🚀 Dependency Injection in ASP.NET Core

Introduction

When building applications using ASP.NET Core, we often create objects inside classes.

Example:

ProductService service = new ProductService();

But this is not a good practice in large applications.

Why?

Because:

• Code becomes tightly coupled

• Hard to test

• Hard to maintain

• Difficult to scale

To solve this problem, ASP.NET Core uses:

Dependency Injection (DI)

In this article, we will learn:

• What is Dependency Injection?

• Why do we use it?

• How it works?

• Step-by-step example

• Output explanation

Everything in simple words.

What is Dependency Injection?

Dependency Injection means:

Instead of creating an object inside a class, we provide (inject) the object from outside.

Simple meaning:

“Don’t create dependency. Receive dependency.”

Why Use Dependency Injection?

Without DI:

• Classes depend on specific implementations

• Difficult to change code

• Hard to write unit tests

With DI:

• Loose coupling

• Easy testing

• Clean code

• Better maintainability

Real-Life Example

Think like this:

• A Car needs an Engine

• Instead of building the engine inside the car

• The engine is provided from outside

That is Dependency Injection.

Step-by-Step Example in ASP.NET Core

Step 1 – Create Interface

Create a folder: Services

Add Interface:

public interface IMessageService
{
    string GetMessage();
}

Step 2 – Create Service Class

public class MessageService : IMessageService
{
    public string GetMessage()
    {
        return "Hello from Dependency Injection!";
    }
}

Step 3 – Register Service in Program.cs

Open Program.cs:

builder.Services.AddScoped<IMessageService, MessageService>();

This tells ASP.NET Core:

• Whenever IMessageService is required

• Provide MessageService object

Step 4 – Use in Controller

using Microsoft.AspNetCore.Mvc;

public class HomeController : Controller
{
    private readonly IMessageService _messageService;

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

    public IActionResult Index()
    {
        var message = _messageService.GetMessage();
        ViewBag.Message = message;
        return View();
    }
}