using System;

public class Program

{

    public static void Main()

    {

        var tester = new Tester();

        tester.Test();

    }

    public class Tester

    {

        public void Test()

        {

            var bar = new Bar();

            TestBar(bar);

            bar.Do();

        }

        private void TestBar(ITask t)

        {

            t.Do();

            t = new Foo();

        }

    }

}

​

public interface ITask{ void Do(); }

public class Bar: ITask { public void Do() => Console.WriteLine("Bar"); }

public class Foo: ITask { public void Do() => Console.WriteLine("Foo"); }