Since Computer Modern Roman does not have a fitting hash symbol, we need to look somewhere else. I experimented with several popular fonts and found that the hash symbol from Liberation Serif does not stand out, has approximately the same brush width and isn't too wide.

Unfortunately, as Liberation Serif is a TrueType font, it can't be readily used with pdfTeX. Below is the code to use it with XeTeX/LuaTeX.
\documentclass{article}
\usepackage{fontspec}
\newfontface\lserif{Liberation Serif}
\newcommand{\Csh}{C{\lserif\#}}
\begin{document}
Some text \Csh{} some text.
\end{document}
Workaround for pdfTeX
Process this file with XeTeX/LuaTeX and save the result as hash-symbol.pdf
:
\documentclass[border=0pt]{standalone}
\usepackage{fontspec}
\begin{document}% I get extra space without this comment
\fontspec{Liberation Serif}\#
\end{document}
Then use this code to include the symbol with pdfTeX:
\documentclass{article}
\usepackage{graphicx}
\newcommand{\Csh}{C\includegraphics{hash-symbol}}
\begin{document}
Some text \Csh{} some text.
\end{document}

The positioning of the symbol might be slightly off, use \kern
and \raisebox
to fine-tune. Also note that this will only work for one font size; to remedy this, \scalebox
might be useful (manual).
\texttt{C\#}
for your second picture. And you'll find this question useful: Prettiest way to typeset “C++”?\newcommand{\textsharp}{$\sharp$}
\texttt{C\#}
. 'C' must be set in the normal font.