Modern .NET — Hosting, Configuration, DI & Deployment

.NET is Microsoft's free, open-source, cross-platform developer platform. Today ".NET" (5, 6, 7, 8, 9, 10…) means the unified, cross-platform successor of .NET Core. The older .NET Framework (1.0–4.8.1) is Windows-only and in maintenance mode.

Building blocks: runtime (CLR, GC, JIT), libraries (BCL), SDK (compilers, dotnet CLI, MSBuild), ASP.NET Core and Windows Desktop runtimes.

Release cadence: a new major version every November. Even numbers are LTS (3 years of support); odd numbers are STS (24 months since .NET 9; previously 18). Monthly patch releases ("Patch Tuesday").

Application startup lifecycle (generic host / ASP.NET Core):

1. Main / top-level statements run.
2. WebApplication.CreateBuilder(args) (or Host.CreateApplicationBuilder) creates the host builder: loads configuration (appsettings.json → appsettings.{Environment}.json → user secrets in Development → environment variables → command line), sets up logging and the DI container.
3. You register services: builder.Services.Add....
4. builder.Build() builds the service provider and the app.
5. You configure the middleware pipeline (app.Use...) and endpoints.
6. app.Run() starts the host: IHostedService.StartAsync for each hosted service, then Kestrel listens.
7. Requests are processed through middleware → endpoint.
8. On shutdown (Ctrl+C / SIGTERM): IHostApplicationLifetime.ApplicationStopping fires, hosted services StopAsync, the container disposes singletons.

Build/compile lifecycle: source → Roslyn → IL assembly (.dll) + metadata + PDB → (publish) → at runtime JIT → native code. With Native AOT, IL is compiled to a native executable at publish time (no JIT, fast startup, smaller memory, but no runtime code generation and limited reflection).

Support lifecycle: Preview → RC (go-live) → GA (November) → patches → end of support (LTS 3 years, STS 2 years).