IfModifiedSinceWebClient

Есть в .NET несколько загадочный (в своей ограниченности), но в то же время удобный класс WebClient. И как показывает практика, его несложно расширить под свою задачу:

"You are now up to date." [IfModifiedSinceWebClient makes it easy to keep a local file in sync with online content - and do so asynchronously]

Recently, I was working with a program that needs to cache the content of a network file on the local disk for faster, easier access. The network file is expected to change periodically according to an unpredictable schedule; the objective is to keep the local file in "close" sync with the online copy without a lot of cost or complex protocols.

...

we can improve things easily: the HTTP specification defines the If-Modified-Since header for exactly this purpose! By including that header with the HTTP request, the server "knows" whether the local file is out of date

blogs.msdn.com/b/delay/archive/2011/10/18/ you-are-now-up-to-date ifmodifiedsincewebclient-makes-it-easy-to-keep-a-local-file-in-sync-with-online-content-and-do-so-asynchronously

Исходный код прилагается.