The code is just horribly wrong. @usr already pinpointed two big mistakes.
Here is corrected code:
// Server Reply
if (networkStream.CanRead) {
// Buffer to store the response bytes.
byte[] readBuffer = new byte[tcpClient.ReceiveBufferSize];
string fullServerReply = null;
using (var writer = new MemoryStream()) {
while (networkStream.DataAvailable) {
int numberOfBytesRead = networkStream.Read(readBuffer, 0, readBuffer.Length);
if (numberOfBytesRead <= 0) {
break;
}
writer.Write(readBuffer, 0, numberOfBytesRead);
}
fullServerReply = Encoding.UTF8.GetString(writer.ToArray());
}
}