Changing fontsize within soul underline

Question

This question is related to http://tex.stackexchange.com/questions/2218/how-to-create-a-newcommand-for-fontsize. I would like to change the fontsize within \ul of the soul package. For example:

\newcommand{\mycfs}[1]{\fontsize{#1pt}{1.2#1pt}\selectfont} 
\soulregister{\mycfs}{1}

\ul{{\mycfs{6} Size 6 font} Normal size font}

This error message occurs: ! Package soul Error: Reconstruction failed.

How can this be fixed?

Answer 1

I would change the definition of \mycfs so that it no longer has an argument:

\documentclass{book}
\usepackage{soul}

\newcommand\mycfs{\fontsize{\mysize}{\the\dimexpr \mysize*1.2\relax}\selectfont}
\newcommand\mysetsize[1]{\edef\mysize{#1pt}}
\soulregister{\mycfs}{0}

\begin{document}
\mysetsize{6}

\ul{\mycfs Size 6 font} Normal size font
\end{document}

Answer 2

\mbox could already fix it for you:

\documentclass{book}
\usepackage{soul}
\newlength{\mysize}
\newcommand{\mycfs}[1]{\setlength{\mysize}{#1pt}%
  \fontsize{\mysize}{1.2\mysize}\selectfont}
\begin{document}
text

\ul{\mbox{\mycfs{6}Size 6 font} Normal size font}

text
\end{document}

Output: