2. First program, namespaces and keywords

Classic program structure (what every C# file looked like before C# 9, and what top-level statements generate for you):

using System;                       // import a namespace so you can write Console instead of System.Console

namespace Divakar.Shop              // logical grouping of types; usually matches the folder
{
    internal class Program          // every method lives inside a type
    {
        static void Main(string[] args)   // entry point: static, named Main, returns void or int (or Task/Task<int>)
        {
            Console.WriteLine("Hello, Divakar!");
            Console.Write("Your name: ");
            string? name = Console.ReadLine();
            Console.WriteLine($"Welcome {name}");   // string interpolation
        }
    }
}

Modern equivalent (C# 10+):

// GlobalUsings.cs — applies to every file in the project
global using System.Text.Json;

// Customer.cs — file-scoped namespace saves one level of indentation
namespace Divakar.Shop.Models;
public class Customer { public string Name { get; set; } = ""; }

// Program.cs — top-level statements: the compiler creates Program and Main
using Divakar.Shop.Models;
var c = new Customer { Name = "Asha" };
Console.WriteLine(JsonSerializer.Serialize(c));

Namespaces:

• Prevent name clashes (Divakar.Shop.Models.Order vs Divakar.Billing.Order).
• using imports a namespace; using Alias = Some.Long.Namespace.Type; creates an alias; using static System.Math; lets you write Sqrt(2).
• Implicit usings (<ImplicitUsings>enable</ImplicitUsings> in the .csproj) auto-import System, System.Linq, System.Collections.Generic, System.IO, System.Threading.Tasks and more.
• A namespace is not an assembly: one assembly (DLL) can contain many namespaces and one namespace can span many assemblies.

Keywords are reserved words with special meaning: class, int, if, return, public, static, new, this, base, null, true... You can't use them as identifiers unless prefixed with @ (int @class = 5;). Contextual keywords are only special in certain places and can otherwise be used as names: var, async, await, get, set, init, value, yield, record, required, where, select, field (C# 14).

Naming conventions: PascalCase for types, methods, properties and namespaces; camelCase for locals and parameters; _camelCase for private fields; interfaces start with I (IRepository); async methods end with Async.