refactoring applications using solid principles

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Refactoring Applicationsusing SOLID Principles

Steve SmithTelerik

ardalis.com @ardalis

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Software Rots

How? Duplication Excess Coupling Quick Fixes Hacks

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Preventive Maintenance

• Refactoring– Eliminate Duplication– Simplify Design

• Automated Tests– Verify correctness– Avoid regressions– Increase Confidence

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Refactoring Process

• Verify existing behavior• Write Characterization Tests if none

exist– Find test points– Break dependencies

• Apply Refactoring• Confirm existing behavior is

preserved

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Characterization Tests

Process1. Write a test you know will fail2. 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

S O L I DPrinciples

http://flickr.com/photos/kevinkemmerer/2772526725/

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Principles of OO Design

0. Don’t Repeat Yourself (DRY)

1.Single Responsibility2.Open/Closed3.Liskov Substitution4.Interface Segregation5.Dependency Inversion

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Don’t RepeatRepeat Yourself

• Duplication in logic calls for abstraction

• Duplication in process calls for automation

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Common Refactorings

• Replace Magic Number/String• Parameterize Method• Pull Up Field• Pull Up Method• Replace Conditional With

Polymorphism• Introduce Method

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Role Checksif(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}

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Single Responsibility PrincipleThe 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

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Example Responsibilities

• Persistence• Validation• Notification• Error Handling• Logging• Class Selection / Construction• Formatting• Parsing• Mapping

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Dependency and Coupling

• Excessive coupling makes changing legacy software difficult

• Breaking apart responsibilities and dependencies is a large part of working with existing code

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Common Refactorings

• Extract Class• Move Method

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Heuristics and Code Smells

• Visual Studio Metrics

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Code Smell: Regions

?More on Regions: http://ardalis.com/regional-differences

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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

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Open / Closed Principle

Open to ExtensionNew behavior can be added in the future

Closed to ModificationChanges to source or binary code are not required

Dr. Bertrand Meyer originated the OCP term in his 1988 book, Object Oriented Software Construction

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Common Refactorings

• Extract Interface / Apply Strategy Pattern

• Parameterize Method• Form Template Method

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OCP Fail

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OCP Failpublic 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;}

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OCP OKprivate IEnumerable<ICustomerRule> _rules;

public bool IsSpecialCustomer(Customer c){ foreach(var rule in _rules) { if(rule.Evaluate(c) == false) return false; } return true;}

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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.

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Common Refactorings

• Collapse Hierarchy• Pull Up / Push Down Field• Pull Up / Push Down Method

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Liskov Substitution Failforeach(var employee in employees){ if(employee is Manager) { Helpers.PrintManager(employee as Manager); break; } Helpers.PrintEmployee(employee);}

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Liskov Substitution OKforeach(var employee in employees){ employee.Print(); // or Helpers.PrintEmployee(employee);}

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Interface Segregation PrincipleThe 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

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Common Refactorings

• Extract Interface

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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);}

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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);}

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Dependency Inversion PrincipleHigh-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#

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Dependency Inversion Principle• Depend on Abstractions– Interfaces, not concrete types

• Inject Dependencies into Classes

• Structure Solution so Dependencies Flow Toward Core– Onion Architecture

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Application Layers

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Data Access EvolutionNo 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

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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

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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

CoreIFooRepository

InfrastructureSqlFooRepository

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DIP “Onion” Architecture

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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

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Common Refactorings

• Extract Class• Extract Interface / Apply Strategy

Pattern• Extract Method• Introduce Service Locator / Container

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DIP Fail

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Some Improvement (Façade)

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DIP OK (Strategy)

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DIP OK (Strategy)

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Summary

• Maintain / Improve Application Code• Follow DRY/SOLID Principles• Use Characterization Tests to “fix”

behavior• Apply Common Refactorings• Re-run Tests After Refactorings

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References

Principles of Object Oriented Design (Pluralsight) http://bit.ly/SFkpmq

Refactoring Catalog http://www.refactoring.com/catalog/index.html

Working Effectively with Legacy Code (Book) Michael C. Feathers

Onion Architecture http://jeffreypalermo.com/blog/the-onion-architecture-part-1

/

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Thank You! – Благодаря!

Find Me Online:Blog: http://ardalis.comTwitter: @ardalishttp://facebook.com/StevenAndrewSmith

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