How to add new line char to rich text box?

Actually the carraiage-return and new-line combination (which is represented by "\r\n" in C++ and C# and Environment.NewLine in .Net) is very standard in the sense that it is what Windows uses. It is also the combination (carraiage-return and new-line) used for end-of-line for printers. The indiviual characters are highly standard. The CrLf combination is the standard (required) end-of-line for email. The Unix/Linux environment uses just a linefeed, but that is inconsistent with the actual definition of the character in printers. A linefeed only moves the carraige down, it does not return the carriage back unless the hardware is set up to work in a non-standard manner. In the Apple OS envirnmnet, a carriage return without a linefeed is used for a new line, and that is sure inconsistent with other operating systems and the standard.
 
Use of symbolic names such as Environment.NewLine is good programming, but the CrLf combination is very standard.

I would suggest that you always use Environment.NewLine because the new line character is not standard.

hi friend,
OntextChanged event of your rich text box write following code:

      private void richTextBox1_TextChanged(object sender, EventArgs e)

        {

            string txt=string.Empty;

            txt=richTextBox1.Text;

            if (txt != string.Empty)

            {

                if (txt.Length % 20 == 0)

                {

                    richTextBox1.AppendText(Environment.NewLine);

                }

            }

        }

Please mark the answer if it helps you.