.Insert(key, result, null, expires, 
                                             Cache.NoSlidingExpiration);
            return result;
        }
        else
            return (T)cache;
    }
}

However, in most cases this is overkill and unnecessary micro optimization.