如何扩展论点#1？

Question

#1如果我像这样使用它，这个论点就会得到很好的扩展：

\DeclareFieldFormat{file}
                   {#1}

但是，如果我像这样使用它：

\usepackage{xstring}

\DeclareFieldFormat{file}
                   {\StrBetween[1,2]{#1}{:}{:}}

然后，\StrBetween宏将应用于文字字符串#1，而不是#1“指向”的内容。我想知道：

1. 为什么会这样？
2. 我怎样才能“取消引用” #1？

编辑：

这是我想要实现的一个最小工作示例：

\documentclass{article}
\usepackage{filecontents}
\usepackage{xstring}
\usepackage{biblatex}

\begin{filecontents}{jobname.bib}
@book{author_book,
title = {Book's title},
author = {Author, Some},
location = {The City},
publisher = {Publisher},
date = {2005},
file = {:name between colons:}
}
\end{filecontents}

% This works fine 
%\DeclareFieldFormat{file}{#1}
% This just outputs an empty string
\DeclareFieldFormat{file}{\StrBetween[1,2]{#1}{:}{:}}

\AtEveryBibitem{%
    \printfield{file}
}

\bibliography{jobname}

\begin{document}
\nocite{*}
\printbibliography
\end{document}

Answer 1

事实证明#1 做工作（感谢@egreg 对此的跟进），你只是不能在声明中换行

\DeclareFieldFormat*{file}{\StrBetween[1,2]{\thefield{file}}{:}{:}}

\AtEveryBibitem{%
    \printfield{file}%
}

Answer 2

应该有不参数之间的空格\DeclareFieldFormat。事实上

\DeclareFieldFormat{file}
                   {#1}

似乎只是偶然的。如果你尝试

\DeclareFieldFormat{file}
                   {XX#1XX}

你得到

Runaway argument?
{:name between colons:}\blx@endunit \addspace \blx@execute \blx@initunit \ETC.
! File ended while scanning use of \blx@theformat.

只要输入正确，我看不出使用#1和之间有什么区别\thefield{file}。

\begin{filecontents}{jobname.bib}
@book{author_book,
title = {Book's title},
author = {Author, Some},
location = {The City},
publisher = {Publisher},
date = {2005},
file = {:name between colons:}
}
\end{filecontents}

\documentclass{article}
\usepackage{xstring}
\usepackage{biblatex}

\AtEveryBibitem{%
    \printfield{file}\addspace
}

\addbibresource{\jobname.bib}

\begin{document}

\nocite{*}

\DeclareFieldFormat{file}{%
  (\StrBetween[1,2]{\thefield{file}}{:}{:})%
}

\printbibliography

\DeclareFieldFormat{file}{%
  [\StrBetween[1,2]{#1}{:}{:}]%
}

\printbibliography

\end{document}

我在第一种情况下添加了括号，在第二种情况下添加了方括号，以显示字段格式已更改。