remove special characters from string in c#.net

public static string RemoveSpecialCharacters(string str) {
   StringBuilder sb = new StringBuilder();
   foreach (char c in str) {
      if ((c >= '0' && c <= '9') || (c >= 'A' && c <= 'Z') | || (c >= 'a' && c <= 'z') | c == '.' || c == '_') {
         sb.Append(c);
      }
   }
   return sb.ToString();
}