C# Language — Basics to Advanced

C# (pronounced "C sharp") is a modern, object-oriented, type-safe language created by Microsoft (lead designer Anders Hejlsberg) and standardised as ECMA-334. It compiles to IL (Intermediate Language) which the .NET runtime (CLR) turns into machine code with a JIT (or ahead of time with Native AOT).

Where C# is used:

• Web APIs and web apps — ASP.NET Core (MVC, Razor Pages, Web API, Blazor)
• Desktop — WPF, WinForms, .NET MAUI, WinUI
• Cloud and microservices — Azure Functions, containers, gRPC
• Games — Unity
• Data, ETL and background workers — Worker Services, console apps

Why interviewers care: almost every .NET round starts with C# fundamentals (value vs reference types, OOP, collections, LINQ, async) before moving to frameworks.

Interview tip: you don't need every row. Know the big ones: generics (2.0), LINQ (3.0), async/await (5.0), nullable reference types (8.0), records (9.0), and the version you use at work.

How a C# program runs

1. Write .cs files inside a project (.csproj).
2. Build — the Roslyn compiler turns C# into IL plus metadata, packaged in an assembly (.dll/.exe).
3. Load — the CLR loads the assembly, verifies the IL and resolves dependencies.
4. JIT compile — each method is compiled to native code the first time it is called (tiered compilation re-optimises hot methods). Native AOT compiles everything ahead of time instead.
5. Execute — managed code runs with type safety, exception handling and the garbage collector managing the heap.
6. Unload / exit — finalisers and IDisposable cleanup run; the process ends.

Object lifecycle: new → constructor runs → object lives on the managed heap while referenced → becomes unreachable → collected by the GC (Gen 0 → 1 → 2) → finaliser (if any) runs. Deterministic cleanup of unmanaged resources is done with Dispose() / using.