Refactoring Day 13 : Extract Method Object
Today’s refactoring comes from Martin Fowlers list of refactorings. You can find his original article here with a brief description.
This is a more infrequent refactoring that I see myself using but it comes in handy at times. When trying to apply an Extract Method refactoring, and multiple methods are needing to be introduced, it is sometimes gets ugly because of multiple local variables that are being used within a method. Because of this reason, it is better to introduce an Extract Method Object refactoring and to segregate the logic required to perform the task.
1: public class OrderLineItem
2: {
3: public decimal Price { get; private set; }
4: }
5:
6: public class Order
7: {
8: private IList<OrderLineItem> OrderLineItems { get; set; }
9: private IList<decimal> Discounts { get; set; }
10: private decimal Tax { get; set; }
11:
12: public decimal Calculate()
13: {
14: decimal subTotal = 0m;
15:
16: // Total up line items
17: foreach (OrderLineItem lineItem in OrderLineItems)
18: {
19: subTotal += lineItem.Price;
20: }
21:
22: // Subtract Discounts
23: foreach (decimal discount in Discounts)
24: subTotal -= discount;
25:
26: // Calculate Tax
27: decimal tax = subTotal * Tax;
28:
29: // Calculate GrandTotal
30: decimal grandTotal = subTotal + tax;
31:
32: return grandTotal;
33: }
34: }
This entails passing a reference to the class that will be returning the computation to a new object that has the multiple methods via the constructor, or passing the individual parameters to the constructor of the method object. I will be showing the former here.
1: public class OrderLineItem
2: {
3: public decimal Price { get; private set;}
4: }
5:
6: public class Order
7: {
8: public IEnumerable<OrderLineItem> OrderLineItems { get; private set;}
9: public IEnumerable<decimal> Discounts { get; private set; }
10: public decimal Tax { get; private set; }
11:
12: public decimal Calculate()
13: {
14: return new OrderCalculator(this).Calculate();
15: }
16: }
17:
18: public class OrderCalculator
19: {
20: private decimal SubTotal { get; set;}
21: private IEnumerable<OrderLineItem> OrderLineItems { get; set; }
22: private IEnumerable<decimal> Discounts { get; set; }
23: private decimal Tax { get; set; }
24:
25: public OrderCalculator(Order order)
26: {
27: OrderLineItems = order.OrderLineItems;
28: Discounts = order.Discounts;
29: Tax = order.Tax;
30: }
31:
32: public decimal Calculate()
33: {
34: CalculateSubTotal();
35:
36: SubtractDiscounts();
37:
38: CalculateTax();
39:
40: return SubTotal;
41: }
42:
43: private void CalculateSubTotal()
44: {
45: // Total up line items
46: foreach (OrderLineItem lineItem in OrderLineItems)
47: SubTotal += lineItem.Price;
48: }
49:
50: private void SubtractDiscounts()
51: {
52: // Subtract Discounts
53: foreach (decimal discount in Discounts)
54: SubTotal -= discount;
55: }
56:
57: private void CalculateTax()
58: {
59: // Calculate Tax
60: SubTotal += SubTotal * Tax;
61: }
62: }
This is part of the 31 Days of Refactoring series. For a full list of Refactorings please see the original introductory post.