decorator_example.cs

public interface IFoo{
  void Bar();
}

public class Foo : IFoo{
  public Bar(){ 
    // whatever...
  }
}

public class DecoratedFoo : IFoo{
  private readonly IFoo _inner;
  public DecoratedFoo(IFoo inner){
    _inner = inner;
  }
  
  public Bar(){ 
    // you can do something here...
    _inner.Bar();
    // ...and guess what, here as well!
  }
}