Lesson 1 / 25

What .NET Is

Explain the SDK, runtime, IL, JIT and base class library, and the .NET release cadence.

A language, a runtime and a library

C# is a statically typed, object-oriented language with strong functional features. It runs on .NET, a free, open-source, cross-platform platform for Windows, Linux and macOS. The C# compiler (Roslyn) turns your code into Intermediate Language (IL) stored in an assembly (.dll). At run time the Common Language Runtime (CLR) loads the assembly and its JIT compiler turns IL into native machine code, method by method, with tiered compilation that re-optimises hot code. The CLR also provides garbage collection, type safety and exception handling. The base class library (BCL) gives you collections, I/O, networking, JSON, threading and much more. Modern .NET (5 and later, unified from .NET Core) is distinct from the older Windows-only .NET Framework 4.x. A new major version ships every November: even-numbered releases such as .NET 10 are Long Term Support (LTS), supported for three years, and odd-numbered releases are Standard Term Support (STS).

From source code to running program

C# compiles to IL; the runtime JIT-compiles IL to native code while the program runs.

A source document flowing into a compiler gear, producing an intermediate package, which flows into a runtime box containing a lightning-bolt compiler and a recycling symbol for garbage collection. — Figure 1.1 — C# source, IL assembly, CLR with JIT and garbage collector.

Checking your installation

The SDK includes the compiler, runtime and the dotnet CLI.

dotnet --info            # SDK and runtime versions, OS, architecture
dotnet --list-sdks
dotnet --list-runtimes

# pin the SDK for a repository so everyone builds with the same version
dotnet new globaljson --sdk-version 10.0.100 --roll-forward latestFeature

Framework is not Core

Tutorials for .NET Framework (Web Forms, web.config, System.Web) do not apply to modern .NET. For new projects, target the current LTS release unless you have a reason not to.

Quick check: What does the JIT compiler in the CLR do?

Turns IL into native machine code while the program runs
Turns C# into IL at build time
Collects unused objects
Downloads NuGet packages

Answer

Turns IL into native machine code while the program runs — Roslyn produces IL at build time; the JIT converts IL to native code at run time.