.NET locking or ConcurrentDictionary?

Question

I'm writing something like a file cache and I'm debating between using lock or ConcurrentDictionary. If multiple threads ask for a key, then an ordinary Dictionary will have issues if two threads try to write to it, so I tried ConcurrentDictionary. There's now a secondary problem, how do you prevent the file being read twice (or more) as each thread tries to get the file. I've added sample code to explain what I mean.

Here's a version using lock and Dictionary

class Program
{
    private static object locking = new object();
    private static Dictionary<string, byte[]> cache;
    
    static void Main(string[] args)
    {
        cache = new Dictionary<string, byte[]>();
        
        Task.Run(() =>
        {
            AddToCache("largefile", "largefile.bin");
        });

        Task.Run(() =>
        {
            AddToCache("largefile", "largefile.bin");
        });
    }
    
    static byte[] AddToCache(string key, string filename)
    {
        lock(locking)
        {
            if (cache.TryGetValue(key, out byte[] data))
            {
                Console.WriteLine("Found in cache");
                return data;
            }

            Console.WriteLine("Reading file into cache");
            data = File.ReadAllBytes(filename);
            cache[key] = data;
            return data;
        }
    }
}

This version does what's expected, it'll protect the dictionary against multiple threads and only read the large file ONCE.

Here's the second version using ConcurrentDictionary:

class Program
{
    private static ConcurrentDictionary<string, byte[]> cache;

    static void Main(string[] args)
    {
        cache = new ConcurrentDictionary<string, byte[]>();

        Task.Run(() =>
        {
            AddToCache("largefile", "largefile.bin");
        });

        Task.Run(() =>
        {
            AddToCache("largefile", "largefile.bin");
        });
    }

    static byte[] AddToCache(string key, string filename)
    {
        return cache.GetOrAdd(key, (s) => 
        {
            Console.WriteLine("Reading file into cache");
            return File.ReadAllBytes(filename); 
        });
    }
}

This version protects the dictionary BUT it reads the large file TWICE which isn't what's required. I think I'm doing something wrong here, but not being familiar with GetOrAdd I'm not sure what.

The first version looks fine, but it's a cut down version of the real code, and the lock would be locking a lot of code. The second version looks much simpler but doesn't prevent multiple reading of the file. Is there a way to do this without the lock blocking a lot of code or is that the only answer?

Somewhat related: [ConcurrentDictionary GetOrAdd async](https://stackoverflow.com/questions/54117652/concurrentdictionary-getoradd-async). — Theodor Zoulias, Mar 13 '23 at 22:55

Guru Stron · Accepted Answer · 2021-10-04T10:44:04.860

The common trick is to use Lazy as value in ConcurrentDictionary so you can make add part of GetOrAdd thread safe. In your case it will look something like this:

private static ConcurrentDictionary<string, Lazy<byte[]>> cache;

static byte[] AddToCache(string key, string filename) => cache
        .GetOrAdd(key, (s) =>
            new Lazy<byte[]>(() =>
            {
                Console.WriteLine("Reading file into cache");
                return File.ReadAllBytes(filename);
            }))
        .Value;

The downside of such approach can be the deffered execution of the value function but since you are already wraping your dictionary access it should not be an issue for you.

.NET locking or ConcurrentDictionary?

1 Answers1