using System;

​

public class B

{

    private readonly A _a;

    public B(A a)

    {

        _a = a;

    }

​

    public void DoSomething()

    {

        Console.WriteLine("B wrote something");

        _a.DoSomethingElse();

    }

}

​

public class A

{

    public void DoSomething()

    {

        // create B, pass in reference to a. I.e This

        var b = new B(this);

        Console.WriteLine("A wrote something");

        b.DoSomething();

    }

​

    public void DoSomethingElse()

    {

        Console.WriteLine("A wrote something else");

    }

}

​

public class Program

{

    public static void Main()

    {

        var a = new A();

        a.DoSomething();

    }

}