You don't write binary code in C# - you can write IL code and embed that in you C#:
https://visualstudiogallery.msdn.microsoft.com/44034a7b-143d-4b51-b7bc-99aa656ba137[
^], but that's about as far as you can go, as the final "binary" version isn't produced until the code is run and the JIT compiler gets it's hands on it.
And why, oh why would you want to "write in binary" anyway? I've been there, don't that, got the T shirt. And one of the major advantages of modern languages is that you don't have to do it again! It's slow, difficult and a maintenance headache.