Code
% https://tex.stackexchange.com/a/159996/13173
\documentclass{article}
\usepackage[normalem]{ulem}
\begin{document}
\begin{enumerate}
\item Initials: \uline{A.B.C.}
\item Age: \uline{50}
\item Address: \uline{Tampere}
\item Condition: \uline{ % showing where \uline{ \hfill\null}}
Entered text here in many lines. % I have also here many comments which causes problems
It can go on many lines. % commments can be here
\hfill\null}
\end{enumerate}
\end{document}
Fig. 1 Current output, Fig. 2 Complication of overloading text at the right-hand-side
Wished output: underline goes all the linewidth but does not pass the right-hand-side margin
Output in TeXLive 2017 because of comments in ulem environment
(/usr/local/texlive/2017/texmf-dist/tex/generic/ulem/ulem.sty) (./test.aux)
Runaway argument?
\@empty
! Paragraph ended before \UL@word was complete.
<to be read again>
\par
l.15 \hfill\null}
?
! Emergency stop.
<to be read again>
\par
l.15 \hfill\null}
Testing Ian's answer
Please, see Fig. 2 about the complication where the underlined text passes through the right-hand-side margin.
OS: Debian 8.7 64 bit
TeXLive: 2017
Engine: Pdflatex
Hardware: Asus Zenbook UX303UA




\itemobject. - - What kind of difficulties can you run if you do not have a space at the end? - - The test condition which should be met is that you should not pass the right-hand-side margin with underline and text as in Fig. 2. – Léo Léopold Hertz 준영 Oct 04 '16 at 11:20\ulinejust as you cannot have a blank line in\textscor insection. – Johannes_B Jun 11 '17 at 10:52