How to add new line into txt file

I'd like to add new line with text to my date.txt file, but instead of adding it into existing date.txt, app is creating new date.txt file..

TextWriter tw = new StreamWriter("date.txt");

// write a line of text to the file
tw.WriteLine(DateTime.Now);

// close the stream
tw.Close();

I'd like to open txt file, add some text, close it, and later after clicking something: open date.txt, add text, and close it again.

So i can get:

Button pressed: txt opened -> added current time, then close it. Another button pressed, txt opened -> added text "OK", or "NOT OK" in the same line, then close it again.

So my txt file will look like that:

2011-11-24 10:00:00 OK
2011-11-25 11:00:00 NOT OK

How can i do this? Thanks!

You could do it easily using

File.AppendAllText("date.txt", DateTime.Now.ToString());

If you need newline

File.AppendAllText("date.txt", 
                   DateTime.Now.ToString() + Environment.NewLine);

Anyway if you need your code do this:

TextWriter tw = new StreamWriter("date.txt", true);

with second parameter telling to append to file.
Check here StreamWriter syntax.

No new line:

File.AppendAllText("file.txt", DateTime.Now.ToString());

and then to get a new line after OK:

File.AppendAllText("file.txt", string.Format("{0}{1}", "OK", Environment.NewLine));

Why not do it with one method call:

File.AppendAllLines("file.txt", new[] { DateTime.Now.ToString() });

which will do the newline for you, and allow you to insert multiple lines at once if you want.