gistfile1.cs

// better like this?
var isMyFavoriteLetter = new List<string> { "A", "B", "C"}.Contains(myFavoriteLetter);

// or better like this?
var isMyFavoriteLetter = myFavoriteLetter == "A" || myFavoriteLetter == "B" || myFavoriteLetter == "C";

A list and using contains seems to match my mental model of the question.

or

var isMyFavoriteLetter = new []{ "A", "B", "C"}.Contains(myFavoriteLetter);

I prefer:

myFavoriteLetter.IsIn("A", "B", "C").

IsIn() is easy to write:

public static bool IsIn(this T p_value, params T[] p_list) {
return p_value.IsIn(p_list.ToList());
}

public static bool IsIn(this T p_value, IList p_list) {
if (p_list == null || p_list.Count == 0)
return false;

return p_list.Contains(p_value);

}