🚀 Dependency Injection in ASP.NET Core – A Complete Beginner-to-Pro Guide

If you’ve been working with ASP.NET Core, you’ve probably seen the term Dependency Injection (DI) everywhere. It’s one of the most powerful features of .NET Core — but also one of the most misundersto

🔹 What is Dependency Injection (DI)?

Dependency Injection is a design pattern that allows classes to get their dependencies (services, repositories, etc.) from the outside, instead of creating them directly.

👉 In simple words:
Instead of a class creating the objects it needs, ASP.NET Core provides them for you.

This makes your code:

• ✅ More testable

• ✅ More maintainable

• ✅ Less tightly coupled

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🔹 Example Without Dependency Injection

public class ProductService  
{  
    private readonly ProductRepository _repo;  

    public ProductService()  
    {  
        _repo = new ProductRepository(); // tightly coupled  
    }  

    public void GetProducts() => _repo.GetAll();  
}  

Here, ProductService directly creates a ProductRepository.
Problem? If tomorrow you switch to SqlProductRepository or ApiProductRepository, you’ll need to change the class.

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🔹 Example With Dependency Injection

public class ProductService  
{  
    private readonly IProductRepository _repo;  

    public ProductService(IProductRepository repo)  
    {  
        _repo = repo;  
    }  

    public void GetProducts() => _repo.GetAll();  
}

Now, ProductService doesn’t care which repository you’re using.
ASP.NET Core injects the right one when the app runs.

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🔹 Registering Services in ASP.NET Core

You register dependencies in the Program.cs (or Startup.cs in older versions).

builder.Services.AddScoped<IProductRepository, SqlProductRepository>();
builder.Services.AddScoped<ProductService>();

When ProductService is called, .NET Core automatically provides an instance of SqlProductRepository.

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🔹 Service Lifetimes in DI

ASP.NET Core DI supports three lifetimes:

• Transient → New instance every time (AddTransient)

• Scoped → One instance per request (AddScoped)

• Singleton → One instance for the entire application (AddSingleton)

⚡ Choosing the right one is crucial for performance and memory usage.

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🔹 Why Dependency Injection Matters?

• Makes unit testing easier with mocks.

• Allows flexibility in swapping implementations.

• Reduces tight coupling and improves maintainability.

• Encourages clean architecture (SOLID principles).

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✅ In short: Dependency Injection is not just a feature of ASP.NET Core; it’s the backbone of writing clean, scalable, and testable code.

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