Microsoft's cutting-edge, quick, cross-platform, and open-source framework for creating a wide range of applications, from web apps and APIs to console apps and cloud-native microservices, is called.NET Core (now a part of the.NET 5+ unified platform). For novices who wish to comprehend what.NET Core is, how it functions, and the structure of an actual ASP.NET Core project, this article provides the most straightforward explanation of the framework.

1. What is .NET Core?
.NET Core is Microsoft’s next-generation application development framework, built to overcome the limitations of the old .NET Framework.

Why was .NET Core created?
The old .NET Framework could run only on Windows, was heavy, and was not suitable for cloud, containers, and modern architecture.

.NET Core solves all of these issues.

Key Features of .NET Core
1. Cross-Platform
You can develop and run apps on:

• Windows
• Linux
• macOS

You can host apps on IIS, Apache, Nginx, Kestrel, Docker, or the cloud.

2. Open Source

• Available on GitHub
• Anyone can read or contribute to the source code
• Community-driven improvements

3. High Performance
One of the fastest web frameworks in the world
Handles more traffic with less hardware
Perfect for APIs, enterprise apps, and large-scale cloud systems.

4. Lightweight & Modular
You install only what you need using NuGet packages, which makes applications fast and optimized.

5. Built-in Dependency Injection
Dependency Injection (DI) is built into the framework — no need for third-party libraries.

DI makes apps:

• Cleaner
• Easier to test
• More modular

6. Regular Updates
Microsoft releases new versions every year, including LTS (Long-Term Support) versions for stability.

2. ASP.NET vs ASP.NET Core — What’s the Difference?
ASP.NET Core is a complete redesign of ASP.NET — not just a small upgrade.

3. Understanding .NET Core Project Structure

When you create a new ASP.NET Core project, you get several important files and folders. Each plays a special role.
3.1 Program.cs

This is the entry point of your application.

What happens here?
Creates and configures the web host

• Registers services (Database, Logging, Authentication)
• Defines the middleware pipeline
• Maps controllers/endpoints

Think of Program.cs as the “main switchboard” that controls your entire app.

3.2 wwwroot Folder
Everything inside this folder is public.

Used for:

• CSS files
• JavaScript
• Images
• Bootstrap files

A browser can directly access these files using URLs.
wwwroot = Your public website folder.

3.3 Controllers Folder
Controllers:
Receive HTTP requests
Run logic
Return responses (JSON, HTML, etc.)

Example actions:

• GET → Read data
• POST → Create data
• PUT → Update data
• DELETE → Remove data

Controllers are like the reception desk of your app.

3.4 appsettings.json
This is your configuration file.

Used for:

• Database connection strings
• API keys
• Logging settings

Email settings
You can also have:
appsettings.Development.json
appsettings.Production.json
appsettings.json is the “control panel” of your project.

3.5 Other Common Folders
Services
Contains business logic.

Data
Contains:

• DbContext
• Migrations
• Entities

Repositories
Handles database CRUD operations.

DTOs
Used to transfer data safely.

These folders are like the “kitchen and back office.”
They do all the behind-the-scenes work.

4. What is Middleware?
Middleware is the heart of ASP.NET Core.
It is a chain of components that process every request and response.

How Middleware Works?
Request → Middleware 1 → Middleware 2 → Middleware 3 → Controller → Response → Back through same middlewares

Key Points About Middleware

• Runs one-by-one in the order you configure.
• Can modify request or response.
• Can stop the request early (called short-circuiting).
• Used for Logging, Authentication, Routing, Error Handling, etc.

Understanding the Complete Request Pipeline
Let’s break down each stage in the simplest way.

1. Request
When the user sends a request:
Method: GET / POST / PUT / DELETE

URL: /api/products/5

Headers: Auth token, content type

Body: JSON data (for POST/PUT)

2. Logging Middleware
Tracks

• Which URL was called
• Who called
• How long did the request take
• What was the final status code

Useful for

• Debugging
• Performance monitoring
• Auditing

3. Routing
Matches URL → Correct controller action.

Without routing, the application does not know where to send a request.

4. Authentication
Authentication answers:
“Who are you?”

Examples

• JWT Token
• Cookies
• OAuth

If invalid → Later returns 401 Unauthorized

5. Authorization
Authorization answers:
“Are you allowed to do this?”

Example

• Admin-only routes
• Checking user roles
• Checking user claims

If not allowed → 403 Forbidden

6. Controller Execution
Here, the actual processing happens:

• Validating data
• Calling database
• Applying business rules
• Returning response (JSON / HTML)

7. Response
Response goes back through the pipeline and finally returns:

• Status code (200/404/401/403/500)
• Headers
• Body (JSON/HTML)

Why Middleware Order Matters?

• Routing should come before authentication
• Authentication must come before authorization
• Static files should be before MVC
• Error handling needs to be at the top

Incorrect order → Errors like:

• 404 Not Found
• 401 Unauthorized
• Authorization not working

When Things Go Wrong - Quick Fix Guide
401 - Unauthorized
Problem: No identity.
Fix: Check token/cookie + authentication config

403 - Forbidden
Problem: User is known but not allowed.
Fix: Add required roles/claims or change policy

404 - Not Found
Problem: Route not matched.
Fix: Check controller routes and middleware order

Pipeline issues
If things randomly break →
Fix: Ensure correct order:
UseRouting()
UseAuthentication()
UseAuthorization()
MapControllers()