CalculatorSteps.cs

[Binding]
public class CalculatorSteps
{
  private readonly Calculator calculator = new Calculator();

  [Given(@"I have entered (.*) into the calculator")]
  public void GivenIHaveEnteredIntoTheCalculator(int operand)
  {
      calculator.Enter(operand);
  }
  
  [When(@"I press add")]
  public void WhenIPressAdd()
  {
    calculator.Add();
  }

  //...

  [Then(@"the result should be (.*) on the screen")]
  public void ThenTheResultShouldBeOnTheScreen(int expectedResult)
  {
    Assert.Equal(expectedResult, calculator.Result);
  }
}