Highlighting line ranges in a lstlisting: partial solution, but expansion issues

Question

In my quest for a better solution for How to change appearance of a particular line when using \lstincludelisting, I came across the linebackground= option Martin Scharrer added to the listings package in his answer to Creating a zebra effect using listings.

The basic idea is to have a macro

% \btLstHL<overlay spec>{range list}
\newcommand<>{\btLstHL}[1]{%
  \only#2{\btIfInRange{\value{lstnumber}}{#1}{\color{blue!30}}{}}%
}%

that expands to a \color{blue!30} if the current line fits into the range or to \empty (?) otherwise. This is then used together with linebackground

\begin{lstlisting}[language=C, gobble=6,linebackgroundcolor={%
    \btLstHL{4}%
    \btLstHL<1>{1-2,5-6}%
    \btLstHL<2>{7}%
  }]
    /**
    * Prints Hello World.
    **/
    #include <stdio.h>

    int main(void) {
       printf("Hello World!");  
       return 0;
    }
\end{lstlisting}

This, however, does not work as intended, as all nonselected lines now get a white background:

The culprit, I guess, is in the implementation of lstlinebackground and how btLstHL gets expanded in its processing.:

    \lst@Key{linebackgroundcolor}{}{%
        \def\lst@linebgrdcolor{#1}%
    }
    \newcommand{\lst@linebgrd}{%
        \ifx\lst@linebgrdcolor\empty\else  % PROBLEM: for \btLstHL does not expand to \empty ???
        \rlap{%
            \lst@basicstyle
            \color{-.}% By default use the opposite (`-`) of the current color (`.`) as background
            \lst@linebgrdcolor{%
            \kern-\dimexpr\lst@linebgrdsep\relax%
            \lst@linebgrdcmd{\lst@linebgrdwidth}{\lst@linebgrdheight}{\lst@linebgrddepth}%
            }%
        }%
        \fi
    }

Apparently, \btLstHL expands to "nothing", but not \empty for nonselected lines, which causes \lst@linebgrd to highlight the background, but with its default color, which is the opposite of the current text color (black).

And at this point I am lost :-(

So how to define \btLstHL that it expands to \empty if the line is not within the given range?

Complete MWE:

\documentclass{beamer}

% beamer class setup
\usecolortheme{rose}
\setbeamertemplate{navigation symbols}{}

% we use UTF8
\usepackage[utf8]{inputenc}
% font setup
\usepackage[T1]{fontenc}
\usepackage[scaled=0.85]{beramono}
\usepackage{xcolor,listings, pgffor}

\makeatletter

  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  %
  % \btIfInRange{number}{range list}{TRUE}{FALSE}
  %
  % Test if int number <number> is element of a (comma separated) list of ranges
  % (such as: {1,3-5,7,10-12,14}) and processes <TRUE> or <FALSE> respectively

  \newcount\bt@rangea
  \newcount\bt@rangeb

  \newcommand\btIfInRange[2]{%
      \global\let\bt@inrange\@secondoftwo%
      \edef\bt@rangelist{#2}%
      \foreach \range in \bt@rangelist {%
          \afterassignment\bt@getrangeb%
          \bt@rangea=0\range\relax%
          \pgfmathtruncatemacro\result{ ( #1 >= \bt@rangea) && (#1 <= \bt@rangeb) }%
          \ifnum\result=1\relax%
              \breakforeach%
              \global\let\bt@inrange\@firstoftwo%
          \fi%
      }%
      \bt@inrange%
  }
  \newcommand\bt@getrangeb{%
      \@ifnextchar\relax%
          {\bt@rangeb=\bt@rangea}%
          {\@getrangeb}%
  }
  \def\@getrangeb-#1\relax{%
      \ifx\relax#1\relax%
          \bt@rangeb=100000%   \maxdimen is too large for pgfmath
      \else%
          \bt@rangeb=#1\relax%
      \fi%
  }

  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  %
  % \btLstHL<overlay spec>{range list}
  %
  \newcommand<>{\btLstHL}[1]{%
    \only#2{\btIfInRange{\value{lstnumber}}{#1}{\color{blue!30}}{}}%
  }%


  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  %
  % \btInputEmph<overlay spec>[listing options]{range list}{file name}
  %
  \newcommand<>{\btLstInputEmph}[3][\empty]{%                                                    
    \only#4{%
      \lstset{linebackgroundcolor=\btLstHL{#2}}%
      \lstinputlisting{#3}%
   }% \only
  }


  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  %% lstlinebgrd.sty
  %% see: https://tex.stackexchange.com/questions/18969/creating-a-zebra-effect-using-listings/18989#18989
  %%
  %% This small package is not yet published/not commonly available.
  %%
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    % Patch line number key to call line background macro
    \lst@Key{numbers}{none}{%
        \def\lst@PlaceNumber{\lst@linebgrd}%
        \lstKV@SwitchCases{#1}%
        {none&\\%
         left&\def\lst@PlaceNumber{\llap{\normalfont
                    \lst@numberstyle{\thelstnumber}\kern\lst@numbersep}\lst@linebgrd}\\%
         right&\def\lst@PlaceNumber{\rlap{\normalfont
                    \kern\linewidth \kern\lst@numbersep
                    \lst@numberstyle{\thelstnumber}}\lst@linebgrd}%
        }{\PackageError{Listings}{Numbers #1 unknown}\@ehc}}

    % New keys
    \lst@Key{linebackgroundcolor}{}{%
        \def\lst@linebgrdcolor{#1}%
    }
    \lst@Key{linebackgroundsep}{0pt}{%
        \def\lst@linebgrdsep{#1}%
    }
    \lst@Key{linebackgroundwidth}{\linewidth}{%
        \def\lst@linebgrdwidth{#1}%
    }
    \lst@Key{linebackgroundheight}{\ht\strutbox}{%
        \def\lst@linebgrdheight{#1}%
    }
    \lst@Key{linebackgrounddepth}{\dp\strutbox}{%
        \def\lst@linebgrddepth{#1}%
    }
    \lst@Key{linebackgroundcmd}{\color@block}{%
        \def\lst@linebgrdcmd{#1}%
    }

    % Line Background macro
    \newcommand{\lst@linebgrd}{%
        \ifx\lst@linebgrdcolor\empty\else
        \rlap{%
            \lst@basicstyle
            \color{-.}% By default use the opposite (`-`) of the current color (`.`) as background
            \lst@linebgrdcolor{%
            \kern-\dimexpr\lst@linebgrdsep\relax%
            \lst@linebgrdcmd{\lst@linebgrdwidth}{\lst@linebgrdheight}{\lst@linebgrddepth}%
            }%
        }%
        \fi
    }

\makeatother

\begin{document}

\begin{frame}[fragile]{Problem}
  \begin{alertblock}{A cool app; unfortunately it has a white background}
    \begin{lstlisting}[language=C, gobble=6,linebackgroundcolor={%
        \btLstHL{4}%
        \btLstHL<1>{1-2,5-6}%
        \btLstHL<2>{7}%
      }]
        /**
        * Prints Hello World.
        **/
        #include <stdio.h>

        int main(void) {
           printf("Hello World!");  
           return 0;
        }
    \end{lstlisting}
  \end{alertblock}
\end{frame}

\end{document}

Answer 1

After understanding better what \ifx actually does (thanks to whlt3!) I came to the conclusion that I cannot define \btLstHL in a way that it does first-level expand to \empty.

My current workaround solution is to go another route and redefine \lst@linebgrdcmd in \btLstHL so it becomes a no-op for nonselected lines:

  % \btLstHL<overlay spec>{range list}
  \newcommand<>{\btLstHL}[1]{%
    \only#2{\btIfInRange{\value{lstnumber}}{#1}%
      {\color{blue!30}}%
      {\def\lst@linebgrdcmd####1####2####3{}}% define as no-op
    }%
  }%

Note: To use this without beamer, just remove the <> from the \newcommand and the \only#2 command including its curly braces.

This way I do get the intended result. However, the solution is somewhat "hacky". I am still open for better approaches.