Definining a new enviroment depending on a parameter

Question

I want to create a new environment which depend if a parameter exists or not. The basic definition, which works fine, is:

\newrgbcolor{bcolor}{1. 1. 1.}%
\newenvironment{pbox}
{\begin{lrbox}{\dummybox}%
\begin{minipage}{0.95\linewidth}
}
{\end{minipage}\end{lrbox}%
{\setlength{\fboxsep}{0.02\linewidth}%
\colorbox{bcolor}{\usebox{\dummybox}}}\vskip1.2em}

In some cases I need to specify the height of the minipage, there for I tried

\usepackage{ifthen}
\newrgbcolor{bcolor}{1. 1. 1.}%
\newenvironment{pbox}[1][0]
{\begin{lrbox}{\dummybox}%
\ifthenelse{\value{#1}=0}
 {\begin{minipage}{0.95\linewidth}}
 {\begin{minipage}[c][#1][c]{0.95\linewidth}}
}
{\end{minipage}\end{lrbox}%
{\setlength{\fboxsep}{0.02\linewidth}%
\colorbox{bcolor}{\usebox{\dummybox}}}\vskip1.2em}

which does not.

In most cases I want to use the pbox without the extra argument to detect the right size automatilly. Only when I specifiy the additional parameter the box should be fixed to this size.

Example:

\begin{pbox}
 This is a normal pbox which height is depending on the content. 
\end{pbox}
\begin{pbox}[10cm]
 This is a pbox which is at least 10cm height
\end{pbox}

How can this be done in a correct way?

Answer 1

This works (etoolbox can do much more than ifthen)

\documentclass[a4paper]{memoir}
\usepackage{xcolor}
\usepackage{etoolbox}
\newsavebox\dummybox
\newenvironment{pbox}[1][]{
  \begin{lrbox}{\dummybox}%
    \ifblank{#1}{%
      \begin{minipage}{0.95\linewidth}%
    }{%
      \begin{minipage}[c][#1][c]{0.95\linewidth}%
    }%
  }{
    \end{minipage}%
  \end{lrbox}%
  \setlength{\fboxsep}{0.02\linewidth}%
  \colorbox{red}{\usebox{\dummybox}}%
  \vskip1.2em
}
\begin{document}
\begin{pbox}
   sdfsdfsd asdfasdf
as

 asdfasdf
\end{pbox}

\begin{pbox}[3cm]
  sdfsdfsd
 asdfasdf
 as

 asdfasdf
\end{pbox}
\end{document}