Specified argument was out of the range of valid values. Parameter name: size & Serial Port Communication

c#, serial-port, sockets, windows-ce

Solution

Your code is a great example of how not to do TCP communication. I've seen this code copied over and over many times, and I'd be very happy to point you to a good tutorial on TCP - too bad I haven't seen one yet :)

Let me point out some errors first:

- TCP doesn't guarantee you the packet arrives as one bunch of bytes. So (theoretically) the `Write` operation could result in a split, requiring two reads on the other side. Sending data without headers over TCP is a very bad idea - the receiving side has no idea how much it has to read. So you've got two options - either write the length of the whole bunch of data before the data itself, or use a control character to end the "packet"

- The first point should also clarify that your reading is wrong as well. It may take more than a single read operation to read the whole "command", or a single read operation might give you two commands at once!

- You're reading `ReceiveBufferSize` bytes into a `10025` long buffer. `ReceiveBufferSize` might be bigger than your buffer. Don't do that - read a max count of `inStream.Length`. If you were coding in C++, this would be a great example of a buffer overflow.

- As you're converting the data to a string, you're expecting the whole buffer is full. That's most likely not the case. Instead, you have to store the return value of the read call - it tells you how many bytes were actually read. Otherwise, you're reading garbage, and basically having another buffer overflow.

So a much better (though still far from perfect) implementation would be like this:

NetworkStream serverStream = clientSocket.GetStream();
byte[] outStream = Encoding.ASCII.GetBytes("PCK|SCAN|5025066840471");

// It would be much nicer to send a terminator or data length first, 
// but if your server doesn't expect that, you're out of luck.
serverStream.Write(outStream, 0, outStream.Length);

// When using magic numbers, at least use nice ones :)
byte[] inStream = new byte[4096];

// This will read at most inStream.Length bytes - it can be less, and it
// doesn't tell us how much data there is left for reading.
int bytesRead = serverStream.Read(inStream, 0, inStream.Length);

// Only convert bytesRead bytes - the rest is garbage
string returndata = Encoding.ASCII.GetString(inStream, 0, bytesRead);

Oh, and I have to recommend this essay on TCP protocol design.

It talks about many of the misconceptions about TCP, most importantly see the Message Framing part.

Problem

Original source