Common ASP.NET Design Patterns - Telerik India DevCon 2013

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An overview of several design patterns that are useful when developing ASP.NET applications.

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  • General, reusable solutions to common problemsNot a complete, finished solutionA template, recipe, or approach to solving certain problemsEach has a specific name, so we can discuss common elements of design using a common vocabulary
  • Learning anything – a musical chord, a martial arts technique, etc.Zero: You used a what? Never heard of it.Awakening:Wow, I just learned how XYZ pattern can improve my design. I’m not really sure where it would work in my code, but I’m definitely looking.Overzealous:I totally “get” the XYZ pattern; I’m adding it everywhere I can shoehorn it into my code. My design’s gonna be better now, for sure!Mastery: In response to some specific design pain points, the XYZ pattern was a logical next step and was achieved through a simple refactoring.
  • Design pattern names provide placeholders or complex abstractions and sets of refactorings. Consider the difference between:We have some tight coupling to the database here. We can probably fix it if we apply these refactorings;extract an interface, extract a method, Extract a class,Replace local with parameterORLet’s apply the Repository pattern to fix it.
  • Used Regularly or Daily by most respondentsFactory (194)Repository (185)Iterator (139)Adapter/Wrapper (122)Observer (113)Command (106)Singleton (105)Strategy (101)Proxy (91)
  • It’s possible for the new Singleton(); line to be called more than once.
  • Here are three example registration commands using an IOC container, in this case StructureMap. If you’re not familiar with IOC containers, they’re basically just factories on steroids, which you configure to create instances of types according to certain rules.In the first case, one new instance of the type is created per thread, or per HTTP request. Thus, in an ASP.NET application, a new DbContext will be created with each new web request, but if we used this same code in a non-web context, such as in an integration test, it would create a single instance per thread. In the second case, the Singleton() call will literally configure the lifetime of the instance to be a singleton, with one and only one instance available for the life of this application. In the last, default case, each new request for an instance will get its own, separate instance of the requested type.The benefit of this approach is that it makes it very clear which objects are configured with which lifetimes, and none of the objects themselves are cluttered with these concerns. The resulting code has less repetition, is simpler, and its classes have fewer responsibilities.
  • Note – this cannot easily be tested without an actual database instance in place.
  • http://en.wikipedia.org/wiki/File:Proxy_concept_en.svg
  • Apply patterns via refactoring,Not Big Design Up Front
  • Show the MvcMusicStore applicationHg clone https://hg.codeplex.com/forks/ssmith/mvcmusicstorerepositorypattern 
  • Talk about the CacheDependencyBug in AdSignia and how it was introduced by moving a hard-coded instantiation to a parameter, and then fixed by replacing the parameter with a Factory (lambda).
  • Common ASP.NET Design Patterns - Telerik India DevCon 2013

    1. 1. Common ASP.NET Design Patterns Steve Smith @ardalis | ardalis.com Telerikhttp://flic.kr/p/4MEeVn
    2. 2. Design Patternshttp://flic.kr/p/8XgqKu
    3. 3. Stages of LearningStage Zero - IgnoranceStage One - AwakeningStage Two - OverzealousStage Three – Masteryhttp://flic.kr/p/6StgW5
    4. 4. Languagehttp://flic.kr/p/6UpMt9
    5. 5. Actual Usage Twitter Poll in 2011 w/342 responses  http://twtpoll.com/r/t7jzrx Votes 250 200 150 100 50 Votes 0
    6. 6. Common Design Patterns0.Singleton1.Strategy2.Repository3.Proxy (Of course there are many others!)4.Command5.Factory
    7. 7. (Singleton)
    8. 8. Singleton: Intent Ensure a class has only one instance Make the class itself responsible for keeping track of its sole instance “There can be only one”
    9. 9. Singleton Structure (not thread-safe)
    10. 10. How Singleton Is Used Call methods on Instance directly Assign variables to Instance Pass as Parameter
    11. 11. Singleton Structure(thread-safe and fast) Source: http://csharpindepth.com/Articles/General/Singleton.aspx
    12. 12. Singleton Consequences The default implementation not thread-safe  Avoid in multi-threaded environments  Avoid in web server scenarios (e.g. ASP.NET) Singletons introduce tight coupling among collaborating classes Shameless Plug: But why would you Telerik JustMock can be intentionally write code Singletons are notoriously difficult to test usedcan mocktest with you to only and test  Commonly regarded as an anti-pattern certainSingletons tools? premium Violate the Single Responsibility Principle  Class is responsible for managing its instances as well as whatever it does Using an IOC Container it is straightforward to avoid the coupling and testability issues
    13. 13. Managing Object Lifetime Using an IOC Container
    14. 14. Strategyhttp://flic.kr/p/6spkwo
    15. 15. Strategy: Intent Encapsulate a family of related algorithms Let the algorithm vary and evolve independently from the class(es) that use it Allow a class to maintain a single purpose  Single Responsibility Principle (SRP)  Also enables Open/Closed Principle (OCP)  Learn more about SRP and OCP here:  http://pluralsight.com/training/Courses/TableOfContents/principles-oo-design
    16. 16. Strategy : Structure
    17. 17. Strategy : Common Usage Dependency Inversion and Dependency Injection Decouple class dependencies and responsibilitiesRefactoring Steps Create interfaces for each responsibility Inject the implementation of the interface via the constructor Move the implementation to a new class that implements the interface
    18. 18. Hidden Dependencies Classes should declare their dependencies via their constructor Avoid hidden dependencies  Anything the class needs but which is not passed into constructor Avoid making non-stateless static calls Avoid directly instantiating classes (except those with no dependencies, like strings)  Instead, move these calls to an interface, and call a local instance of the interface
    19. 19. “New is Glue” “new” creates tight coupling between classesBe conscious of theconsequences of using“new”If you need loosecoupling, replace“new” with StrategyPatternhttp://flic.kr/p/aN4Zv
    20. 20. Repository
    21. 21. Data Access EvolutionNo separation of concerns: Compile Time Runtime Data access logic baked directly into UI  ASP.NET Data Source Controls  Classic ASP scripts User Interface Data access logic in UI layer via codebehind  ASP.NET Page_Load event  ASP.NET Button_Click event Database
    22. 22. Data Access : Helper Classes Compile Time Calls to data made through a utility Runtime Example: Data Access Application Block (SqlHelper) User Interface Logic may still live in UI layer Helper Class Or a Business Logic Layer may make calls to a Data Access Layer which might then call the helper Database
    23. 23. What’s Missing? Compile Time Abstraction! Runtime No way to abstract away data access User Interface Tight coupling Leads to Big Ball of Mud Core Infrastructure system IFooRepository SqlFooRepository Solution:  Depend on interfaces, not concrete implementations  What should we call such Database interfaces? Repositories!
    24. 24. Repository A Data Access Pattern Separates persistence responsibility from business classes Enables  Single Responsibility Principle  Separation of Concerns  Testability Frequently Separate Interfaces for Each Entity in Application  E.g. CustomerRepository, OrderRepository, etc.  Or using Generics: IRepository<TEntity> May also organize Repositories based on  Reads vs. Writes  Bounded Contexts
    25. 25. Repository - Example
    26. 26. Repository - Example (1) Create/extend an interface to represent the data access you need (2) Copy the implementation from your class into a new class implementing this interface.(3)Inject the interface using theStrategy Pattern. Use the local fieldof this interface type in place ofprevious implementation code.
    27. 27. Where do Repositories Live? Place interfaces in Core  Core includes Model classes and most business logic  Core has no dependencies (ideally) Place implementation in Infrastructure  Infrastructure references Core UI layer references both Core and Infrastructure  Responsible for creating object graph, either manually or via an IOC Container
    28. 28. Proxy
    29. 29. Proxy A class that controls access to another Implemented via subclass or via delegation using a common interface Frequent use cases:  Remote Proxy for web services / network calls  Lazy loading implementations  Security / access control
    30. 30. Proxy - Structure
    31. 31. Adding Caching to a Repository Caching is frequently added to query methods in repositories Caching logic is a separate concern and should live in a separate class from the main data access logic Proxy pattern provides a straightforward means to control access to the “real” data, versus the cache
    32. 32. Proxy – CachedRepository Implementation
    33. 33. Implementing with IOC// Strategy Pattern with Proxy Pattern (using composition)x.For<IAlbumRepository>().Use<CachedAlbumRepository>() .Ctor<IAlbumRepository>().Is<EfAlbumRepository>();
    34. 34. Commandhttp://flic.kr/p/5Yb5i
    35. 35. Command Represent an action as an object Decouple performing the action from the client that is issuing the command Common scenarios:  Delayed execution  Logging activity  Enabling Undo / Revoke Transaction functionality
    36. 36. Command
    37. 37. Command : Usage Great for representing state transitions (the arrows below) Pending Archived Published
    38. 38. Factoryhttp://flic.kr/p/vGpC2
    39. 39. Factory: Intent Separate object creation from the decision of which object to create Defer creation of objects  Only create them if and when needed Add new classes and functionality without breaking client code  Only factory needs to know about new types available Store possible objects to create outside of program  Configuration  Persistent Storage  Plug-in assemblies
    40. 40. Practice PDD Pain Driven DevelopmentIf it’s causing pain, fix it.
    41. 41. Design Patterns in ActionCODE WALKTHROUGH
    42. 42. A Tale from theREAL WORLD
    43. 43. References Design Patterns, http://amzn.to/95q9ux Design Patterns Explained, http://amzn.to/cr8Vxb Design Patterns in C#, http://amzn.to/bqJgdU Head First Design Patterns, http://amzn.to/aA4RS6Pluralsight Resources N-Tier Application Design in C# http://bit.ly/Msrwig Design Patterns Library http://bit.ly/vyPEgK SOLID Principles of OO Design http://bit.ly/OWF4la My Blog  http://ardalis.com
    44. 44. Summary Design Patterns can be used during refactoring or initial design to improve code quality Become comfortable with a number of design patterns, but focus on the most common ones Use design patterns to address pain in your design, such as tight coupling or excessive repetition Design patterns can be combined in powerful ways, such as Strategy-Proxy-Repository

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