1) Close current project (if any is open), just to make things clean
2) From the menu, click on File/New...
3) In the "File" tab, Document mode: does not have "C#"!
a) If you select C/C++ and the filename ends in .cs, you'll get a popup window with title "Mismatched Extensions" and contents "Filename extension, cs, does not match selected Document Mode, C/C++. Continue?"
b) If you select Automatic, no error message is displayed (good, isn't it?), but if you later click on the menu entry "Document", you'll see that the mode selected is "pp" (whatever that is). Click on Select Mode (under Document) and you'll not find C# there, either.