C# Response.WriteFile vs Response.TransmitFile filesize issues

Question

I have a 5Mb pdf on the server dowloading this file using a writeFile gives me a 15Mb download, where as the transmitfile gives the correct 5Mb filesize...

Is this due to some sort of uncompression into memory on the server for the writeFile? Just wonder if anyone had seen the same thing happening...

(ps only noticed it since we went to iis7??)

code being...

if (File.Exists(filepath))
{
    HttpContext.Current.Response.Clear();
    HttpContext.Current.Response.ContentType = "application/octet-stream";
    HttpContext.Current.Response.AddHeader("content-disposition","attachment;filename=\""+Path.GetFileName(filepath)+"\"");
    HttpContext.Current.Response.AddHeader("content-length", new FileInfo(filepath).Length.ToString());

    //HttpContext.Current.Response.WriteFile(filepath);
    HttpContext.Current.Response.TransmitFile(filepath);

    HttpContext.Current.Response.Flush();
    HttpContext.Current.Response.Close();
}

Answer 1

TransmitFile - Writes the specified file directly to an HTTP response output stream without buffering it in memory.

WriteFile - Writes the specified file directly to an HTTP response output stream.

I would say the difference occurs because Transmit file doesn't buffer it. Write file is using buffering (Afiak), basically temporarily holding the data before transmitting it, as such it cannot guess the accurate file size because its writing it in chunks.