how to find the timestamp of an online pdf file using c#?

I am writing an application that would download and replace a pdf file only if the timestamp is newer than that of the already existing one ...

I know its possible to read the time stamp of a file on a local computer via the code line below,

MessageB开发者_StackOverflowox.Show(File.GetCreationTime("C:\\test.pdf").ToString());

is it possible to read the timestamp of a file that is online without downloading it .. ?

Unless the directory containing the file on the site is configured to show raw file listings there's no way to get a timestamp for a file via HTTP. Even with raw listings you'd need to parse the HTML yourself to get at the timestamp.

If you had FTP access to the files then you could do this. If just using the basic FTP capabilities built into the .NET Framework you'd still need to parse the directory listing to get at the date. However there are third party FTP libraries that fill in the gaps such as editFTPnet where you get a FTPFile class.

Updated:

Per comment:

If I were to set up a simple html file with the dates and filenames written manually , I could simply read that to find out which files have actually been updated and download just the required files . is that a feasible solution ..

That would be one approach, or if you have scripting available (ASP.NET, ASP, PHP, Perl, etc) then you could automate this and have the script get the timestamp of the files(s) and render them for you. Or you could write a very simple web service that returns a JSON or XML blob containing the timestamps for the files which would be less hassle to parse than some HTML.

It's only possible if the web server explicitly serves that data to you. The creation date for a file is part of the file system. However, when you're downloading something over HTTP it's not part of a file system at that point.

HTTP doesn't have a concept of "files" in the way people generally think. Instead, what would otherwise be a "file" is transferred as response data with a response header that gives information about the data. The header can specify the type of the data (such as a PDF "file") and even specify a default name to use if the client decides to save the data as a file on the client's local file system.

However, even when saving that, it's a new file on the client's local file system. It has no knowledge of the original file which produced the data that was served by the web server.