Question

I am confused by a code listing in a book i am reading, C# 3 in a Nutshell, on threading.
In the topic on Thread Safety in Application Servers, below code is given as an example of a UserCache:

static class UserCache
{
    static Dictionary< int,User> _users = new Dictionary< int, User>();

    internal static User GetUser(int id)
    {
        User u = null;

        lock (_users) // Why lock this???
            if (_users.TryGetValue(id, out u))
                return u;

        u = RetrieveUser(id); //Method to retrieve from databse

        lock (_users) _users[id] = u; //Why lock this???
            return u;
    }
}

The authors explain why the RetrieveUser method is not in a lock, this is to avoid locking the cache for a longer period.
I am confused as to why lock the TryGetValue and the update of the dictionary since even with the above the dictionary is being updated twice if 2 threads call simultaneously with the same unretrieved id.

What is being achieved by locking the dictionary read?
Many thanks in advance for all your comments and insights.

Answer 1

The Dictionary<TKey, TValue> class is not threadsafe.

If one thread writes one key to the dictionary while a different thread reads the dictionary, it may get messed up. (For example, if the write operation triggers an array resize, or if the two keys are a hash collision)

Therefore, the code uses a lock to prevent concurrent writes.