ASP.Net Cache helper

CacheHelper.cs

/// <summary>
/// Implements a cache helper.
/// Taken from http://codereview.stackexchange.com/questions/25907/asp-net-caching-helper (comments added).
/// </summary>
public static class CacheHelper
{
    /// <summary>
    /// Retrieve the specified item from the cache if it is cached. Otherwise, insert it to the cache.
    /// Note: of the cache is not available, the <paramref name="initializer"/> method may be called on every call.
    /// </summary>
    /// <typeparam name="T">Type of the item to cache.</typeparam>
    /// <param name="key">The cache key used to reference the item.</param>
    /// <param name="initializer">The method to call to retrieve the item to cache</param>
    /// <param name="absoluteExpiration">
    /// The time at which the added object expires and is removed from the cache. If you are using sliding
    /// expiration, the absoluteExpiration parameter must be NoAbsoluteExpiration.
    /// </param>
    /// <returns>The retrieved cache item, or null if the key is not found.</returns>
    public static T GetCached<T>(string key, Func<T> initializer, DateTime absoluteExpiration)
    {
        return GetCached(key, initializer, absoluteExpiration, Cache.NoSlidingExpiration);
    }

    /// <summary>
    /// Retrieve the specified item from the cache if it is cached. Otherwise, insert it to the cache.
    /// Note: of the cache is not available, the <paramref name="initializer"/> method may be called on every call.
    /// </summary>
    /// <typeparam name="T">Type of the item to cache.</typeparam>
    /// <param name="key">The cache key used to reference the item.</param>
    /// <param name="initializer">The method to call to retrieve the item to cache</param>
    /// <param name="slidingExpiration">
    /// The interval between the time the added object was last accessed and the time at which that object expires.
    /// If this value is the equivalent of 20 minutes, the object expires and is removed from the cache 20 minutes
    /// after it is last accessed. If you are using absolute expiration, the slidingExpiration parameter must be
    /// NoSlidingExpiration.
    /// </param>
    /// <returns>The retrieved cache item, or null if the key is not found.</returns>
    public static T GetCached<T>(string key, Func<T> initializer, TimeSpan slidingExpiration)
    {
        return GetCached(key, initializer, Cache.NoAbsoluteExpiration, slidingExpiration);
    }

    /// <summary>
    /// Retrieve the specified item from the cache if it is cached. Otherwise, insert it to the cache.
    /// Note: of the cache is not available, the <paramref name="initializer"/> method may be called on every call.
    /// </summary>
    /// <typeparam name="T">Type of the item to cache.</typeparam>
    /// <param name="key">The cache key used to reference the item.</param>
    /// <param name="initializer">The method to call to retrieve the item to cache</param>
    /// <param name="absoluteExpiration">
    /// The time at which the added object expires and is removed from the cache. If you are using sliding
    /// expiration, the absoluteExpiration parameter must be NoAbsoluteExpiration.
    /// </param>
    /// <param name="slidingExpiration">
    /// The interval between the time the added object was last accessed and the time at which that object expires.
    /// If this value is the equivalent of 20 minutes, the object expires and is removed from the cache 20 minutes
    /// after it is last accessed. If you are using absolute expiration, the slidingExpiration parameter must be
    /// NoSlidingExpiration.
    /// </param>
    /// <returns>The retrieved cache item, or null if the key is not found.</returns>
    public static T GetCached<T>(string key, Func<T> initializer, DateTime absoluteExpiration, TimeSpan slidingExpiration)
    {
        var httpContext = HttpContext.Current;

        if (httpContext != null)
        {
            key = string.Intern(key);
            lock (key) // locking on interned key
            {
                var obj = httpContext.Cache[key];
                if (obj == null)
                {
                    obj = initializer();
                    httpContext.Cache.Add(key, obj, null, absoluteExpiration, slidingExpiration, CacheItemPriority.Default,
                                          null);
                }
                // taking care of value types
                if (obj == null && (typeof(T)).IsValueType)
                {
                    return default(T);
                }
                return (T)obj;
            }
        }
        else
        {
            return initializer(); // no available cache
        }
    }
}