using System;

public class Program

{

    public void Main()

    {

        var game = SaveGame(new Derived());

        Console.WriteLine("game returned: " + game.GetType().Name);

    }

    public static Base SaveGame(Base game)

    {

        Console.WriteLine("game saved: " + game.GetType().Name);

        //do something with derived-specific variables, like

        if(game is Derived derived)

            derived.hitpoints = 15;

        return game;

    }

}

​

public class Base { }

​

public class Derived : Base

{

    public int hitpoints = 10;

}