Expanding an argument of code added by \g@addto@macro

Question

I would like to have the following (it's a part of a large code block):

\def\C{foobar}
\def\A{}
\g@addto@macro\A{\B{\C}}

Now \show\A outputs \B{\C}

I would like to modify the 3rd line so that \show\A outputs \B{foobar}

I believe that some trick with \expandafter should help, but I'm not familiar with it. (Note: expl3 would surely help here, but I'd like to avoid it.)

Full MWE:

\documentclass{article}

\begin{document}
\makeatletter

\def\C{foobar}
\def\A{}
\g@addto@macro\A{\B{\C}}

\show\A

\makeatother
\end{document}

Answer 1

\documentclass{article}

\begin{document}
\makeatletter

\def\C{\D}
\def\D{foobar}
\def\A{}
\edef\next{\noexpand\g@addto@macro\noexpand\A{\noexpand\B{\C}}}
\next
\show\A

\expandafter\g@addto@macro\expandafter\A\expandafter{\expandafter\B\expandafter{\C}}
\show\A
\makeatother
\end{document}

The variants are different: the \edef will expand fully, the \expandafter only one level.

Answer 2

For reference, a LaTeX3 solution would look something like

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\tl_new:N \C
\tl_set:Nn \C { foobar }
\tl_new:N \A
\tl_gput_right:Nx \A { \exp_not:N \B { \exp_not:V \C } }
\ExplSyntaxOff

\show\A

\end{document}

(We are not told anything about \B, so I've assuming it might be expandable.) I have assumed \C is a variable, and have deliberately only expanded it to it's value: you could miss out the \exp_not:V and have x-type expansion.

Answer 3

\def\C{foobar}

\def\A{}
    
\expandafter\g@addto@macro\expandafter\A\expandafter{%
 \expandafter\noexpand\expandafter\B\expandafter{\C}}
    

or define

\def\C{foobar}
    
\def\A{}
    
\edef\X{\noexpand\B{\C}}

    \expandafter\g@addto@macro\expandafter\A\expandafter{\X}