Budgets.cs

Budget {
  public double CalculateTotal() { ... }
}

/* ---- */
OneClient {
  ...
    budget.CalculateTotal();
  ...
}
/* ---- */
OtherClient {
    ...
    budget.CalculateTotal();
    ...
}



/* new changes => Take into account spanish VAT (IVA) */

// Refactor step 1
Budget {
  public double CalculateTotal() { // used by clients in production code
    // ...
    var taxes = 0.0;
    return total + taxes;  
  }
}

// Refactor step 2
Budget {
  public double CalculateTotal() { // used by clients in production code
    // ...
    return AddTaxes(total);  
  }
  
  private double AddTaxes(double total) {
    var taxesPercentage = 0.0;
    var taxes = total * taxesPercentage;
    return total + taxes;
  }
}

// Refactor step 3, enables testing new functionality
Budget {
  public Budget(double taxesPercentage = 0.0) { // used by clients in production code
    this.taxesPercentage = taxesPercentage;
  }
  
  public static Budget WithSpanishTaxes() { // for testing purposes
    return new Budget(0.21);
  }
  
  public double CalculateTotal() { // used by clients in production code
    // ...
    return AddTaxes(total);   
  }
  
  private double AddTaxes(double total) {
    return total * (1.00 + this.taxesPercentage);
  }
}

// Final state: new feture working
Budget {
  public Budget() {  // used by clients in production code
    this.taxesPercentage = 0.21;
  }
  
  public static Budget WithSpanishTaxes() { // used by tests
    return new Budget();
  }
  
  public double CalculateTotal() { // used by clients in production code
    // ...
    return total * (1.00 + this.taxesPercentage); // AddTaxes was inlined
  }
}