Static methods updating a Dictionary<T,U> in ASP.NET - is it safe to lock() on the dictionary itself?

Question

I have a class that maintains a static dictionary of cached lookup results from my domain controller - users' given names and e-mails.

My code looks something like:

private static Dictionary<string, string> emailCache = new Dictionary<string, string>();

protected string GetUserEmail(string accountName)
{
    if (emailCache.ContainsKey(accountName))
    {
        return(emailCache[accountName]);
    }

    lock(/* something */)
    {
        if (emailCache.ContainsKey(accountName))
        {
            return(emailCache[accountName]);
        }

        var email = GetEmailFromActiveDirectory(accountName);
        emailCache.Add(accountName, email);
        return(email);
    }
}

Is the lock required? I assume so since multiple requests could be performing lookups simultaneously and end up trying to insert the same key into the same static dictionary.

If the lock is required, do I need to create a dedicated static object instance to use as the lock token, or is it safe to use the actual dictionary instance as the lock token?

Answer 1

Collections in .NET are not thread safe so the lock is indeed required. An alternative to using the dictionary one could use Concurrent dictionaries introduced in .NET 4.0

http://msdn.microsoft.com/en-us/library/dd287191.aspx