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++”? – Leo Liu Feb 14 '12 at 13:03\newcommand{\textsharp}{$\sharp$}– egreg Feb 14 '12 at 13:07\texttt{C\#}. 'C' must be set in the normal font. – Andrey Vihrov Feb 14 '12 at 13:24\newcommand{\csharp}{C\raisebox{.8ex}{\scriptsize\#}}?:)– Paulo Cereda Feb 14 '12 at 13:29#needs to be aligned with the C instead. – Filip Ekberg Feb 14 '12 at 13:48.8exin Paulo's comment to adjust the height. 0.5 gets it pretty close to level with the top of the C. – qubyte Feb 14 '12 at 14:06