CalculatorController and CalculatorService modified.cs

    // in Services\Calculator\CalculatorService.cs
    public class CalculatorService : ICalculator
    {
        // this was added
        private readonly IPrinter _printer;
        public CalculatorService( IPrinter printer)
        {
            this._printer = printer;
        }

        public int Substract(int a, int b)
        {
            return a - b;
        }

        public int Sum(int a, int b)
        {
            // every time we sum, a ping pong is printed
            this._printer.PingPong();

            return a + b;
        }
    }
    
    // in Controllers\CalculatorController.cs
    [ApiController]
    [Route("calculator")]
    public class CalculatorController : Controller
    {
        //this was added
        private readonly IPrinter _printer;
    
        private readonly ICalculator _calculator;

        public CalculatorController(IPrinter printer, ICalculator calculator)
        {
            // this was added
            this._printer = printer;
            this._calculator = calculator;
        }

        [HttpGet("sum")]
        public IActionResult Sum([FromQuery] int a, [FromQuery] int b)
        {
            // this was added
            this._printer.PingPong();

            // printing the result is not needed, just to print it from the CalculatorService
            this._calculator.Sum(a, b);

            return Ok();
        }
    }