C＃从URL读取文件信息

Question

In my ASP.NET MVC application I'm reading an external file from URL and saving it into a directory on my server. I do this in a loop for every few seconds to have actual data if the file was modified.

What I need is to recognize that the LastWrittenTime of the file accessed via the URL is different from file already downloaded to the server. I cannot use FileInfo class because "URI formats are not supported".

So how do I get the last written time of the file from the URL without needing to download the full file for every loop?

Answer 1

Assuming your HTTP Server allows this.

System.Net.WebRequest req = System.Net.HttpWebRequest.Create("http:\\your\url.ext");
req.Method = "HEAD";
using (System.Net.WebResponse resp = req.GetResponse())
{
    DateTime LastModified;
    if(DateTime.TryParse(resp.Headers.Get("Last-Modified"), out LastModified))
    { 
        //Check if date is good and then go to full download method.
    }
}

When this method doesn't work because the server doesn't allow it. Then the only way of doing this is by fully downloading the file.

Answer 2

I'm afraid you can't.

When it's a webpage you want to download, you might hope that the server would set the LastModified attribute of a WebResponse correctly to get the date the page was last modified. But even this is not something you can always rely on.

When it's a file, there's no way to tell the date unless you saved the file on your own disk. (Like you are doing now.) And even then, depending on server settings, you might be getting the date the file was created on your disc, not on the server you're downloading from.