How to deal with costly building operations using MemoryCache?

We solved this issue by combining Lazy<T> with AddOrGetExisting to avoid a need for a lock object completely. Here is a sample code (which uses infinite expiration):

public T GetFromCache<T>(string key, Func<T> valueFactory) 
{
    var newValue = new Lazy<T>(valueFactory);
    // the line belows returns existing item or adds the new value if it doesn't exist
    var value = (Lazy<T>)cache.AddOrGetExisting(key, newValue, MemoryCache.InfiniteExpiration);
    return (value ?? newValue).Value; // Lazy<T> handles the locking itself
}

That’s not complete. There are gotchas like “exception caching” so you have to decide about what you want to do in case your valueFactory throws exception. One of the advantages, though, is the ability to cache null values too.