Improving The Quality of Existing Software

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Presented at DevReach 2013.

As developers, most of our time is spent working on existing software. Sure, occasionally we get the chance to fire up a new Solution in Visual Studio, and that can be exciting. But after the first day, we find ourselves once more having to deal with the work we did yesterday. And today, we know more than we did yesterday, so there are things we’d do differently, if we had it to do over.
Over time, software rots. If we’re not diligent, our beautiful code can degrade into a worthless mess. Keeping our code in working condition is no different than changing the oil in our car – it’s preventive maintenance. In this session, Steve will look at some common places to look for signs of degradation in existing applications, and steps to take to improve the code. Examples will use C# and primarily ASP.NET.

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Improving The Quality of Existing Software

  1. 1. Improving the Quality of Existing Software Steve Smith Telerik ardalis.com @ardalis
  2. 2. Software Rots
  3. 3. Technical Debt • Low quality code and shortcuts in our applications • Technical debt, like real debt, has direct cost and interest
  4. 4. http://www.jimhighsmith.com/
  5. 5. Preventive Maintenance • Refactoring – Eliminate Duplication – Simplify Design • Automated Tests – Verify correctness – Avoid regressions – Increase Confidence
  6. 6. When should you refactor? • While delivering value
  7. 7. Refactoring Should Not Change System Behavior
  8. 8. Refactoring Process • Verify existing behavior • Write Characterization Tests if none exist – Find test points – Break dependencies • Apply Refactoring • Confirm existing behavior is preserved
  9. 9. Characterization Tests Process 1. Write a test you know will fail 2. Use the output of the failing test to determine the existing behavior to assert 3. Update the test with the new value/behavior 4. Run the test again – it should pass
  10. 10. S O L I DPrinciples http://flickr.com/photos/kevinkemmerer/2772526725
  11. 11. Principles of OO Design 0. Don’t Repeat Yourself (DRY) 1. Single Responsibility 2. Open/Closed 3. Liskov Substitution 4. Interface Segregation 5. Dependency Inversion
  12. 12. Don’t Repeat Repeat Yourself • Duplication in logic calls for abstraction • Duplication in process calls for automation
  13. 13. Common Refactorings • Replace Magic Number/String • Parameterize Method • Pull Up Field • Pull Up Method • Replace Conditional With Polymorphism • Introduce Method
  14. 14. Role Checks if(user.IsInRole(“Admins”) { // allow access to resource } // favor privileges over role checks // ardalis.com/Favor-Privileges-over-Role-Checks var priv = new ContentPrivilege(user, article); if(priv.CanEdit()) { // allow access }
  15. 15. Single Responsibility Principle The Single Responsibility Principle states that every object should have a single responsibility, and that responsibility should be entirely encapsulated by the class. Wikipedia There should never be more than one reason for a class to change. Robert C. “Uncle Bob” Martin
  16. 16. Example Responsibilities • Persistence • Validation • Notification • Error Handling • Logging • Class Selection / Construction • Formatting • Parsing • Mapping
  17. 17. Dependency and Coupling • Excessive coupling makes changing legacy software difficult • Breaking apart responsibilities and dependencies is a large part of working with existing code
  18. 18. Common Refactorings • Extract Class • Extract Method • Move Method
  19. 19. Heuristics and Code Smells • Visual Studio Metrics
  20. 20. Code Smell: Regions More on Regions: http://ardalis.com/regional-differences
  21. 21. Open / Closed Principle The Open / Closed Principle states that software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification. Wikipedia
  22. 22. Open / Closed Principle Open to Extension New behavior can be added in the future Closed to Modification Changes to source or binary code are not required Dr. Bertrand Meyer originated the OCP term in his 1988 book, Object Oriented Software Construction
  23. 23. Common Refactorings • Extract Interface / Apply Strategy Pattern • Parameterize Method • Form Template Method
  24. 24. OCP Fail
  25. 25. OCP OK
  26. 26. OCP Fail public bool IsSpecialCustomer(Customer c) { if(c.Country == “US” && c.Balance < 50) return false; if(c.Country == “DE” && c.Balance < 25) return false; if(c.Country == “UK” && c.Balance < 35) return false; if(c.Country == “FR” && c.Balance < 27) return false; if(c.Country == “BG” && c.Balance < 29) return false; if(c.Age < 18 || c.Age > 65) return false; if(c.Income < 50000 && c.Age < 30) return false; return true; }
  27. 27. OCP OK private IEnumerable<ICustomerRule> _rules; public bool IsSpecialCustomer(Customer c) { foreach(var rule in _rules) { if(rule.Evaluate(c) == false) return false; } return true; }
  28. 28. Liskov Substitution Principle The Liskov Substitution Principle states that Subtypes must be substitutable for their base types. Agile Principles, Patterns, and Practices in C# Named for Barbara Liskov, who first described the principle in 1988.
  29. 29. Common Refactorings • Collapse Hierarchy • Pull Up / Push Down Field • Pull Up / Push Down Method
  30. 30. Liskov Substitution Fail foreach(var employee in employees) { if(employee is Manager) { Helpers.PrintManager(employee as Manager); break; } Helpers.PrintEmployee(employee); }
  31. 31. Liskov Substitution OK foreach(var employee in employees) { employee.Print(); // or Helpers.PrintEmployee(employee); }
  32. 32. Interface Segregation Principle The Interface Segregation Principle states that Clients should not be forced to depend on methods they do not use. Agile Principles, Patterns, and Practices in C# Corollary: Prefer small, cohesive interfaces to “fat” interfaces
  33. 33. Common Refactorings • Extract Interface
  34. 34. Keep Interfaces Small and Focused
  35. 35. Membership Provider
  36. 36. ISP Fail (sometimes) public IRepository<T> { T GetById(int id); IEnumerable<T> List(); void Create(T item); void Update(T item); void Delete(T item); }
  37. 37. ISP OK (for CQRS for example) public IRepository<T> : IReadRepository<T>, IWriteRepository<T> { } public IReadRepository<T> { T GetById(int id); IEnumerable<T> List(); } public IWriteRepository<T> void Create(T item); void Update(T item); void Delete(T item); }
  38. 38. Dependency Inversion Principle High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions. Agile Principles, Patterns, and Practices in C#
  39. 39. Dependency Inversion Principle • Depend on Abstractions – Interfaces, not concrete types • Inject Dependencies into Classes • Structure Solution so Dependencies Flow Toward Core – Onion Architecture (a.k.a. Ports and Adapters)
  40. 40. Application Layers
  41. 41. Data Access Evolution No separation of concerns:  Data access logic baked directly into UI  ASP.NET Data Source Controls  Classic ASP scripts  Data access logic in UI layer via codebehind  ASP.NET Page_Load event  ASP.NET Button_Click event User Interface Database Compile Time Runtime
  42. 42. Data Access : Helper Classes Calls to data made through a utility  Example: Data Access Application Block (SqlHelper)  Logic may still live in UI layer  Or a Business Logic Layer may make calls to a Data Access Layer which might then call the helper User Interface Database Compile Time Runtime Helper Class
  43. 43. What’s Missing? Abstraction!  No way to abstract away data access  Tight coupling  Leads to Big Ball of Mud system  Solution:  Depend on interfaces, not concrete implementations  What should we call such interfaces? Repositories! User Interface Database Compile Time Runtime Core IFooRepository Infrastructure SqlFooRepository
  44. 44. DIP Architecture (aka Ports and Adapters)
  45. 45. Common Dependencies • Framework • Third Party Libraries • Database • File System • Email • Web Services • System Resources (Clock) • Configuration • The new Keyword • Static methods • Thread.Sleep • Random See also responsibilities: • Persistence • Validation • Notification • Error Handling • Logging • Class Selection / Construction • Formatting • Parsing • Mapping
  46. 46. Common Refactorings • Extract Class • Extract Interface / Apply Strategy Pattern • Extract Method • Introduce Service Locator / Container
  47. 47. DIP Fail
  48. 48. Some Improvement (Façade)
  49. 49. DIP OK (Strategy)
  50. 50. DIP OK (Strategy)
  51. 51. Self-Improvement and Quality • How fast can you produce: – Code you believe to be of high quality – Code that maybe gets the job done, but you believe to be of low quality • Which one can you produce more quickly? • Why? • How can we develop our skills and our tools so that building quality is natural and easier than not doing so?
  52. 52. 0 2 4 6 8 10 12 14 16 Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 Week 9 User Stories Completed High Quality Low Quality Column1
  53. 53. 0 2 4 6 8 10 12 14 16 18 20 Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 Week 9 User Stories Completed High Quality Low Quality Column1
  54. 54. Summary • Maintain / Improve Application Code • Follow DRY/SOLID Principles • Use Characterization Tests to “fix” behavior • Apply Common Refactorings • Re-run Tests After (and during) Refactorings • Train and Practice to Write Better Code Faster
  55. 55. References http://bit.ly/SFkpmq  Refactoring Catalog  http://www.refactoring.com/catalog/index.html  Onion Architecture  http://jeffreypalermo.com/blog/the-onion-architecture-part-1/
  56. 56. Books  Refactoring http://amzn.to/110tscA  Refactoring to Patterns http://amzn.to/Vq5Rj2  Working Effectively with Legacy Code http://amzn.to/VFFYbn  Code Complete http://amzn.to/Vq5YLv  Clean Code http://amzn.to/YjUDI0
  57. 57. Thank you! @ardalis ardalis.com http://facebook.com/stevenandrewsmith

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