how to convert IEnumerable<string> to byte[] in c#?

IEnumerable<string> myStrings = new[] { "hello", "world"};  
 var myBytes = myStrings.SelectMany(s => Encoding.UTF8.GetBytes(s)).ToArray();  

This will use UTF8 as encoding for the bytes. You might need to use a different encoding depending on what you want to do with the bytes afterwards.

Note that - as written above - this is not directly reversible because you lose the information where one string ends and where the next one starts.

If you want to, you could a NewLine (or any other identifier that you could use afterwards to split on) after each string to keep them separate:

var myBytes = myStrings.Select(s => s + Environment.NewLine) .  
         SelectMany(s => Encoding.UTF8.GetBytes(s)).ToArray();