Question

In my program I need to process files. My program could use several threads to process files and therefore I need some sort of locking, because each file should not be processed by more than one thread at a time.

private object lockObj = new object();

public void processFile(string file)
{
  lock(lockObj)
  {
    //... actuall processing
  }
}

With the above code only one file can be processed at a time, but two threads should be able to process two different files at a time, but not the same file.

My first idea was to create a Dictionary with the key beeing the file and the value beeing the lock-object.
But I was wondering if it would also be possible to lock the string file? Any thoughts on this?

PS: sorry for not beeing able to find a better title

Answer 1

My first idea was to create a Dictionary with the key beeing the file and the value beeing the lock-object. But I was wondering if it would also be possible to lock the string file? Any thoughts on this?

If the strings will be created at runtime, locking on the string will not be safe. It would be better to make a dictionary of objects, and use those to lock.

That being said, you should consider using a ConcurrentDictionary<string,object> for this dictionary, as it will prevent a race condition (or other lock) in your dictionary itself. By using GetOrAdd you can safely get the appropriate object to use for locking.

That being said, a different approach may be appropriate here. You could use a ConcurrentQueue or BlockingCollection to provide a list of items to process, and then have a fixed number of threads process them. This will prevent synchronization problems from occurring in the first place.