query about O.S. for .net....

It should certainly be beneficial to know something about programming for other major operating systems such as iOS, Android and Linux, as well as Windows.

This gives you 'more strings to your bow' when applying for jobs.

The only problem is that it is difficult to remember stuff unless you're using it every day. If you try to spead your knowledge too thin, you might end up as a 'jack of all trades but master of none'.

I'd therefore make sure that you have good specialist knowledge in at least one of these areas, preferably the one that interests you the most or is most likely to land you a decent job.