C# and .NET — The Platform, the Language and the Toolchain

C# is a statically-typed, object-oriented language created by Microsoft and first released in 2002. It runs on the .NET platform — a runtime and class library that abstracts the operating system and provides a managed execution environment. Understanding the relationship between C# (the language you write) and .NET (the platform that runs it) is the foundation for everything in this series. The ASP.NET Core Web API stack you will build in Parts 3 and 4, the Angular frontend in Part 5, and the SQL Server integration in Part 6 all run on this foundation.

The .NET Platform #

C# Source Code (.cs files)
        ↓  C# Compiler (Roslyn)
Intermediate Language (IL / MSIL) inside .dll or .exe
        ↓  CLR: JIT Compiler (at runtime, per method)
Native Machine Code (x64, ARM64, etc.)
        ↓
Execution on any supported OS

Key components:
  CLR    — Common Language Runtime (memory management, GC, JIT, exceptions)
  BCL    — Base Class Library (System.*, Collections, IO, Net, Threading, ...)
  SDK    — dotnet CLI, MSBuild, project templates, publish tools
  NuGet  — package manager (like npm for JavaScript, pip for Python)

.NET versions — always use .NET 8 (current LTS):
  .NET Framework 1.0–4.8  — Windows only, legacy — do NOT use for new projects
  .NET Core 1.0–3.1       — Cross-platform (renamed to just .NET from v5)
  .NET 5 / 6 / 7 / 8 / 9 — Unified cross-platform runtime (current)

Your First C# Program #

# Create a new console application
dotnet new console -n HelloWorld
cd HelloWorld

# Project structure:
# HelloWorld/
# ├── HelloWorld.csproj   ← MSBuild project file (like package.json)
# ├── Program.cs          ← your code (top-level statements)
# ├── obj/                ← intermediate build files (git-ignore this)
# └── bin/                ← compiled output (git-ignore this)

// Program.cs — top-level statements (.NET 6+)
// No class declaration or static void Main() needed

Console.WriteLine("Hello, World!");
Console.WriteLine($".NET version: {Environment.Version}");

// The compiler automatically generates this behind the scenes:
// namespace HelloWorld;
// internal class Program
// {
//     static void Main(string[] args)
//     {
//         Console.WriteLine("Hello, World!");
//         Console.WriteLine($".NET version: {Environment.Version}");
//     }
// }
// You never write this boilerplate by hand for entry-point files.

dotnet run
# Hello, World!
# .NET version: 8.0.x

The .csproj Project File #

<!-- HelloWorld.csproj — MSBuild XML project definition -->
<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>                   <!-- produces a console executable -->
    <TargetFramework>net8.0</TargetFramework>       <!-- target .NET 8 LTS -->
    <Nullable>enable</Nullable>                     <!-- nullable reference type warnings -->
    <ImplicitUsings>enable</ImplicitUsings>         <!-- auto-imports System, Linq, etc. -->
  </PropertyGroup>

</Project>

<!-- An ASP.NET Core Web API project uses Microsoft.NET.Sdk.Web instead -->
<!-- and does not need the OutputType element -->

Essential dotnet CLI Commands #

Common Mistakes #

Mistake 1 — Targeting .NET Framework instead of .NET 8 #

❌ Wrong — legacy target that is Windows-only:

<TargetFramework>net472</TargetFramework>   <!-- .NET Framework 4.7.2 — legacy! -->

✅ Correct:

<TargetFramework>net8.0</TargetFramework>   <!-- .NET 8 LTS — modern, cross-platform -->

Mistake 2 — Committing obj/ and bin/ folders to git #

dotnet new creates a .gitignore automatically that excludes bin/ and obj/. Always use it — these folders contain compiled binaries regenerated by dotnet build and should never be in source control.