|By "Unix file" most people typically mean a plain ASCII text file where each line is terminated by an LF character (ASCII 10, or '\n') instead of the Windows CR+LF pair ("\r\n").|
The StreamWriter.WriteLine method will write Environment.NewLine at the end. Environment.NewLine is defined as "\r\n" on Windows and "\n" on Unix.
The StreamReader.ReadLine method will read one line that is terminated by either "\n" or "\r" or the pair "\r\n". So if you are reading a Unix text file on Windows, it's all done seamlessly.