class ClassExample 
{
  ...
 
  public ClassExample(SerialPort x) {
      ...
  }
 
  public ClassExample(string portname, int baudrate) : this( new SerialPort(portname, baudrate) ) {}
 
  public ClassExample(string connection) {
    //connection is in the form "portname:baudrate"
    string portname;
    int baudrate;
    ...(parse connection into portname and baudrate)...
    this(portname, baudrate); //###ERROR HERE###
  }

}