Formatting a block of text with newlines embedded within it:

// 1) Remember to use Environment.NewLine on every block of text 
// we format that we want platform correct newlines inside of
string line;
line = String.Format("FirstLine {0} SecondLine {0} ThirdLine {0}",
              Environment.NewLine);

// get a temp file to work with
string file = Path.GetTempFileName( );
FileStream stream = File.Create(file);
byte[] bytes = Encoding.Unicode.GetBytes(line);
stream.Write(bytes,0,bytes.Length);
// close the file
stream.Close( );

// remove the file (good line to set a breakpoint to check out the file
// we created)
File.Delete(file);

You need to use a different newline character than the default one used by StreamWriter (which happens to be Environment.NewLine). You can set the newline that a StreamWriter will use once so that all WriteLines performed by the StreamWriter use that newline instead of having to manually do it each time:

// 2) Set up a text writer and tell it to use the certain newline
// string
// get a new temp file
file = Path.GetTempFileName( );
line = "Double spaced line";
StreamWriter streamWriter = new StreamWriter(file);
// make this always write out double lines
streamWriter.NewLine = Environment.NewLine + Environment.NewLine;
// WriteLine on this stream will automatically use the newly specified
// newline sequence (double newline in our case)
streamWriter.WriteLine(line);
streamWriter.WriteLine(line);
streamWriter.WriteLine(line);
// close the file
streamWriter.Close( );
// remove the file (good line to set a breakpoint to check out the file
// we created)
File.Delete(file);

Normal WriteLine calls:

// 3) Just use any of the normal WriteLine methods as they use the 
// Environment.NewLine by default
line = "Default line";
Console.WriteLine(line);