The Dependency Inversion Principle (DIP)

The Dependency Inversion Principle states that:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g., interfaces).
2. Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.

Source: Wikipedia

Other SOLID Principles

• Single Responsibility principle
• Open Closed Principle
• Liskov Substitution Problem
• Interface Segregation Principle

Example without DIP

public class UserRepository
{
    public string GetUserById(int id)
    {
        // Code to fetch user from the database
        return "User";
    }
}

public class UserService
{
    private readonly UserRepository _userRepository;
    public UserService()
    {
        _userRepository = new UserRepository();
    }
    public string GetUser(int id)
    {
        return _userRepository.GetUserById(id);
    }
}

One problem with this code is , theUserService is tightly coupled with the UserRepository. Another problem is, if you want to use different implementation of UserReposoitory, you can not easily do it. For instance you want to swap your UserRepsoitory to UserRepositoryWithDifferentDatabase.

With DIP

public interface IUserRepository
{
    string GetUserById(int id);
}

public class UserRepository:IUserRepository
{
    public string GetUserById(int id)
    {
        // Code to fetch user from the database
        return "User";
    }
}

public class UserService
{
    private readonly IUserRepository _userRepository;
    public UserService(IUserRepository userRepository)
    {
        _userRepository = userRepository;
    }
    public string GetUser(int id)
    {
        return _userRepository.GetUserById(id);
    }
}


public class DIPTest
{
    public static void Main()
    {
        UserService userService = new UserService(new UserRepository());
        Console.WriteLine(userService.GetUser(2));
    }
}

Now, UserService is depend on abstraction (IUserRepository). UserService is not responsible for instantiating the UserRepository. It is loosely coupled now. If you have another repository (eg. UserRepositoryWithDifferentDatabase), which implements the IUserRepository, you can pass it's instance instead of UserRepository. User service don't care if you pass the instance of UserRepository or UserRepositoryWithDifferentDatabase, because both implements the same interface.

What is inversion?

You might be wondering what is inversion? What are we inverting here? First we need to understand the traditional approach. Traditionally high level module depends on the low level module. If we consider our example, our dependency direction is UserService -> UserRepository, UserService is depending on the UserRepository.

Using the DIP, UserService depends on IUserRepository. So we have changed the flow of dependency (UserService -> IUserRepository)

I personally find this name “Dependency Inversion” very confusing, specially the term “Inversion”. But it is what it is. I would encourage you to search more about it.

References:

https://stackoverflow.com/questions/27978841/what-is-meant-by-inversion-in-dependency-inversion

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