Why asynchronous ASHX generated images doesn't always work in IE6?

Question

If you use an WebHandler inheriting IHttpAsyncHandler, you shouldn't notice that under undetermined specific circumstances the browser MS IE6 won't display it, the request will never finish. Is there a fix for it?

Answer 1

I'll answer it myself, but i took 3 days to solve it when I first met this problem.

When the image is requested through the "src" property of an "img" HTML tag, in certain conditions the browser MS IE6 needs the Content-Length to finish the request and display the result.

Synchronous ASHX generated images, automatically includes the "Content-Length" HTTP header, but the Asynchronous version does not. So when you are writing the output, write it first to an Memory Stream, read the total length, write it as an HTTP header, and then write the Memory Stream to the output.

Like this:

using (Image resizedImage = generateImage())
{
    using (MemoryStream memoryStream = new MemoryStream())
    {
        resizedImage.Save(memoryStream, ImageFormat.Jpeg);
        context.Response.AddHeader("Content-Length", memoryStream.Length.ToString());
        memoryStream.WriteTo(context.Response.OutputStream);
    }
}

I have tcpdumped both the Synchronous and the Asynchronous versions of my code and I noticed 2 more differences between them:

1) The asynchronous handler divides the answer in 3 TCP packets, instead of a single one.

2) The synchronous version uses a different "Keep-Alive" header (I can't remember which one)