How to make underlined-centered titles at KOMA-script?

Question

So I tried to make underlined-centered titles:
The answer for the centered came here and here.

Now my question is, how I make it underlined: with \ul it doesn't work (the file don't compile, and with uline I get ugly underline:

And here is the MWE:

\documentclass{scrartcl}

\usepackage{fontspec,adforn,ulem,soul}

\makeatletter

\addtokomafont{section}{\centering}  
\addtokomafont{subsection}{\centering}
\renewcommand{\sectionlinesformat}[4]{%
    %\@tempswafalse
        \ifstr{#1}{section}{%
                {\@hangfrom{{{#3}{#4}\adforn{12}}}}
        }
        {\ifstr{#1}{subsection}{%
                {\@hangfrom{\uline{{{#3}{#4}\adforn{24}}}}}%
        }
    {\@hangfrom{\hskip#2#3}{#4}}}%
}\makeatother

\begin{document}

\part{ABC}

\section{abc}

\subsection{abcd efgh abcd efgh iiii hhjjgghgg abcd efgh abcd efgh iiii hhjjgghgg abcd efgh abcd efgh iiii hhjjgghgg}
\end{document}

P.S. For the parts I got answer here but it also don't work.
and here is about using \ul or \uline.
And if I'm putting parbox this what I get:
and this is the relevant line:

{\ifstr{#1}{subsection}{%
                {\@hangfrom{\uline{\parbox{\linewidth}{{#3}{#4}\adforn{24}}}}}%
        }

Do you have any idea how it could work and look properly and the text can be more then one line centered and underlined?

Thank you!!

Answer 1

To visualize the problems with underlining in your case please see the following code. It is based on my answer and uses underlining with command \ul{} from package soul. Please see also that your used \adforn{} is placed in the title for section and subsection:

\documentclass{scrartcl}

\usepackage{fontspec,adforn}
\usepackage{soul}


\begin{document}

\renewcommand\raggedsection{\centering}

\part{ABC}

\section{\ul{abc test test test test test test test test test test test test 
  test test test test test test test test test test test test test test 
  test test test test test test} \adforn{12}} % <=======================

\subsection{\ul{def} \adforn{24}} % <===================================

\section{\ul{abc test test test test test test test test test test test test 
  test test test test test test test test test test test test test test 
  test test test test test test \protect\adforn{12}}} % <===============
\end{document} 

with the result:

As you can see \ul{...} is able to underline long text inside a \section if the \adforn{} is not included to \ul (see first red circle in image above)!

Now you can try to include \adforn into \ul like

\subsection{\ul{def \adforn{24}}}

with the result of 9 errors.

As you can see marked with the second red circle in the image above you can use \protect\adfarn to be able to compile without errors but the result is ugly (second red circle).

Conclusion:

• You can have underlining with centering without using \adforn or
• you can use \adforn with centering but without underlining

I suggest to omit underlining!

Based on the second MWE in this answer you can do

\documentclass{scrartcl}

\usepackage{fontspec}
\usepackage{adforn}
\usepackage{soul} % <===================================================

\renewcommand\raggedsection{\centering}

\let\originalsectionlinesformat\sectionlinesformat
\renewcommand{\sectionlinesformat}[4]{%
  \originalsectionlinesformat{#1}{#2}{#3}{\ul{#4}% <====================
    \ifstr{#1}{section}{\adforn{12}}
      {\ifstr{#1}{subsection}{\adforn{24}}{}}%
  }%
}

\usepackage{blindtext}% only for dummy text
\begin{document}
\section{abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc}
\blindtext
\subsection{def}
\blindtext
\end{document}

to get underlining heading for \section and \subsection without automaticly added \adforn symbol. But see: that could result in ugly typography, depending on the content of the used heading text ...