public static string RemoveSpecialCharacters(this 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();
}
test