Is ConcurrentDictionary.GetOrAdd() guaranteed to invoke valueFactoryMethod only once per key?

Question

Problem: I need to implement object cache. The cache need to be thread-safe and need to populate values on demand(lazy loading). The values are retrieved vi

Answer 1

As others have already pointed out, valueFactory may be invoked more than once. There is a common solution that mitigates this issue - have your valueFactory return a Lazy instance. Although it's possible that multiple lazy instances will be created, the actual T value will only be created when you access Lazy.Value property.

Specifically:

// Lazy instance may be created multiple times, but only one will actually be used.
// GetObjectFromRemoteServer will not be called here.
var lazyObject = dict.GetOrAdd("key", key => new Lazy(() => GetObjectFromRemoteServer()));

// Only here GetObjectFromRemoteServer() will be called.
// The next calls will not go to the server
var myObject = lazyObject.Value;

This method is further explained in Reed Copsey's blog post