When writing a user defined package, how can I decide whether the \documentclass using it is book or article?

Question

I want to write my own package as a collection of other packages I often use.

Some declarations in my package only work for book, some for article, and the remaining work for both.

In (La)TeX, how can we make a conditional declaration ?

\ProvidesPackage{MyPackage}

\newcommand\ContentsDir{Contents/}

\newcommand\ChapterDir{\ContentsDir}

%==============================================
%== It will be defined for book document class
%==============================================
\newcommand\IncludeChapter[1]{%
\renewcommand\ChapterDir{\ContentsDir#1/}%
\include{\ContentsDir#1}%
}

\newcommand\IncludeOnlyChapter[1]{%
\includeonly{\ContentsDir#1}%
}

\newcommand\Chapter[1]%
{%
\chapter{#1}%
%\addcontentsline{toc}{chapter}{#1}%
}%
%==============================================



%==============================================
%== Common commands
%==============================================
\newcommand\SectionDir{\ChapterDir}

\newcommand\InputSection[1]{%
\renewcommand\SectionDir{\ChapterDir#1/}
\input{\ChapterDir#1}%
}
%----------------------------------------------
\newcommand\SubSectionDir{\SectionDir}

\newcommand\InputSubSection[1]{%
\renewcommand\SubSectionDir{\SectionDir#1/}
\input{\SectionDir#1}%
}
%----------------------------------------------
\newcommand\InputSubSubSection[1]{%
\input{\SubSectionDir#1}%
}
%----------------------------------------------
\newcommand\Section[1]%
{%
\section{#1}%
%\addcontentsline{toc}{section}{#1}%
}%
%----------------------------------------------
\newcommand\SubSection[1]%
{%
\subsection{#1}%
%\addcontentsline{toc}{chapter}{#1}%
}%
%----------------------------------------------
\newcommand\SubSubSection[1]%
{%
\subsubsection{#1}%
%\addcontentsline{toc}{chapter}{#1}%
}%
%----------------------------------------------

Answer 1

To answer the question in your title,

\@ifclassloaded{article}{%
  % code for article
}{%
  \@ifclassloaded{book}{%
    % code for book
  }{%
    % else clause
  }%
}

For more generic information about conditionals, see this question. For some more information about writing your own package, see the standard documentation file clsguide.

Update: Ulrich makes a good suggestion; for your particular case, it's probably sufficient to write instead

\@ifundefined{chapter}{%
  % code for article
}{%
  % code for book
}