stream.CopyTo - file empty. asp.net

There is nothing wrong with your code. The fact you say "I've checked the stream.Length before copying and its not empty" makes me wonder about the stream position before copying.

If you've already consumed the source stream once then although the stream isn't zero length, its position may be at the end of the stream - so there is nothing left to copy.

If the stream is seekable (which it will be for a MemoryStream or a FileStream and many others), try putting

stream.Position = 0

just before the copy. This resets the stream position to the beginning, meaning the whole stream will be copied by your code.

I would recommend to put the following before CopyTo()

fileStream.Position = 0

Make sure to use the Flush() after this, to avoid empty file after copy.

fileStream.Flush()