using static System.Console;

​

DoStuff @do = new(2);

IDoStuff iDo = (IDoStuff)@do;

WriteLine("a");

iDo.Do();

WriteLine("b");

ReadKey(true);

​

internal class DoStuff

{

    public int X { get; }

    public void Do() => WriteLine(X);

    public DoStuff(int X) => this.X = X;

}

​

internal interface IDoStuff

{

    void Do();

}