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DIVAKAR UPADHYAY
DIVAKAR UPADHYAY

Posted on Originally published at devladder.pages.dev

First program, namespaces and keywords (C# Language)

#dotnet #csharp #beginners #interview

šŸ‘‰ Full lesson with practice questions on DevLadder (free, lesson 2 of 38 in the C# Language: Basics to Advanced course)

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
        }
    }
}
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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));
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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.

Interview questions on C# Language

What is the difference between a value type and a reference type?

A value type variable holds the data itself (int, bool, decimal, DateTime, any struct/enum). Assigning it copies the value. A reference type variable holds a reference to an object on the managed heap (class, string, arrays, delegates, record). Assigning it copies the reference, so two variables can point to the same object.

var a = new List<int>(); var b = a; b.Add(1);   // a.Count == 1: same object
int x = 1; int y = x; y++;                       // x is still 1
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Value types are stored inline (stack for locals, inside the parent object for fields); reference-type objects always live on the heap and are cleaned up by the GC.

What is the difference between string and StringBuilder?

string is immutable: every concatenation creates a new string object, so building a string in a loop causes many allocations (O(nĀ²) copying). StringBuilder keeps a mutable buffer and appends in place; call ToString() once at the end. Use StringBuilder for loops or many appends, plain strings/interpolation for a handful of pieces.

Explain const vs readonly vs static readonly.

  • const: compile-time constant, implicitly static, only primitives/strings; the value is baked into callers at compile time.
  • readonly: instance field assigned at declaration or in a constructor; can differ per object.
  • static readonly: one value per type, computed at runtime (e.g. static readonly HttpClient Client = new()).

Gotcha: if library A has public const int Max = 10 and app B uses it, changing it to 20 in A does nothing in B until B is recompiled. Use static readonly for values that may change.

Read the full lesson with progress tracking and the rest of the course on DevLadder: free step-by-step tutorials and interview questions for developers.

Next in the course: Data types, numbers and type conversion ā†’

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