how to append text to lines of texbox?-Collection of common programming errors

Setting Lines may trigger the _TextChanged event again. What is the error you are getting? If you see a StackOverflowException, this is the cause.

You could possibly add this to get around the problem, or take the bool flag approach Daniel mentioned in his answer.

text.TextChanged -= textBox1_TextChanged;

text.Lines = currentLines;

text.TextChanged += textBox1_TextChanged;

Also, perhaps this question discussing the difference between programmatic changes and user driven changes is of interest to you.