Dependency Injection in ASP.NET Core

Introduction

When building applications, we often create objects inside classes. But this creates tight coupling, making code hard to maintain.

ASP.NET Core solves this problem using Dependency Injection (DI).

Dependency Injection is one of the most important concepts in modern development.

In this article, you will learn:

What Dependency Injection is
Why we use it
Types of DI
Step-by-step example
Real-world usage

What is Dependency Injection?

Dependency Injection means providing required objects (dependencies) from outside instead of creating them inside the class.

Without Dependency Injection ❌

public class StudentController
{
    private StudentService service = new StudentService();
}

Problem

Tight coupling
Hard to test
Hard to maintain

With Dependency Injection ✔

public class StudentController
{
    private readonly StudentService _service;

    public StudentController(StudentService service)
    {
        _service = service;
    }
}

Benefit

Loose coupling
Easy testing
Better structure

Types of Dependency Injection

ASP.NET Core supports 3 types:

1️⃣ Transient

builder.Services.AddTransient<StudentService>();

👉 New object every time

2️⃣ Scoped

builder.Services.AddScoped<StudentService>();

👉 One object per request

3️⃣ Singleton

builder.Services.AddSingleton<StudentService>();

👉 Same object for entire application

Step-by-Step Example

Step 1: Create Interface

public interface IStudentService
{
    string GetStudentName();
}

Step 2: Create Service

public class StudentService : IStudentService
{
    public string GetStudentName()
    {
        return "Abhay";
    }
}

Step 3: Register Service

builder.Services.AddScoped<IStudentService, StudentService>();

Step 4: Use in Controller

public class HomeController : Controller
{
    private readonly IStudentService _service;

    public HomeController(IStudentService service)
    {
        _service = service;
    }

    public IActionResult Index()
    {
        var name = _service.GetStudentName();
        return Content(name);
    }
}

Output

Abhay

Real-World Example

Dependency Injection is used in:

Database services
Logging
API services
Authentication

Why Dependency Injection is Important?

Makes code flexible
Easy to change implementation
Improves testing
Industry standard

When to Use Which Lifetime?

Scenario	Use
Lightweight service	Transient
Database context	Scoped
Global service	Singleton

Common Mistakes

Using Singleton for database
Not using interface
Creating object manually

Best Practice

Always use interfaces
Use Scoped for DB
Avoid tight coupling

Conclusion

Dependency Injection is a powerful concept in ASP.NET Core that helps build clean and maintainable applications.

In this article, we learned:

What DI is
Types of DI
Step-by-step example
Real-world usage