sectsty underline and titlecap not working together

Question

So I'm trying to make the format of the subsection be normal font size, underlined, and have title case. Below is an example, stolen from here:

\documentclass{article}
\usepackage[normalem]{ulem}
\usepackage{sectsty} % Used for section formatting
\usepackage{titlecaps} % used for title case

\sectionfont{\centering\bfseries\normalsize\titlecap}

\begin{document}
\section*{Materials and methods}

\subsectionfont{\normalfont\normalsize\underline\titlecap}
\subsection*{Sampling about something magic}

\subsectionfont{\normalfont\normalsize\titlecap}
\subsection*{A very long sectional heading title that will probably have to be split over two lines}
Lorem ipsum
\end{document}

As you can see, the section Sampling about something magic makes has the underline, but not the title case. The next section with a long title has the title case once you remove the underline. How do I get both at the same time?

Also, more random note, if the \titlecap isn't at the end of the format arguments, it will not compile. Not sure if that helps out or not.

Answer 1

Not with sectsty nor titlecap, but with xparse and titlesec:

\documentclass{article}
\usepackage[normalem]{ulem}
\usepackage{titlesec} % Used for section formatting
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\titlecap}{m}
 {
  \berggeist_titlecap:Nn \tl_use:N { #1 }
 }

\cs_new_protected:Nn \berggeist_uline:n { \uline { #1 } }
\cs_generate_variant:Nn \berggeist_uline:n { V }

\NewDocumentCommand{\ultitlecap}{m}
 {
  \berggeist_titlecap:Nn \berggeist_uline:V { #1 }
 }

\NewDocumentCommand{\addtitlecapexceptions}{m}
 {
  \clist_gput_right:Nn \g_berggeist_titlecap_exceptions_clist { #1 }
 }

\seq_new:N \l__berggeist_titlecap_in_seq
\seq_new:N \l__berggeist_titlecap_out_seq
\tl_new:N \l__berggeist_titlecap_out_tl
\clist_new:N \g_berggeist_titlecap_exceptions_clist

\cs_new_protected:Nn \berggeist_titlecap:Nn
 {
  \seq_set_split:Nnn \l__berggeist_titlecap_in_seq { ~ } { #2 }
  \seq_clear:N \l__berggeist_titlecap_out_seq
  \seq_map_inline:Nn \l__berggeist_titlecap_in_seq
   {
    \clist_if_in:NnTF \g_berggeist_titlecap_exceptions_clist { ##1 }
     {
      \seq_put_right:Nn \l__berggeist_titlecap_out_seq { ##1 }
     }
     {
      \seq_put_right:Nf \l__berggeist_titlecap_out_seq { \tl_mixed_case:n { ##1 } }
     }
   }
  \tl_set:Nx \l__berggeist_titlecap_out_tl
   {
    \seq_use:Nn \l__berggeist_titlecap_out_seq { ~ }
   }
  #1 \l__berggeist_titlecap_out_tl
 }
\cs_generate_variant:Nn \seq_put_right:Nn { Nf }

\ExplSyntaxOff

\addtitlecapexceptions{a,an,and,or,to}

\titleformat{\section}[block]
  {\filcenter\bfseries\normalsize}
  {\thesection}
  {1em}
  {\titlecap}

\begin{document}

\section*{Materials and methods}

\titleformat{\subsection}[block]
  {\normalfont\normalsize}
  {\thesubsection}
  {1em}
  {\titlecap}

\subsection*{Sampling about something magic}

\titleformat{\subsection}[block]
  {\normalfont\normalsize}
  {\thesubsection}
  {1em}
  {\ultitlecap}

\subsection*{A very long sectional heading title that
  will probably have to be split over two lines}
Lorem ipsum

\end{document}

But, please, don't inflict underlining to your readers.