.NET: CLR, Ecosystem & Project Structure

.NET is Microsoft's cross-platform, open-source framework for building web, desktop, mobile, cloud, and IoT applications. .NET 8 (LTS) and .NET 9 are current. The CLR (Common Language Runtime) provides JIT compilation, garbage collection, and type safety.

.NET Ecosystem Overview

• ASP.NET Core: web APIs, MVC, Razor Pages, Blazor — runs on Linux/Mac/Windows

• Entity Framework Core: ORM for SQL databases (PostgreSQL, SQL Server, SQLite)

• MAUI: cross-platform native UI (iOS, Android, Windows, macOS)

• Blazor: C# in the browser (WebAssembly) or server-side rendering

• .NET CLI: dotnet new, build, run, test, publish — the primary tool

• NuGet: package manager (nuget.org). Packages declared in .csproj.

• LTS vs STS: even versions (.NET 8, 10) are LTS (3 years); odd versions (9, 11) are STS (18 months)

Project Structure

# Create new projects
dotnet new webapi -n MyApi --use-controllers   # REST API with controllers
dotnet new webapi -n MyApi                     # Minimal API (no controllers)
dotnet new mvc -n MyMvc                        # MVC with Razor views
dotnet new classlib -n MyLib                   # Class library
dotnet new xunit -n MyLib.Tests                # xUnit test project

# Solution (groups multiple projects)
dotnet new sln -n MySolution
dotnet sln add src/MyApi/MyApi.csproj
dotnet sln add tests/MyApi.Tests/MyApi.Tests.csproj

# Common commands
dotnet run                   # run the project
dotnet watch run             # run with hot reload
dotnet build                 # compile
dotnet test                  # run tests
dotnet add package Npgsql.EntityFrameworkCore.PostgreSQL
dotnet publish -c Release -o ./publish

<!-- MyApi.csproj — project file -->
<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <RootNamespace>MyApi</RootNamespace>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="9.0.0" />
    <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="9.0.0" />
    <PackageReference Include="Swashbuckle.AspNetCore" Version="7.2.0" />
  </ItemGroup>
</Project>

Key .NET Concepts

// Namespaces and using
using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.DependencyInjection;

// Records (immutable data classes — C# 9+)
public record User(int Id, string Name, string Email);

// var — type inference
var numbers = new List<int> { 1, 2, 3 };
var user = new User(1, "Alice", "alice@example.com");

// LINQ — integrated query language
var admins = users
    .Where(u => u.Role == "admin")
    .OrderBy(u => u.Name)
    .Select(u => new { u.Id, u.Name })
    .ToList();

// Nullable reference types (enabled in modern .NET)
string? nullable = null;
string nonNullable = "hello";  // compiler warns if this could be null

// Pattern matching
object obj = "hello";
if (obj is string s && s.Length > 3)
    Console.WriteLine(s.ToUpper());

// Switch expression
string label = score switch {
    >= 90 => "A",
    >= 80 => "B",
    >= 70 => "C",
    _ => "F"
};