How to Implement Dependency Injection Manually in Go

Manually implement dependency injection in Go by defining interfaces for your dependencies and passing concrete implementations into your struct via a constructor function.

type Repository interface {
    Get(id string) (*User, error)
}

type UserService struct {
    repo Repository
}

func NewUserService(repo Repository) *UserService {
    return &UserService{repo: repo}
}

func (s *UserService) GetUser(id string) (*User, error) {
    return s.repo.Get(id)
}

// Usage
func main() {
    realRepo := &PostgresRepository{}
    service := NewUserService(realRepo)
    _ = service.GetUser("123")
}

Dependency injection is a technique where you provide a program's required components from the outside rather than letting it create them internally. This makes your code easier to test because you can swap real components with fake ones. Think of it like handing a worker the specific tools they need for a job instead of forcing them to build their own tools first.