You could do this:
char c = ...
bool isVowel = "aeiouAEIOU".IndexOf(c) >= 0;
or this:
char c = ...
bool isVowel = "aeiou".IndexOf(c.ToString(), StringComparison.InvariantCultureIgnoreCase) >= 0;
Once you add international support for things like éèe???ê?e???и
etc. this string will get long, but the basic solution is the same.
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