Custom environment with minipages causes "Illegal parameter number in definition of \end" [duplicate]

Question

Using MiKTeX with PDFLaTeX on Windows

I want to create an environment that contains two minipages; the first one has a description item, and the second has a small text (or picture). This piece of code works as I want:

{
\noindent
\begin{minipage}{0.5\textwidth}
\begin{description}
    \item[Label] Description
\end{description}
\end{minipage}\hfill
\begin{minipage}{0.3\textwidth}
    Second minipage
\end{minipage}
}

I wanted to create a macro or environment to make it easy for me to introduce this kind of content. So I create the following environment with two arguments:

\newenvironment{myenvironment}[2]% 1:label, 2:second minipage text
{%
    \noindent
    \begin{minipage}{0.5\textwidth}
    \begin{description}
        \item[#1] % Description
}%
{%
    \end{description}
    \end{minipage}\hfill
    \begin{minipage}{0.3\textwidth}
        #2
    \end{minipage}
}

But when I try to use it with

\begin{myenvironment}{Label}{Second minipage text}
Description text
\end{myenvironment}

I get Illegal parameter number in definition of \endmyenvironment. pointing to the line that finishes the newenvironment declaration. After some hours struggling with this I tried using double hashes (##) and got two new different errors: You can't use `macro parameter character #' in restricted horizontal mode and You can't use `macro parameter character #' in internal vertical mode. This time the errors were pointing to the lines when I wanted to use the environment, but had no complaints on the definition.

What am I doing wrong?

Answer 1

The ## had nothing to do with my problem. It seems that you cannot use arguments inside of the closing or ending code of a custom environment. There are workarounds for this, but in my opinion this should be a must have feature. Using a possible workaround ---an auxiliary variable---, I arrived to this working environment:

\newenvironment{myenvironment}[2]%
{%
    \def\myenvargumentII{#2}    
    \noindent
    \begin{minipage}{0.5\textwidth}
    \begin{description}
        \item[#1] % Description
}%
{%
    \end{description}
    \end{minipage}\hfill
    \begin{minipage}{0.3\textwidth}
        \myenvargumentII
    \end{minipage}
}

This solution arrived late. In the meantime I had managed to solve my problem with a \newcommand^a with 3 arguments:

\newcommand{\myminipages}[3]% 1:label, 2:secondminipage, 3:Description
{%
    \noindent
    \begin{minipage}{0.5\textwidth}
        \begin{description}
            \item[#1] #3
        \end{description}
    \end{minipage}\hfill
    \begin{minipage}{0.3\textwidth}
        #2
    \end{minipage}
    \medskip % vertical space
}

Which can be used with:

\myminipages{Label}{Second minipage text}{Description text}

Hope it helps.

---

[a] It seems that there is no particular advantage of the newcommand approach over the newenvironment approach, at least for this use. More here and here.