I have codes below on asp net currently detected as a potential XSS attacks during code scanning.

public static UserDTO GetUserByUserId(int userId){
    UserDTO user = service.GetUser(userId);
    return user;
}

The method sends unvalidated data to a web browser on line 826, which
can result in the browser executing malicious code.

I did try to add HttpUtility.HtmlEncode() for string values but now it gives same error for nullable properties.

 public static UserDTO GetUserByUserId(int userId){
        UserDTO user = service.GetUser(userId);

        UserDTO newuser = new UserDTO();
        newuser.id=user.id;  //int id
        newuser.name = HttpUtility.HtmlEncode(user.name);  //name string
        newuser.age = user.age;  //age int?  >>> this line causes xss

        return newuser;
    }

How do I apply for the XSS prevention in this case for C#?