Why does this compile normally with \def but hangs with \edef?

Question

Why does the MWE

\documentclass{article}
\begin{document}
\makeatletter
\def\CharBx{\textbf{\f@series}} 
\makeatother
\CharBx
\end{document}

compile normally (it returns 'bx'), while the MWE

\documentclass{article}
\begin{document}
\makeatletter
\edef\CharBx{\textbf{\f@series}} 
\makeatother
\CharBx
\end{document}

doesn't compile at all, but just hangs?

Answer 1

\textbf (as many other commands in LaTeX is made robust using LaTeX's \protect system which you circumvent by manually using plain TeX's \edef instead of LaTeX's \protected@edef. With that your code works fine:

\documentclass{article}
\begin{document}
\makeatletter
\protected@edef\CharBx{\textbf{\f@series}} 
\makeatother
\CharBx
\end{document}

Of course this will show a bold m and not bx since \f@series gets expanded at the point of definition, while \textbf as a non-expandable command does not have any effect during the \edef.