agila-learning-examples/en/csharp-practice-20261101/lesson-2/main.cs

using System;
using System.Collections.Generic;
class Program {
    class Plan { public int Done = 2; }
    static void Main() {
        var plan = new Plan();
        var alias = plan;
        alias.Done = 3;
        Console.WriteLine(plan.Done);
    }
}