How do you tell whether one design is better than another? This talk provides the answer. By walking through a simple example business problem and its software solution, we examine a few key principles that can be applied to make designs simpler, easier to maintain and change, and overall better.

2. OO SOLID
Applying SOLID principles to improve object-oriented designs

3. Say we have a mechanic
shop...

4. ServiceOrder
AddPart(Part)
Total()
1..*
Part
Name
Price

5. !
public class Part
{
private decimal price;
public Part(string name, decimal price)
{
this.price = price;
}
public decimal Price
{
get { return price; }
set { price = value; }
}
}

6. !
public class ServiceOrder {
private List<Part> parts;
public ServiceOrder() {
parts = new List<Part>();
}
public void AddPart(Part part) {
parts.Add(part);
}
public decimal Total() {
decimal total = 0;
foreach (Part aPart in parts) {
total += aPart.Price;
}
return total;
}
}

7. Now we want to specify additional
quantities of each part.
(because we are lazy)

8. ServiceOrder
partsAndQtys : Dictionary
AddPart(Part)
Total()
1..*
Part
Name
Price

9. !
public class ServiceOrder
{
private Dictionary<Part, int> partsAndQuantities;
public ServiceOrder() {
partsAndQuantities = new Dictionary<Part, int>();
}
public void AddPart(Part part) {
this.AddPart(part, 1);
}
public void AddPart(Part part, int quantity) {
int currentQuantity = 0;
if (partsAndQuantities.ContainsKey(part)) {
currentQuantity = partsAndQuantities[part];
}
int newQuantity = currentQuantity + quantity;
partsAndQuantities.Add(part, newQuantity);
}
public decimal Total() {
decimal total = 0;
foreach (Part aPart in partsAndQuantities.Keys) {
int quantity = partsAndQuantities[aPart];
total += aPart.Price * quantity;
}
return total;
}
}

10. ServiceOrder
AddPart(Part, Qty)
Total()
1
1..*
LineItem
Part
Quantity
Part
*
1
Name
Price

11. !
public class LineItem
{
private Part part;
private int quantity;
public LineItem(Part part, int quantity) {
this.part = part;
this.quantity = quantity;
}
public Part Part {
get { return part; }
}
public int Quantity {
get { return quantity; }
private set { quantity = value; }
}
}

12. public class ServiceOrder
{
private List<LineItem> lineItems;
public ServiceOrder() {
lineItems = new List<LineItem>();
}
public void AddPart(Part part) {
this.AddPart(part, 1);
}
public void AddPart(Part part, int quantity) {
LineItem lineItem = new LineItem(part, quantity);
lineItems.Add(lineItem);
}
public decimal Total() {
decimal total = 0;
foreach (LineItem aLineItem in lineItems) {
total += aLineItem.Part.Price * aLineItem.Quantity;
}
return total;
}
}

13. S ingle Responsibility Principle

14. Now we want to add a markup
for each part.
(because we want to track
profit margin)

15. ServiceOrder
AddPart(Part, Qty)
Total()
1
1..*
Part
LineItem
Part
Quantity
*
1
Name
MarkupPercentage
WholesalePrice

16. !
public class Part
{
private string name;
private decimal wholesalePrice;
private int markupPercentage;
public Part(string name, decimal price) :this(name, price, 0)
{}
public Part(string name, decimal price, int markupPercentage) {
this.name = name;
this.wholesalePrice = price;
this.markupPercentage = markupPercentage;
}
public string Name {
get { return name; }
}
public decimal WholesalePrice {
get { return wholesalePrice; }
}
public int MarkupPercentage {
get { return markupPercentage; }
}
}

17. !
public class ServiceOrder
{
private List<LineItem> lineItems;
public ServiceOrder() {
lineItems = new List<LineItem>();
}
public void AddPart(Part part) {
this.AddPart(part, 1);
}
public void AddPart(Part part, int additionalQuantity) {
LineItem lineItem = new LineItem(part, additionalQuantity);
lineItems.Add(lineItem);
}
public decimal Total() {
decimal total = 0;
foreach (LineItem aLineItem in lineItems) {
total += aLineItem.Part.WholesalePrice
* (aLineItem.Part.MarkupPercentage/100m + 1)
* aLineItem.Quantity;
}
return total;
}
}

18. ServiceOrder
AddPart(Part, Qty)
Total()
1
1..*
Part
LineItem
Part
Quantity
LineTotal()
*
1
Name
MarkupPercentage
WholesalePrice

19. !
// New methods in the Part class
public decimal Price {
get { return wholesalePrice * MarkupMultiplier(); }
}
!
private decimal MarkupMultiplier() {
return (markupPercentage / 100m + 1);
}
!
!
// And one in the LineItem class
public decimal LineTotal() {
return part.Price * quantity;
}

20. !
// In the ServiceOrder class...
public decimal Total() {
decimal total = 0;
foreach (LineItem aLineItem in lineItems) {
total += aLineItem.LineTotal();
}
return total;
}

21. O pen/Closed Principle

22. Now we want to add labor costs.
(because we want to
make more money)

23. Part
Name
MarkupPercentage
WholesalePrice
Price()

24. Part
Name
MarkupPercentage
WholesalePrice
Labor
Price
Price()

25. !
public class Labor : Part
{
public Labor(string name, decimal price)
: base(name, price)
{}
}

26. Item
Name
CalculatePrice()
Labor
Price
CalculatePrice()
Part
MarkupPercentage
WholesalePrice
CalculatePrice()

27. !
public interface Item
{
string Name { get; }
decimal CalculatePrice();
}

28. L iskov Substitution Principle

29. Now we want to generate
receipts.
(because we want to
seem professionalish)

30. Presentation Layer
Business Layer
ReceiptGenerator
ServiceOrder
PrintReceipt()
AddPart(Part, Qty)
Total()

31. !
public class ReceiptGenerator
{
private FixItMechanicsSystem system;
private EpsonTMT88IVPrinter epsonTMT88IVPrinter;
private string initString = "sasllkslsl398383fa";
public void PrintReceipt() {
ServiceOrder order = system.FinalizeOrder();
string receiptString = "Total........ $" + order.Total();
epsonTMT88IVPrinter.Initialize(initString);
epsonTMT88IVPrinter.PrintBwMed239(receiptString);
epsonTMT88IVPrinter.Flush();
}
}

32. Presentation Layer
ReceiptGenerator
PrintReceipt()
Business Layer
Receipt
Total()
ServiceOrder
AddPart(Part, Qty)
Total()

33.
// In the ReciptGenerator class.
public void PrintReceipt() {
IReceipt receipt = system.FinalizeOrder();
string receiptString = "Total........ $" + receipt.Total();
epsonTMT88IVPrinter.Initialize(initString);
epsonTMT88IVPrinter.PrintBwMed239(receiptString);
epsonTMT88IVPrinter.Flush();
}

34. I nterface Segregation Principle

35. We need to print to a new
printer.
(because we’re growing and they
don’t make the old one anymore)

36. ReceiptGenerator
PrintReceipt()
EpsonT88IVPrinter
1
Initialize(String)
PrintBwMed239(String)
Flush()

37. !
public class ReceiptGenerator
{
private FixItMechanicsSystem system;
private EpsonTMT88IV epsonTMT88IVPrinter;
private string initString = "sasllkslsl398383fa";
public void PrintReceipt() {
Receipt receipt = system.FinalizeOrder();
string receiptString = "Total........ $" + receipt.Total();
epsonTMT88IVPrinter.Initialize(initString);
epsonTMT88IVPrinter.PrintBwMed239(receiptString);
epsonTMT88IVPrinter.Flush();
}
}

38. ReceiptGenerator
PrintReceipt()
EpsonPrinterAdapter
1
EpsonT88IVPrinter
1
GenericPrinter
Print(String)
SamsungPrinterAdapter
1
SamsungSRP350
Initialize(String)
Initialize(String)
PrintBwMed239(String)
ProcessPages(String)
Flush()

39.
public class ReceiptGenerator
{
private FixItMechanicsSystem system;
private IGenericPrinter printer;
public void PrintReceipt() {
Receipt receipt = system.FinalizeOrder();
string receiptString = "Total........ $" + receipt.Total();
printer.Print(receiptString);
}
}

40.
public class EpsonPrinterAdapter : IGenericPrinter
{
private EpsonTMT88IV epsonTMT88IVPrinter;
private string initString = "sasllkslsl398383fa";
public void Print(string printString) {
epsonTMT88IVPrinter.Initialize(initString);
epsonTMT88IVPrinter.PrintBwMed239(printString);
epsonTMT88IVPrinter.Flush();
}
}

41.
public class SamsungPrinterAdapter : IGenericPrinter
{
private SamsungSRP350 samsungSRP350Printer;
private string initString = "aksreiufgu";
public void Print(string printString) {
samsungSRP350Printer.Init(initString);
samsungSRP350Printer.ProcessPages(printString);
}
}

42. D ependency Inversion Principle

43. S ingle Responsibility Principle
O pen/Closed Principle
L iskov Substitution Principle
I nterface Segregation Principle
D ependency Inversion Principle

44. Questions?

45. X
Poor
Design
• Duplication
• Ambiguity
• Complexity

46. ✓
Good
Design
✓ Low Coupling
✓ High Cohesion

47. Questions?