In StringBuilder copied from richTextBox fast way to findout end of line ?

Question

richTextBox rtb = new richTextBox(); StringBuilder stb = new StringBuilder(); stb.Append(rtb.Text); In a foreach(char ch in stb.ToString()) statement I want to know if we have find end of line. Any fast way to do this? Could this be done without converting .ToString() by someway? It's important for me to read the characters separately and insert or remove a specific number of continous spaces(not all) at the start of any line, rapidly.