The BinaryWriter class is written such that it will dispose the underlying stream that you pass to it when you dispose the BinaryWriter unless you use the optional constructor parameter to tell it not to. Since the binary writer is disposing the underlying memory stream and you are also disposing of it in the using
block it is being disposed of twice.
Now, this isn't really an issue for most classes, as they should be (and memory stream is) written to work just fine if disposed twice, as long as you don't use it after it's dispose (you're not) so you can safely ignore the warning. If you want the warning to go away you can just take the memory stream out of the using since it's already being disposed.