VS 2010 Check if line is empty or contains text.

I've been trying to find a solution myself before I post here but I couldn't find anything.

I have some text in a textbox. For example:

Now, you can obviously see that after the "Thanks", I have an empty line. I want to create
a code to check if the last line is empty or not (Even spaces is considered empty). So, let's look at Text A and Text B.

Text A.

Text B.

in Text B the last line is "Thanks". Therefore, it will add a new line after it saying "John.", for example.
in Text A the last line is " " (empty. there is a line, but it's empty. even if the line had 15 spaces, like
" ", it would be considered as empty). Then, it will overwrite that line and replace it with "John".

Thanks for any help.

Merry Christmas.