Are their any conventions (either written or just generally understood) for when to use a forward slash (/) or a hyphen (-) when reading arguments/flags from a command line?
C:\> myprogram.exe -a
C:\> myprogram.exe /a
The two seem to be interchangeable in my experience, but I haven't used enough command line tools to say I've spotted any rules or patterns.
Is there a good reason that either of them are used at all? Could I theoretically use an asterisk (*) if I wanted to?
You can (theoretically) use whatever you want, as the parameters are just strings passed to your command-line program.
Windows convention seems to prefer the use of the forward slash ipconfig /all, though there are programs that take a hyphen gacutil -i or even a sort-of environment variable syntax setup SKUUPGRADE=1.
*Nix convention seems to prefer the hyphen -v for single-letter parameters, and double hyphen --verbose for multi-letter parameters.
I tend to prefer hyphens, as they are more OS-agnostic (forward slashes are path delimiters in some OSes) and used in more modern Windows apps (nuget, for example).
Edit:
This would be a good place to recommend a library that does .NET command-line argument parsing: http://commandline.codeplex.com/