Dotnet Core - DI(Dependency Injection) vs DIP(Dependency Inversion Principle)

 🧩 The “D” in SOLID — Dependency Inversion Principle (DIP)

Dependency Inversion Principle (DIP) is the design principle behind the idea of Dependency Injection.

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🧠 What DIP States

High-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.

In simpler words:

• Instead of classes depending on concrete implementations, they should depend on interfaces or abstract classes.
• This makes the system more flexible and loosely coupled.

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🧱 Example Without DIP (Bad Design)

public class NotificationService
{
    private EmailService _emailService = new EmailService();

    public void Notify(string message)
    {
        _emailService.SendEmail(message);
    }
}

public class EmailService
{
    public void SendEmail(string message)
    {
        Console.WriteLine($"Email sent: {message}");
    }
}

Problem:

• NotificationService depends directly on a concrete EmailService.
• If you want to switch to SmsService, you must modify NotificationService.

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✅ Example With DIP (Good Design)

public interface IMessageService
{
    void Send(string message);
}

public class EmailService : IMessageService
{
    public void Send(string message) => Console.WriteLine($"Email: {message}");
}

public class SmsService : IMessageService
{
    public void Send(string message) => Console.WriteLine($"SMS: {message}");
}

public class NotificationService
{
    private readonly IMessageService _messageService;

    public NotificationService(IMessageService messageService)
    {
        _messageService = messageService;
    }

    public void Notify(string message)
    {
        _messageService.Send(message);
    }
}

Now:

• NotificationService depends on the abstraction IMessageService.
• You can easily switch between implementations (Email, SMS, Push).

✅ This is Dependency Inversion Principle in action.

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🧩 How Dependency Injection (DI) Fits In

Now that we understand DIP as a principle, here’s where DI comes in:

Concept	What it is
DIP (Principle)	A guideline that says “depend on abstractions, not concretions.”
DI (Pattern)	A technique or mechanism to implement DIP in code.

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🔄 Relationship Between DIP and DI

Let’s visualize it:

DIP → is the principle (what you should do)
DI  → is the method (how you do it)

So:

• DIP is about how classes should relate to each other (architectural rule).
• DI is about how to supply those dependencies (technical mechanism).

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🧰 Example: Applying DI to Implement DIP

In .NET Core:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddScoped<IMessageService, EmailService>();
builder.Services.AddScoped<NotificationService>();

var app = builder.Build();

Here, DI container ensures NotificationService receives an implementation of IMessageService.
That’s Dependency Injection in practice.
And since both classes depend on abstractions, that’s Dependency Inversion Principle applied.

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🧠 Summary Comparison

Aspect	Dependency Inversion Principle (DIP)	Dependency Injection (DI)
Type	Principle (part of SOLID)	Design Pattern / Technique
Purpose	To reduce coupling between high-level and low-level modules	To supply dependencies to classes at runtime
Focus	Architecture & relationships between classes	Implementation & object creation
Enforced By	Developer’s design	Framework or DI container
In .NET Core	Interfaces like IService, IRepository	AddScoped, AddSingleton, AddTransient

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💬 Analogy

Concept	Analogy
DIP	“Don’t depend on specific brands; depend on a plug standard.”
DI	“Use a socket that provides power to any plug that fits.”

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🧾 Quick Recap

✅ DIP (Dependency Inversion Principle) = design principle (the “D” in SOLID)

✅ DI (Dependency Injection) = pattern / implementation of that principle

💡 DIP gives the rule, DI provides the tool to achieve it