Wrap tikz matrix/tikzcd inside vcenter

Question

How could I create an environment that wraps a tikzcd (basically a tikz matrix) inside \vcenter{...} while keeping the syntax intact?

What I tried: According to How do I wrap a macro definition in an environment? I need to use environ or NewDocumentComment{+b} (I prefer the later since it seems to be "native" now). But then the environment turns into a macro and I have troubles with & (see Problem with defining shortcuts for TikZ matrices).

/!\ Note that I don't want to change & into \&! So I tried to adapt https://tex.stackexchange.com/a/611535/116348 to environments, but not sure how to do.

MWE:

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}
\usepackage{tikz}
\usepackage{tikz-cd}
\usetikzlibrary{shapes,shapes.geometric,shapes.misc,positioning}
\NewDocumentEnvironment{vtikzcd}{O{}+b}{
  \ensuremath{%
    \vcenter{%
      \hbox{
        \begin{tikzcd}[#1]
          #2
        \end{tikzcd}
      }%
    }%
  }%
}
\begin{document}
% Ok
\begin{tikzcd}
  A & B\
  C & D
\end{tikzcd}
% Fails:
\begin{vtikzcd}
  A & B\
  C & D
\end{vtikzcd}
\end{document}

I tried to define:

\newenvironment{vtikzcd}{
  \begingroup% To avoid ampersand issues https://tex.stackexchange.com/a/611535/116348
  \NewDocumentEnvironment{tmpZX}{O{}+b}{%
    \endgroup%
    \ensuremath{%
      \vcenter{%
        \hbox{%
          \begin{tikzcd}[##1]%
            ##2%
          \end{tikzcd}%
        }%
      }%
    }%
  }{}%
  \catcode`&=13
  \begin{tmpZX}%
}{\end{tmpZX}}

but it does not compile.

EDIT Instead of using \vcenter, I managed to center the diagram appropriately (including one line diagrams) by editing instead the style, using something like:

baseline={([yshift=-.27em]current bounding box.center)},1-row diagram/.style={%
      /tikz/baseline={([yshift=-.27em]current bounding box.center)}%
    }]

I'm not sure exactly which value I should pick for yshift tought. That said, I'm still interested by this question in case I want later to wrap my environment with any command.

Answer 1

When & is used in TikZ matrices (which tikzcd environments is a particular case of), its category code is changed to active (13). But this is not possible when the code is passed as the argument to another command.

So your

\NewDocumentEnvironment{vtikzcd}{O{}+b}{% <--- this is necessary
  \ensuremath{% <--- this is not necessary
    \vcenter{% <--- this is not necessary
      \hbox{% <--- this is necessary
        \begin{tikzcd}[#1]
          #2
        \end{tikzcd}% <--- this is necessary
      }% <--- this is not necessary
    }% <--- this is not necessary
  }% <--- this is necessary
}
{}

doesn't work (I added the necessary % and {} that you missed), because the environment's content is grabbed as the argument to a command.

So you should not use the b argument type, if you don't want to add ampersand replacement=\& and type in \& inside vtikzcd.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-cd}
\newsavebox{\vtikzcdbox}
\NewDocumentEnvironment{vtikzcd}{O{}}{%
  \begin{lrbox}{\vtikzcdbox}
    $\vcenter\bgroup\hbox\bgroup
    \begin{tikzcd}[#1]
}{%
  \end{tikzcd}%
  \egroup\egroup$
  \end{lrbox}
  \usebox{\vtikzcdbox}%
}
\begin{document}
% no vcenter
X\begin{tikzcd}
  A & B\
  C & D
\end{tikzcd}X
% vcenter
X\begin{vtikzcd}
  A & B\
  C & D
\end{vtikzcd}X
\end{document}

If you want to pursue the b way, you can replace all & with \&.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-cd}
\ExplSyntaxOn
\NewDocumentEnvironment{vtikzcd}{O{}+b}
 {
  \ensuremath{\vcenter{\hbox{
    \begin{tikzcd}[ampersand~replacement=&,#1]
    \tl_set:Nn \l_tmpa_tl { #2 }
    \tl_replace_all:Nnn \l_tmpa_tl { & } { & }
    \tl_use:N \l_tmpa_tl
    \end{tikzcd}
  }}}
 }
 {}
\ExplSyntaxOff
\begin{document}
% no vcenter
X\begin{tikzcd}
  A & B\
  C & D
\end{tikzcd}X
% vcenter
X\begin{vtikzcd}
  A & B\
  C & D
\end{vtikzcd}X
\end{document}

Don't overuse the b argument type. It's a possibility, not the best one in all cases.