Error "This stream does not support seek operations" in C#

You probably want something like this. Either checking the length fails, or the BinaryReader is doing seeks behind the scenes.

HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(url);
WebResponse myResp = myReq.GetResponse();

byte[] b = null;
using( Stream stream = myResp.GetResponseStream() )
using( MemoryStream ms = new MemoryStream() )
{
  int count = 0;
  do
  {
    byte[] buf = new byte[1024];
    count = stream.Read(buf, 0, 1024);
    ms.Write(buf, 0, count);
  } while(stream.CanRead && count > 0);
  b = ms.ToArray();
}

edit:

I checked using reflector, and it is the call to stream.Length that fails. GetResponseStream returns a ConnectStream, and the Length property on that class throws the exception that you saw. As other posters mentioned, you cannot reliably get the length of a HTTP response, so that makes sense.

Use a StreamReader instead:

HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(url);
WebResponse myResp = myReq.GetResponse();

StreamReader reader = new StreamReader(myResp.GetResponseStream());
return reader.ReadToEnd();

(Note - the above returns a String instead of a byte array)

You can't reliably ask an HTTP connection for its length. It's possible to get the server to send you the length in advance, but (a) that header is often missing and (b) it's not guaranteed to be correct.

Instead you should:

1. Create a fixed-length byte[] that you pass to the Stream.Read method
2. Create a List<byte>
3. After each read, call List.AddRange to append the contents of your fixed-length buffer onto your byte list

Note that the last call to Read will return fewer than the full number of bytes you asked for. Make sure you only append that number of bytes onto your List<byte> and not the whole byte[], or you'll get garbage at the end of your list.