Why does hiding a table row using xithen cause the alignment of multirow to break?

Question

TLDR: I try to write a class that shows/hides table rows conditionally. When a row is hidden just before \multicolumn is used, an error is triggered.

Let me first give you an M(N)WE for simpler dicussion:

\documentclass{article}
\RequirePackage{xifthen}
% Put some data in macros. This wil be imported later using \input{...}
\newcommand{\TaskA}{Some description comes here}
\newcommand{\TaskB}{Some other description}
\newcommand{\TaskC}{}
\newcommand{\HoursA}{1}
\newcommand{\HoursB}{3}
\newcommand{\HoursC}{0}
\newcommand{\printline}[1]{\ifthenelse{ \equal{\csname Task#1\endcsname}{} }{}{%
    #1 & \csname Task#1\endcsname & \csname Hours#1\endcsname & 42 \%
}}
\begin{document}
\section{Correct behavior}
\begin{tabular}{|l l | c c }
A & Some description comes here & 1 & 42 \
B & Some other description & 3 & 42 \
\multicolumn{2}{|r|}{Sum:} & 4 & 82 \
\end{tabular}
\section{Error is created}
\begin{tabular}{| l l | c c }
\printline{A}
\printline{B}
\printline{C}
\multicolumn{2}{|r|}{Sum:} & 4 & 82 \
\end{tabular}
\end{document}

I wanted to write a class that outputs a table depending on the values of a set of macros. I dropped here the iteration thing and wrote it explicitly for sake of simplicity. If a condition is met (in the MWE only an empty task name is checked but the check is more complex in the final class) the complete row should be hidden.

I added one table that is rendered completely manually and that is working perfectly fine. However once I use macro to typeset a single row, that does no longer work as expected. First, I get a LaTeX error

Misplaced \omit.
\multispan ->\omit 
                   \@multispan 
l.28 \multicolumn{2}{|r|}{Sum:}
                                & 4 & 82 \\

Additionally, the output has no real cell spanning over two columns but the first column is somehow added as well.

Fun fact if one disables the row B and enables C (just switch the macro names), the effect seems not to happen. So I suspect that I am triggering an issue with the \multicolumn macro.

After reading this post and trying out the approach with DeclareExpandableDocumentCommand, I suspect the problem is the \ifthenelse structure. Am I required to go with basic (plain) TeX here?

---

Edit 1:
I got some anseers/comments (thank you!) but I have the problem that I cannot simply expand these to my needs (with my state of knowledge). So I might need to specify what tests I need more explictly: First, you must knw that there are not one but 2 time entries (call them HoursX and HoursExtX). The row should be dropped if any of these are true:

• The task name (TaskX) is empty
• The task name is undefined (optional)
• The HoursX and the HoursExtX are zero
• The HoursX and the HoursExtX are empty
• Optionally if HoursX and HoursExtX are both undefined

So I need the option to generate logical statements and check for empty string and numerical zero.

Answer 1

You need an expandable test and \ifthenelse isn't one of them.

\documentclass{article}
\usepackage{etoolbox}
% Put some data in macros. This wil be imported later using \input{...}
\newcommand{\TaskA}{Some description comes here}
\newcommand{\TaskB}{Some other description}
\newcommand{\TaskC}{}
\newcommand{\HoursA}{1}
\newcommand{\HoursB}{3}
\newcommand{\HoursC}{0}
\newcommand{\printline}[1]{%
  \ifcsempty{Task#1}{}{#1 & \csname Task#1\endcsname & \csname Hours#1\endcsname & 42 \}%
}
\begin{document}
\section{Correct behavior}
\begin{tabular}{|l l | c c }
A & Some description comes here & 1 & 42 \
B & Some other description & 3 & 42 \
\multicolumn{2}{|r|}{Sum:} & 4 & 82 \
\end{tabular}
\section{No error}
\begin{tabular}{| l l | c c }
\printline{A}
\printline{B}
\printline{C}
\multicolumn{2}{|r|}{Sum:} & 4 & 82 \
\end{tabular}
\end{document}

A higher level implementation where we also check whether \Task#1 is undefined, so you don't need to, say, define \TaskC to empty.

\documentclass{article}
% Put some data in macros. This wil be imported later using \input{...}
\newcommand{\TaskA}{Some description comes here}
\newcommand{\TaskB}{Some other description}
\newcommand{\HoursA}{1}
\newcommand{\HoursB}{3}
\newcommand{\HoursC}{0}
\ExplSyntaxOn
\prg_generate_conditional_variant:Nnn \tl_if_blank:n { v } { p,T,F,TF }
\NewExpandableDocumentCommand{\printline}{m}
 {
  \bool_lazy_and:nnT
   { \tl_if_exist_p:c { Task#1 } } % \Task#1 exists
   { ! \tl_if_blank_p:v { Task#1 } } % \Task#1 is not empty
   { #1 & \csname Task#1\endcsname & \csname Hours#1\endcsname & 42 \}
 }
\ExplSyntaxOff
\begin{document}
\section{Correct behavior}
\begin{tabular}{|l l | c c }
A & Some description comes here & 1 & 42 \
B & Some other description & 3 & 42 \
\multicolumn{2}{|r|}{Sum:} & 4 & 82 \
\end{tabular}
\section{No error}
\begin{tabular}{| l l | c c }
\printline{A}
\printline{B}
\printline{C}
\multicolumn{2}{|r|}{Sum:} & 4 & 82 \
\end{tabular}
\end{document}