System.String.IndexOf (char)
string.IndexOf(char)ってculture insensitiveなんだよねー。
CompareInfo ci = CultureInfo.InvariantCulture.CompareInfo;
Console.WriteLine ("AE".IndexOf ('\u00C6'));
Console.WriteLine ("AE".IndexOf ("\u00C6"));
Console.WriteLine (ci.IndexOf ("AE", '\u00C6'));
-1
0
0