C# Cheat Sheet

C# is a modern, strongly typed programming language designed for building desktop, web, cloud, and cross-platform applications on the .NET ecosystem. This C# cheatsheet provides a structured, example-driven reference to core language features, syntax, object-oriented constructs, LINQ, and asynchronous programming.

Compilation & Execution

dotnet new console
dotnet run

csc Program.cs
Program.exe

Minimal Program

using System;

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

Top-level statements (.NET 6+)

Console.WriteLine("Hello, C#");

Built-in Types

int     a = 10;
double  b = 3.14;
float   c = 1.5f;
bool    d = true;
char    e = 'A';
string  f = "text";

sizeof(int);
sizeof(double);

Nullable Types

int? value = null;

if (value.HasValue)
{
    Console.WriteLine(value.Value);
}

int result = value ?? 0;

Variables & Type Inference

var x = 10;
var y = "text";

dynamic z = 5;
z = "now string";

Control Flow

if / else

if (x > 10)
{
    result = 1;
}
else
{
    result = 0;
}

switch

switch (x)
{
    case 1:
        break;
    case 2:
        break;
    default:
        break;
}

switch expression

var label = x switch
{
    1 => "one",
    2 => "two",
    _ => "other"
};

Loops

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

while (x > 0)
{
    x--;
}

foreach (var item in collection)
{
    Console.WriteLine(item);
}

Methods

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

int result = Add(2, 3);

Method Overloading

static int Add(int a, int b);
static double Add(double a, double b);

Optional & Named Parameters

static int Multiply(int a, int b = 2)
{
    return a * b;
}

Multiply(5);
Multiply(b: 3, a: 4);

Classes

class Counter
{
    private int value;

    public Counter()
    {
        value = 0;
    }

    public void Increment()
    {
        value++;
    }

    public int Get()
    {
        return value;
    }
}

Properties

class User
{
    public string Name { get; set; }
    public int Age { get; private set; }

    public void SetAge(int age)
    {
        Age = age;
    }
}

Structs

struct Point
{
    public int X;
    public int Y;
}

Point p = new Point { X = 10, Y = 20 };

Records

record Person(string Name, int Age);

var person = new Person("Name", 30);

Inheritance

class Animal
{
    public void Speak() { }
}

class Dog : Animal
{
}

Polymorphism & Virtual Methods

class Base
{
    public virtual void Run() { }
}

class Derived : Base
{
    public override void Run() { }
}

Interfaces

interface ILogger
{
    void Log(string message);
}

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

Enums

enum Status
{
    Ok,
    Error,
    Pending
}

Arrays

int[] numbers = { 1, 2, 3 };
numbers[0] = 10;

Collections

List

using System.Collections.Generic;

List<int> list = new List<int> { 1, 2, 3 };
list.Add(4);

Dictionary

Dictionary<int, string> map = new Dictionary<int, string>();
map[1] = "one";

LINQ

using System.Linq;

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

var names = users.Select(u => u.Name);

var grouped = users.GroupBy(u => u.Age);

Query syntax

var result =
    from n in numbers
    where n > 5
    select n;

Delegates

delegate int Operation(int a, int b);

Operation op = Add;
int r = op(2, 3);

Func & Action

Func<int, int, int> sum = (a, b) => a + b;
Action<string> log = message => Console.WriteLine(message);

Events

class Notifier
{
    public event Action OnNotify;

    public void Notify()
    {
        OnNotify?.Invoke();
    }
}

Exception Handling

try
{
    int x = int.Parse("10");
}
catch (FormatException)
{
}
finally
{
}

Async / Await

using System.Threading.Tasks;

async Task<int> GetValueAsync()
{
    await Task.Delay(100);
    return 42;
}

int value = await GetValueAsync();

Tasks

Task task = Task.Run(() =>
{
    Console.WriteLine("running");
});

await task;

File I/O

using System.IO;

File.WriteAllText("file.txt", "content");
string text = File.ReadAllText("file.txt");

Pattern Matching

if (obj is int number)
{
    Console.WriteLine(number);
}

var result = obj switch
{
    int n => n * 2,
    string s => s.Length,
    _ => 0
};

Value vs Reference Types

int a = 10;
int b = a;
b = 20;

class Box { public int Value; }

Box x = new Box { Value = 10 };
Box y = x;
y.Value = 20;

Span (Overview)

Span<int> span = stackalloc int[3] { 1, 2, 3 };