Why my macro is fragile in a title? Can I make it more solid?

Question

The follwoing code stops the compilation even if the output is the good one if the message are ignored.

Why my macro is fragile? Can I make here more solid ?

\documentclass[12pt,a4paper]{article}
\makeatletter
    \def\txtIf{@ifstar\tnsalgo@txtIf@star\tnsalgo@txtIf@no@star}
    \def\tnsalgo@txtIf@no@star{\textbf{Si}}
    \def\tnsalgo@txtIf@star{\texttt{Si}}
\makeatother
\begin{document}
\subsubsection{Conditional: \protect\txtIf{}} % <-- OK
\subsubsection{Conditional: \txtIf{}}         % <-- KO
\end{document}

Log :

This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex 2020.9.12)  12 SEP 2020 13:22
entering extended mode
 \write18 enabled.
 file:line:error style messages enabled.
 %&-line parsing enabled.
**pdftex-titel-pb.tex
(./pdftex-titel-pb.tex
LaTeX2e <2020-02-02> patch level 5
L3 programming layer <2020-09-06>
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/article.cls
Document Class: article 2019/12/20 v1.4l Standard LaTeX document class
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/size12.clo
File: size12.clo 2019/12/20 v1.4l Standard LaTeX file (size option)
)
\c@part=\count168
\c@section=\count169
\c@subsection=\count170
\c@subsubsection=\count171
\c@paragraph=\count172
\c@subparagraph=\count173
\c@figure=\count174
\c@table=\count175
\abovecaptionskip=\skip47
\belowcaptionskip=\skip48
\bibindent=\dimen134
)
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def
File: l3backend-pdftex.def 2020-09-11 L3 backend support: PDF output (pdfTeX)
\l__kernel_color_stack_int=\count176
\l__pdf_internal_box=\box45
)
(./pdftex-titel-pb.aux)
\openout1 = `pdftex-titel-pb.aux'.
LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 11.
LaTeX Font Info:    ... okay on input line 11.
LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 11.
LaTeX Font Info:    ... okay on input line 11.
LaTeX Font Info:    Checking defaults for OT1/cmr/m/n on input line 11.
LaTeX Font Info:    ... okay on input line 11.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 11.
LaTeX Font Info:    ... okay on input line 11.
LaTeX Font Info:    Checking defaults for TS1/cmr/m/n on input line 11.
LaTeX Font Info:    ... okay on input line 11.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 11.
LaTeX Font Info:    ... okay on input line 11.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 11.
LaTeX Font Info:    ... okay on input line 11.
./pdftex-titel-pb.tex:13: Argument of @sect has an extra }.
<inserted text> 
                \par 
l.13 \subsubsection{Conditional: \txtIf{}}
? 
Runaway argument?
{\normalfont \normalsize \bfseries }{@firstoftwo {\tnsalgo@txtIf@star \ETC.
./pdftex-titel-pb.tex:13: Paragraph ended before @sect was complete.
<to be read again> 
                   \par 
l.13 \subsubsection{Conditional: \txtIf{}}
? 
[1
{/usr/local/texlive/2020/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
(./pdftex-titel-pb.aux) ) 
Here is how much of TeX's memory you used:
 378 strings out of 480876
 6653 string characters out of 5907597
 243537 words of memory out of 5000000
 16086 multiletter control sequences out of 15000+600000
 535693 words of font info for 31 fonts, out of 8000000 for 9000
 1141 hyphenation exceptions out of 8191
 24i,4n,25p,181b,114s stack positions out of 5000i,500n,10000p,200000b,80000s
</usr/local/texlive/2020/texmf-dist/fonts/type1/public
/amsfonts/cm/cmbx12.pfb></usr/local/texlive/2020/texmf-dist/fonts/type1/public/
amsfonts/cm/cmr12.pfb>
Output written on pdftex-titel-pb.pdf (1 page, 17436 bytes).
PDF statistics:
 16 PDF objects out of 1000 (max. 8388607)
 10 compressed objects within 1 object stream
 0 named destinations out of 1000 (max. 500000)
 1 words of extra memory for PDF output out of 10000 (max. 10000000)

Answer 1

For an in-depth discussion of what makes a command "robust" or "fragile", see What is the difference between Fragile and Robust commands?

In addition to \protecting a fragile command, as you do in \subsubsection{Conditional: \protect\txtIf{}}, there are (at least) two ways you can succeed in placing the command in question in the argument of a "moving" command:

• Make the command robust from the outset, by defining with \DeclareRobustCommand instead of with either \def or \newcommand.

• Make the command robust after it's been created, by loading the etoolbox package and executing

  \robustify{\fragilecommand}
  

  E.g., \robustify{\txtIf}.