System.Globalization.CultureTableRecord.ValidateCulturePieceToLower C# (CSharp) Method

        private static string ValidateCulturePieceToLower(string testString, string paramName, int maxLength)
        {
            if(testString.Length > maxLength)
            {
                throw new ArgumentException(
                    String.Format(CultureInfo.CurrentCulture, Environment.GetResourceString("Argument_NameTooLong"), testString, maxLength), paramName);
            }

            StringBuilder sb = new StringBuilder(testString.Length);

            for(int ich = 0; ich < testString.Length; ich++) {
                char ch = testString[ich];

                if(ch <= 'Z' && ch >= 'A') {
                    sb.Append((char)(ch - 'A' + 'a'));
                }
                else if(((ch <= 'z' && ch >= 'a') ||
                    (ch <= '9' && ch >= '0') ||
                    (ch == '_') ||
                    (ch == '-'))) {
                    sb.Append(ch);
                }
                else {
                    throw new ArgumentException(
                        String.Format(CultureInfo.CurrentCulture, Environment.GetResourceString("Argument_NameContainsInvalidCharacters"), testString), paramName);
                }
            }

            return(sb.ToString());

        }