Where to Save License File

To hide license file should not be the only protection you have but it may discourage very casual crackers to play with your application (finding, probably by case, a vulnerability).

You may move this file to application data folder (for example Environment.SpecialFolder.CommonApplicationData). This is just little bit more safe (because application data won't be deleted when uninstall) but it's still possible for them to manually find and delete it. If application will be installed by low privileges users there isn't much you can do (you can't try to hide license somewhere in Registry.)

Now it's a game between you and crackers. They'll win, always. You'll only make life of legitimate users more hard so read cum grano salis. Where you may store license data:

Registry. Pro: easy to do. Cons: easy to crack and for low privileges user it's valid only for one user per time. A registry key (in a per-user base) can be somehow hidden if it has \0 in its name. Take a look to this nice post. File. Pro: easy to do and IMO little bit more safe than Registry. Cons: easy to crack (but you can hide it more, see later).

Application itself (appending data to your executable, few words about that on this post). Pro: harder to detect. Cons: an antivirus may see this as...a virus and an application update may delete license too (of course if you don't handle this situation properly) so it'll make your code and deployment more complicated.

File. If you're going with a file (it doesn't matter where it's located) you may consider to make crackers life (little bit) harder. Two solutions come to my mind now:

Alternate Data Streams. File is attached to another file and they won't see it with just a search in Windows Explorer. Of course there are utilities to manage them but at least they have to explicitly search for it. Hide it inside application data (a bitmap, for example, using steganography). They just don't know it's license data, what's more safe? Problem is they can easy decompile your C# program to see what you do (read about Code Obfuscation).

Probably many others (fantasy here is our master) but don't forget...crackers will find it (if they really want) so you have to balance your effort.