What's the use of offset while reading from NetworkStream?

Question

I wrote a simple echo server with sockets. While making it I found that these two code gives the same result.

  var buffer = new byte[bytesToRead];
        var bytesRead = 0;
        while (bytesRead < bytesToRead)
        {
            var bytesReceived = await networkStream.ReadAsync(buffer, 0, (bytesToRead - bytesRead))
                .ConfigureAwait(false);
            if (bytesReceived == 0)
                throw new Exception("Socket Closed");
            bytesRead += bytesReceived;
        }

        return buffer;

---

 var buffer = new byte[bytesToRead];
        var bytesRead = 0;
        while (bytesRead < bytesToRead)
        {
            var bytesReceived = await networkStream.ReadAsync(buffer, bytesRead, (bytesToRead - bytesRead))
                .ConfigureAwait(false);
            if (bytesReceived == 0)
                throw new Exception("Socket Closed");
            bytesRead += bytesReceived;
        }

        return buffer;

The difference is in one I always set the

offset=0

while on the other I set

offset=bytesRead

(offset is 2nd parameter of networkStream.ReadAsync(byte[] buffer, int offset ,int count) ).

So what I concluded is that it doesn't matter what's the offset is as both code works but then how these both codes are working?

does offset is ignored in network stream?

Answer 1

As mentioned in the comments, In stream the second parameter is the offset into the buffer array not the stream That means if you keep the offset 0 then your are overwriting the buffer. Also network stream does not allow seeking. And I was getting same results because I was trying this on very short message length, in which case the loop effectively runs a single iteration to read the entire message, so the 2 code snippets appear to give the same result. I tried it on a much longer message and results changes.

So, the correct way is the second way in which I used offset as bytesRead.