Cached Value Factory with Expiration

MemoizeWithExpire.cs

async static Task Main()
{
	(await GetCachedContentAsync (new Uri ("http://foo"))).Dump();
  //(await GetCachedContentAsync (new Uri ("http://foo"))).Dump();
}

static Task<string> GetContent (Uri uri) {
 return Task.Delay (1000).ContinueWith (_ => "slow");
}

static Task<string> GetCachedContentAsync (Uri uri) {
 return _getCachedContent (uri)();
}

static readonly Func<Uri, Func<Task<string>>> _getCachedContent = 
	Memoize<Uri, Func<Task<string>>> (
		uri => WithExpiry (() => GetContent (uri), TimeSpan.FromSeconds(5)));

static Func<TKey, TValue> Memoize<TKey, TValue> (Func<TKey, TValue> getValueFunc)
{
	var cache = new Dictionary<TKey, TValue>();
	return (key =>
	{
		TValue result;
		lock (cache)
		{
			if (cache.TryGetValue (key, out result)) return result;
			return cache [key] = getValueFunc (key);
		}
	});
}

static Func<TResult> WithExpiry<TResult> (Func<TResult> payloadFunc, TimeSpan lifetime)
{
	object locker = new object();
	TResult payload = default(TResult);
	DateTime expiry = DateTime.MinValue;

	return () =>
	{
		lock (locker)
		{
			if (DateTime.UtcNow > expiry)
			{
				payload = payloadFunc();
				expiry = DateTime.UtcNow + lifetime;
			}
			return payload;
		}
	};
}