1. What C# and .NET are

C# is the language; .NET is the platform (runtime + base class library + SDK). You write C#, the compiler produces IL, and the .NET runtime executes it.

• CLR (Common Language Runtime): JIT, GC, type safety, exception handling, threading.
• BCL (Base Class Library): System, System.Collections.Generic, System.IO, System.Net.Http, ...
• CTS / CLS: the common type system that lets C#, F# and VB.NET interoperate.

// Program.cs — a complete program using top-level statements (C# 9+)
Console.WriteLine("Hello, Divakar!");
var name = args.Length > 0 ? args[0] : "world";
Console.WriteLine($"Hello, {name}");

dotnet new console -n HelloApp
cd HelloApp
dotnet run -- Divakar

Interview tip: "Managed code" = code whose execution is managed by the CLR (memory, security, exceptions). "Unmanaged code" = native code outside the CLR, e.g. Win32 DLLs called via P/Invoke.