Return Object Based on Query Strings

I'm trying to return objects based on query strings.
For example, I would like api/users/{id}?fields=username,email,reputation to return an object of type User that contains only the three included properties (username, email, reputation).

On a side note, comma separated query strings are not possible by default in .NET Core.
Here's a tutorial on making that work.

Following the guide above, I have a list of strings. How can I create an object that only includes the properties that match the names of those strings?
For only a few strings I can do this (thanks Ben Hall):

List<string> listOfStrings ...; // Strings from query
User user = GetUser(id); // User from db 

User newUser = new User();

if (listOfStrings.Contains("username"))
    newUser.username = user.username;
if (listOfStrings.Contains("email"))
    newUser.email = user.email;
if (listOfStrings.Contains("reputation"))
    newUser.reputation = user.reputation;

But for a long list of strings (my user class has 30+ properties) how would I go about doing this?

For reference, Facebook Graph API does this.