He gave 4 examples of how it's done incorrectly, but zero actual examples of doing it correctly.
> If you need to perform a case mapping on a string, you can use LCMapStringEx with LCMAP_LOWERCASE or LCMAP_UPPERCASE, possibly with other flags like LCMAP_LINGUISTIC_CASING. If you use the International Components for Unicode (ICU) library, you can use u_strToUpper and u_strToLower.
All other options are going to result in edge cases where you're not handling it properly. It's like trying to programmatically split a full name into a first name and a last name: language doesn't work like that.
for (int i = 0; i < strlen(s); i++) {
s[i] ^= 0x20;
} s[i] &= ~0x20;
Jokes aside, I was kinda hoping for a good answer that doesn't rely on a Windows API or an external library, but I'm not sure there is one. It's a rather complex problem when you account for more than just ASCII and the English language.