C# Cheat Sheet — Complete Syntax, OOP Concepts, and Examples

Table of Contents:

C# Cheat Sheet — Quick Reference Guide for Developers

C# (C-Sharp) is a modern, object-oriented programming language developed by Microsoft. It runs on the .NET framework and is widely used for building desktop apps, web apps, games, and enterprise software.

This C# Cheatsheet gives you a quick and complete overview of all major topics — from basic syntax to advanced OOP and LINQ — with easy-to-understand examples for practical learning.

C# Basics for Beginners

Concept	Description	Example
File Extension	`.cs`	`Program.cs`
Namespace	Organizes classes	`namespace DemoApp { }`
Entry Point	Main method	`static void Main()`
Comments	Single or multi-line	`// Comment` or `/* Comment */`

Example:

using System;

namespace HelloWorld {
    class Program {
        static void Main() {
            Console.WriteLine("Hello, C#!");
        }
    }
}

Data Types in C#

Type	Keyword	Size (bytes)	Example
Integer	`int`	4	`int age = 25;`
Floating-point	`float`, `double`, `decimal`	4 / 8 / 16	`float price = 12.5F;`
Character	`char`	2	`char grade = 'A';`
Boolean	`bool`	1	`bool isValid = true;`
String	`string`	Variable	`string name = "John";`
Object	`object`	Any type	`object data = 42;`

C# Variables and Constants

int x = 10;
const double PI = 3.1416;
var message = "Hello!";

var allows type inference at compile time.
const defines immutable values.

Operators in C#

Category	Operators	Example
Arithmetic	`+ - * / %`	`x + y`
Relational	`== != > < >= <=`	`if(a > b)`
Logical	`&&
Assignment	`= += -= *= /=`	`x += 5;`
Conditional	`?:`	`result = (a > b) ? a : b;`
Null-Coalescing	`??`	`value = name ?? "Guest";`

Conditional Statements in C#

if (x > 0)
    Console.WriteLine("Positive");
else if (x < 0)
    Console.WriteLine("Negative");
else
    Console.WriteLine("Zero");

Switch Example:

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

Loops in C#

Type	Syntax	Example
for	`for(init; condition; increment)`	`for(int i=0;i<5;i++)`
while	`while(condition)`	`while(i<10)`
do-while	`do { } while(condition);`	Executes once
foreach	`foreach(var item in collection)`	Loops through arrays or lists

Example:

foreach (int n in new int[]{1,2,3})
    Console.WriteLine(n);

C# Functions (Methods)

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

Static Methods: Belong to the class, not instance.
Overloading: Same name, different parameters.
Default Parameters: void Greet(string name="User").

C# Object-Oriented Programming (OOP)

Concept	Definition	Example
Class	Blueprint for objects	`class Car { }`
Object	Instance of class	`Car myCar = new Car();`
Encapsulation	Data hiding via access modifiers	`private int age;`
Inheritance	Reusing base class	`class Child : Parent`
Polymorphism	Method overriding	`virtual` and `override`
Abstraction	Hiding complexity	`abstract class Shape {}`

Example:

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

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

C# Collections

Type	Namespace	Example
Array	`System`	`int[] nums = {1,2,3};`
List	`System.Collections.Generic`	`List<int> list = new List<int>();`
Dictionary	`System.Collections.Generic`	`Dictionary<int,string> dict = new();`
Queue	FIFO	`Queue<string> q = new();`
Stack	LIFO	`Stack<int> s = new();`

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LINQ (Language Integrated Query)

LINQ simplifies querying data in collections.

int[] numbers = { 1, 2, 3, 4, 5 };
var even = from n in numbers where n % 2 == 0 select n;

foreach(var n in even)
    Console.WriteLine(n);

File Handling in C#

using System.IO;

File.WriteAllText("file.txt", "Hello C#");
string content = File.ReadAllText("file.txt");
Console.WriteLine(content);

Exception Handling in C#

try {
    int x = 0;
    int y = 10 / x;
}
catch (DivideByZeroException e) {
    Console.WriteLine(e.Message);
}
finally {
    Console.WriteLine("Cleanup code");
}

Asynchronous Programming in C#

async Task FetchData() {
    await Task.Delay(1000);
    Console.WriteLine("Data loaded");
}

C# Keywords Summary Table

Keyword	Purpose
`public`	Accessible from anywhere
`private`	Accessible only within class
`protected`	Accessible in derived class
`static`	Belongs to class, not instance
`const`	Constant value
`readonly`	Assigned only once
`virtual`	Overridable method
`override`	Replaces base method
`abstract`	Must be implemented in child class
`interface`	Defines contract
`using`	Imports namespaces or disposes resources

FAQ — C# Programming

Q1: Is C# only used for Windows applications?
No, modern .NET Core allows C# apps to run on Windows, Linux, and macOS.

Q2: What is the difference between C++ and C#?
C++ is a compiled system-level language, while C# runs on the .NET runtime and is designed for managed, high-level development.

Q3: Can I use C# for game development?
Yes, Unity Engine uses C# for all its game scripts.

Q4: Is C# easy to learn for beginners?
Yes! Its syntax is clean, modern, and similar to C, Java, and JavaScript.

Q5: Where can I practice C# online?
Try the PHPOnline C# Compiler for running C# code instantly.