solid-patterns

SOLID Patterns — Rules

Use these rules when creating new classes, services or repositories in Delphi.

SOLID Principles

SRP — Single Responsibility

• One class = one responsibility
• Separate: TCustomerValidator, TCustomerRepository, TCustomerService
• Do not put business logic in forms

OCP — Open/Closed

• Use interfaces for extension
• New features = new classes implementing existing interface

LSP — Liskov Substitution

• Any implementation of ICustomerRepository must work interchangeably

ISP — Interface Segregation

• IReadableRepository<T> separated from IWritableRepository<T>
• Small and cohesive interfaces

DIP — Dependency Inversion

• Always use constructor injection
• Dependencies via interfaces, never concrete classes

Repository Creation Pattern

//1. Interface no Domain
ICustomerRepository = interface
  ['{30394253-A077-4363-A974-6477DDAB5671}']
  function FindById(AId: Integer): TCustomer;
  procedure Save(ACustomer: TCustomer);
end;

//2. Implementation on Infrastructure
TCustomerRepository = class(TInterfacedObject, ICustomerRepository)
  constructor Create(AConnection: TFDConnection);
end;

Service Creation Pattern

//1. Interface no Application
ICustomerService = interface
  ['{0231C34F-7161-49D0-88C2-D326B67ABB38}']
  procedure CreateCustomer(const AName, ACpf: string);
end;

//2. Implementation with constructor injection
TCustomerService = class(TInterfacedObject, ICustomerService)
  constructor Create(ARepository: ICustomerRepository);
end;

Checklist

• Interfaces defined in the Domain
• Constructor injection for all dependencies
• Guard clauses at the beginning of methods
• Try/finally for temporary objects
• XMLDoc in public methods
• No global variables, no with, no generic catch