RemoveSpecialCharacters

public static string RemoveSpecialCharacters(string str) {
  StringBuilder sb = new StringBuilder();
        for (int i = 0; i < str.Length; i++)
        {
            if ((str[i] >= '0' && str[i] <= '9') || (str[i] >= 'A' && str[i] <= 'z' || (str[i] == '.' || str[i] == '_')))
                sb.Append(str[i]);
        }

        return sb.ToString();
}