How can I check a new line and space characters

Hello,
I am controlling each newline(rows) in a textbox with a db. If a line has only space characters or just a newline (when you just hit enter), I want to know this. For example
---------
1123         
34324
43523          //3.th line

--------
When I hit enter on the 3.th line, it creates one more line but it has no character. How can I check this line?


 if (str[dongu].ToString() == "")
            {
              // do something when recognize last row.
             }