using System;

​

public class FooController

{

    public string? foo { get; set; }

​

    public virtual string DoSomethingWithFoo()

    {

        return $"I did something with \"{foo}\"";

    }

}

​

public class BarController : FooController

{

    public new int? DoSomethingWithFoo()

    {

        return foo?.Length;

    }

}

​

public class Program

{

    public static void Main()

    {

        var x = new BarController();

        x.foo = "hello world";

        var y = x.DoSomethingWithFoo();

        Console.WriteLine(y);

    }

}