Why can't I insert a blank line in this code?

Question

In the code below, everything is OK until I insert a blank line instead of one of the \\'s. Upon inserting a blank line, I'll get some errors. How can I get rid of these errors?

\documentclass{article}
\newsavebox\mybox
\newcommand{\Class}[1]
{ 
\sbox{\mybox}{\begin{tabular}{l}#1\end{tabular}}
\fbox{\parbox{\wd\mybox}{\texttt{#1}}}
}
\begin{document}
\Class
{double Width\\
 double Height\\
 double Perimeter\\ 
 double Area\\
}

\usebox\mybox
\end{document}

Edit:
My indented output is:

Answer 1

The command \texttt does not allow paragraph breaks in its argument. You could use \ttfamily instead but a blank line inside a tabular doesn't do anything useful, so it is probably better to have the error message to warn of the incorrect input.

Perhaps this which makes a blank line do \\ ?

\documentclass{article}
\newsavebox\mybox
\usepackage{array}
\newcommand{\Class}[1]
{%
\sbox{\mybox}{\begin{tabular}{>{\expandafter\def\csname par\endcsname{\\}}l}#1\end{tabular}}%
\fbox{\parbox{\wd\mybox}{\par{\def\par{\\}\ttfamily{#1}}}}%
}
\begin{document}
\Class
{double Width\\
 double Height

 double Perimeter\\ 
 double Area\\
}

\usebox\mybox
\end{document}

Answer 2

You can simply \fbox the tabular and use \\ for blank lines; to get typewriter type use \ttfamily; the \fbox keeps the font change local.

\documentclass{article}

\newcommand{\Class}[1]{%
  \fbox{\ttfamily\begin{tabular}{l}#1\end{tabular}}%
}

\begin{document}
\Class
{double Width\\
 double Height\\
 \\
 double Perimeter\\
 double Area\\
}

\end{document}

Beware of spurious spaces and comment end of lines in definitions.