public static class MemoryCacheExtensions
{
    public static T AddOrGetExistingLazy<T>(this MemoryCache cache, string key, Func<T> valueFactory, CacheItemPolicy cacheItemPolicy)
    {
        var newValue = new Lazy<T>(valueFactory);
        var value = (Lazy<T>)cache.AddOrGetExisting(key, newValue, cacheItemPolicy);
        return (value ?? newValue).Value; // Lazy<T> handles the locking itself
    }

    public static void SetLazy<T>(this MemoryCache cache, string key, T value, CacheItemPolicy cacheItemPolicy)
    {
        var cacheValue = new Lazy<T>(() => value);
        cache.Set(key, cacheValue, cacheItemPolicy);
    }
}