How to convert a sound signal function to wave file?
i don't consider this a "bad" question.  But there is a lot that nano needs to deal with.

first, you must be able to think about conceptually and mathematically converting your continuous-time signal

$$ x(t) = \sin(\Omega t) $$

into a discrete-time signal

$$ x[n] = \sin(\omega n) $$

how $n$ is related to $t$ and how $\omega$ is related to $\Omega$.  this requires knowledge of the sample rate.

finally, after creating an array of samples, you can write that array to a .wav file using a library such as this one from Erik de Castro Lopo.  you might have to massage the code to make it work for C#.

or you can learn about the format of the particular .wav file (nowadays they can be float or int format) and write that file using the C# counterparts to fopen(), fwrite(), and fclose().  but i dunno shit about C#.