C# Core Study Guide

Overview

C# (pronounced C-Sharp) is a modern, object-oriented programming language developed by Microsoft. It is widely used for:

Backend development with ASP.NET Core
Desktop applications
Cloud services
Game development with Unity
Enterprise software
APIs and microservices

C# runs on the .NET platform and provides powerful features such as:

Automatic memory management
Strong type safety
Asynchronous programming
LINQ for data querying
Cross-platform support

Why Learn C#

Advantages

Feature	Description
Object-Oriented	Encourages reusable and maintainable code
Cross-Platform	Works on Windows, Linux, and macOS
High Performance	Optimized runtime with JIT compilation
Rich Ecosystem	Large .NET ecosystem and libraries
Enterprise Ready	Widely used in enterprise systems
Cloud Friendly	Excellent integration with Azure

.NET Ecosystem

Installing the Environment

Install .NET SDK

Official website: https://dotnet.microsoft.com/download

Verify installation:

dotnet --version

Your First C# Program

using System;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello, World!");
    }
}

Keyword	Meaning
using	Imports namespaces
class	Defines a class
static	Belongs to the class itself
void	No return value
Main	Entry point of application

Variables and Data Types

int age = 25;
string name = "John";
double salary = 5000.50;
bool isActive = true;

Type	Description	Example
int	Integer	10
double	Floating point	10.5
decimal	Financial calculations	99.99m
bool	True/False	true
char	Single character	'A'
string	Text	"Hello"

Type Conversion

Implicit Casting

int number = 10;
double value = number;

Explicit Casting

double price = 9.99;
int rounded = (int)price;

Parsing

string text = "123";
int number = int.Parse(text);

Operators

Arithmetic Operators

Operator	Meaning
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulus

Comparison Operators

int a = 10;
int b = 20;

Console.WriteLine(a < b);
Console.WriteLine(a == b);

Conditional Statements

if Statement

int age = 18;

if (age >= 18)
{
    Console.WriteLine("Adult");
}
else
{
    Console.WriteLine("Minor");
}

switch Statement

int day = 2;

switch(day)
{
    case 1:
        Console.WriteLine("Monday");
        break;

    case 2:
        Console.WriteLine("Tuesday");
        break;

    default:
        Console.WriteLine("Unknown");
        break;
}

Loops

for Loop

for(int i = 0; i < 5; i++)
{
    Console.WriteLine(i);
}

while Loop

int count = 0;

while(count < 5)
{
    Console.WriteLine(count);
    count++;
}

foreach Loop

string[] fruits = { "Apple", "Orange", "Banana" };

foreach(var fruit in fruits)
{
    Console.WriteLine(fruit);
}

Methods

static int Add(int a, int b)
{
    return a + b;
}

int result = Add(5, 3);
Console.WriteLine(result);

Object-Oriented Programming (OOP)

Principle	Description
Encapsulation	Hiding internal details
Inheritance	Reusing code from base classes
Polymorphism	Multiple behaviors
Abstraction	Hiding complexity

Classes and Objects

class Person
{
    public string Name;
    public int Age;

    public void Introduce()
    {
        Console.WriteLine($"My name is {Name}");
    }
}

Person person = new Person();
person.Name = "Alice";
person.Age = 30;
person.Introduce();

Constructors

class Car
{
    public string Brand;

    public Car(string brand)
    {
        Brand = brand;
    }
}

Properties

class Employee
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Department { get; }
}

Access Modifiers

Modifier	Access Level
public	Accessible everywhere
private	Accessible only inside class
protected	Accessible in derived classes
internal	Accessible within assembly

Inheritance

class Animal
{
    public void Eat()
    {
        Console.WriteLine("Eating...");
    }
}

class Dog : Animal
{
    public void Bark()
    {
        Console.WriteLine("Barking...");
    }
}

Polymorphism

class Animal
{
    public virtual void MakeSound()
    {
        Console.WriteLine("Animal sound");
    }
}

class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Bark");
    }
}

Abstraction

abstract class Shape
{
    public abstract double CalculateArea();
}

Interfaces

interface ILogger
{
    void Log(string message);
}

class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

Exception Handling

try
{
    int result = 10 / 0;
}
catch(Exception ex)
{
    Console.WriteLine(ex.Message);
}
finally
{
    Console.WriteLine("Finished");
}

Collections

List<string> names = new List<string>();
names.Add("John");
names.Add("Alice");

Dictionary<int, string> users = new Dictionary<int, string>();
users.Add(1, "Admin");

LINQ

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

var evenNumbers = numbers.Where(n => n % 2 == 0);

foreach(var number in evenNumbers)
{
    Console.WriteLine(number);
}

Async and Await

public async Task FetchDataAsync()
{
    await Task.Delay(1000);

    Console.WriteLine("Data fetched");
}

Benefits:

Non-blocking operations
Better scalability
Improved responsiveness

File Handling

File.WriteAllText("data.txt", "Hello");

string content = File.ReadAllText("data.txt");

Dependency Injection (DI)

services.AddScoped<IUserService, UserService>();

Benefits: loose coupling, easier testing, better maintainability.

Nullable Types

int? number = null;

string name = input ?? "Default";

Memory Management

C# uses automatic garbage collection to free unused memory.

Best practices:

Dispose of unmanaged resources with using statements
Avoid memory leaks by releasing subscriptions and event handlers

Recommended Project Structure

/src
  /Controllers
  /Services
  /Repositories
  /Models
  /Interfaces
  /DTOs

Real-World Use Cases

Use Case	Technology
REST APIs	ASP.NET Core
Enterprise Apps	.NET
Cloud Services	Azure
Games	Unity
Desktop Apps	WPF / WinForms

Interview Questions

Beginner

What is C#?
Difference between class and object?
What is inheritance?
What is polymorphism?
What is an interface?

Intermediate

Difference between abstract class and interface?
What is dependency injection?
Explain async/await.
What is LINQ?
What is garbage collection?

Advanced

Explain CLR and JIT.
What are delegates and events?
What is middleware in ASP.NET Core?
Explain memory management in .NET.
What are expression trees?

Learning Roadmap

Recommended Resources

Official C# Documentation
LeetCode
HackerRank
Exercism
CodeWars

Overview​

Why Learn C#​

Advantages​

.NET Ecosystem​

Installing the Environment​

Install .NET SDK​

Your First C# Program​

Variables and Data Types​

Type Conversion​

Implicit Casting​

Explicit Casting​

Parsing​

Operators​

Arithmetic Operators​

Comparison Operators​

Conditional Statements​

if Statement​

switch Statement​

Loops​

for Loop​

while Loop​

foreach Loop​

Methods​

Object-Oriented Programming (OOP)​

Classes and Objects​

Constructors​

Properties​

Access Modifiers​

Inheritance​

Polymorphism​

Abstraction​

Interfaces​

Exception Handling​

Collections​

LINQ​

Async and Await​

File Handling​

Dependency Injection (DI)​

Nullable Types​

Memory Management​

Recommended Project Structure​

Real-World Use Cases​

Interview Questions​

Beginner​

Intermediate​

Advanced​

Learning Roadmap​

Recommended Resources​