Why does HTTP headers doesn't get created when

问题:

I'm using an .aspx page to serve an image file from the file system according to the given parameters.

Server.Transfer(imageFilePath);

When this code runs, the image is served, but no Last-Modified HTTP Header is created. as opposed to that same file, being called directly from the URL on the same Server.

Therefor the browser doesn't issue an If-Modified-Since and doesn't cache the response.

Is there a way to make the server create the HTTP Headers like normally does with a direct request of a file (image in that case) or do I have to manually create the headers?

回答1:

When you make a transfer to the file, the server will return the same headers as it does for an .aspx file, because it's basically executed by the .NET engine.

You basically have two options:

• Make a redirect to the file instead, so that the browser makes the request for it.

• Set the headers you want, and use Request.BinaryWrite (or smiiliar) to send the file data back in the response.

回答2:

I'll expand on @Guffa's answer and share my chosen solution.

When calling the Server.Transfer method, the .NET engine treats it like an .aspx page, so It doesn't add the appropriate HTTP Headers needed (e.g. for caching) when serving a static file.

There are three options

• Using Response.Redirect, so the browser makes the appropriate request
• Setting the headers needed and using Request.BinaryWrite to serve the content
• Setting the headers needed and calling Server.Transfer

I choose the third option, here is my code:

        try
        {
            DateTime fileLastModified = File.GetLastWriteTimeUtc(MapPath(fileVirtualPath));
            fileLastModified = new DateTime(fileLastModified.Year, fileLastModified.Month, fileLastModified.Day, fileLastModified.Hour, fileLastModified.Minute, fileLastModified.Second);
            if (Request.Headers["If-Modified-Since"] != null)
            {
                DateTime modifiedSince = DateTime.Parse(Request.Headers["If-Modified-Since"]);

                if (modifiedSince.ToUniversalTime() >= fileLastModified)
                {
                    Response.StatusCode = 304;
                    Response.StatusDescription = "Not Modified";
                    return;
                }
            }
            Response.AddHeader("Last-Modified", fileLastModified.ToString("R"));
        }
        catch
        {
            Response.StatusCode = 404;
            Response.StatusDescription = "Not found";
            return;
        }
        Server.Transfer(fileVirtualPath);