Why is ASP.NET replacing a Content-Length header with a Transfer-Encoding header when manually flushing a response?

Question

Our web application (ASP.NET Web Forms) has a page that will display a recently generated PDF file to users. Because the PDF file is sometimes quite large, we've implemented a "streaming" approach to send it down to the client browser in chunks.

Despite sending the data down in chunks, we know the full size of the file prior to sending it, so we set the Content-Length header appropriately. This has been working in our production environment for awhile (and continues to work in our test environment with a virtually identical configuration) until today. The issue reported was that Chrome would attempt to open the PDF file but would hang with the "Loading" animation stuck.

Because everything was still working fine in our test environment I was able to use Firebug to take a look at the response headers that were coming back in both environments. In the test environment, I was seeing a proper 'Content-Length' header, while in production that had been replaced with a Transfer-Encoding: chunked header. Chrome doesn't like this, hence the hang-up.

I've read some articles and posts talking about how the Transfer-Encoding header can show up when no Content-Length header is provided, but we are specifying the Content-Length header and everything still appears to work while running the same code for the same PDF file on a test server.

Both test and production servers are running IIS 7.5 and both have Dynamic and Static Compression enabled.

Here is the code in question:

var fileInfo = new FileInfo(fileToSendDown);
Response.ClearHeaders();
Response.ContentType = "application/pdf";            
Response.AddHeader("Content-Disposition", "filename=test.pdf");
Response.AddHeader("Content-Length", fileInfo.Length.ToString());
var buffer = new byte[1024];
using (var fs = File.Open(file, FileMode.Open, FileAccess.Read, FileShare.Read))
{
    int read;
    while ((read = fs.Read(buffer, 0, 1024)) > 0)
    {
        if (!response.IsClientConnected) break;
        Response.OutputStream.Write(buffer, 0, read);
        Response.Flush();
    }
}

I was fortunate to see the same behavior on my local workstation so using the debugger I have been able to see that the 'Transfer-Encoding: chunked' header is being set on the 2nd pass through the while loop during the call to 'Flush'. At that point, the response has both a Content-Length header and Transfer-Encoding header, but somehow by the time the response reaches the browser Firebug is only showing the Transfer-Encoding header.

UPDATE

I think I've tracked this down to using a combination of sending the data down in "chunks" AND attaching a 'Filter' to the HttpResponse object (we were using a filter to track the size of viewstate being sent down to each page). There's no sense in us using an HTTP filter when sending a PDF down to the browser, so clearing the filter here has resolved our issue. I decided to dig in a little deeper purely out of curiosity and have updated this question should anyone else ever stumble onto this problem in the future.

I've got a simple app up on AppHarbor that reproduces the issue: http://transferencodingtest.apphb.com/. If you check both the 'Use Filter?' and 'Send In Chunks?' boxes you should be able to see the 'transfer-encoding: chunked' header show up (using Chrome dev tools, Firebug, Fiddler, whatever). If either of the boxes are not checked, you'll get a proper content-length header. The underlying code is up on github so you can see what's going on behind the scenes:

https://github.com/appakz/TransferEncodingTest

Note that to repro locally you'd need to setup a local website in IIS 7.5 (7 may also work, I haven't tried). The ASP .NET development server that ships with Visual Studio DOES NOT repro the issue.

I've added some more details to a blog post here: 'Content-Length' Header Replaced With 'Transfer-Encoding: Chunked' in ASP .NET

Answer 1

From an article on MSDN it seems that you can disable chunked encoding:

appcmd set config /section:asp /enableChunkedEncoding:False

But it's mentioned under ASP settings, so it may not apply to a response generated from an ASP.NET handler.