Be aware always to use overloaded methods of ToLower/ToUpper of System.String class that accepts CultureInfo instance, if you don't want your application to fail in some circumstances when a user regional setting differs from English. In my case I had bug for users with Turkish regional settings, because uppercase "I" lowered to "ı" but not "i" as was expected. The only case when you might use overload with no parameters of ToLower/ToUpper methods is when you need to display lowered or uppercased strings onto UI. Thanks to ToLowerInvariant/ToUpperInvariant in .NET 2.0, but not worked for me, my app requires running under .NET 1.1.