C# Cheat Sheet

Microsoft's flagship .NET language — behind Unity games, Windows desktop apps, ASP.NET web backends, and huge chunks of enterprise software. Strongly typed, object-oriented, with modern conveniences like LINQ built in. Copy-paste examples plus live Run/Submit exercises.

Your first program

using System;

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

Variables

int age = 25;
string name = "Sam";
double price = 19.99;
var city = "NYC";         // var infers the type — still statically typed
const double Pi = 3.14159; // constants can't change

if / else

int age = 20;
if (age >= 18) {
    Console.WriteLine("Adult");
} else if (age >= 13) {
    Console.WriteLine("Teen");
} else {
    Console.WriteLine("Child");
}

Loops

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

int n = 3;
while (n > 0) {
    Console.WriteLine(n);
    n--;
}

Lists & LINQ

C#'s List<T> is the go-to resizable collection, and LINQ lets you query it almost like SQL.

using System.Collections.Generic;
using System.Linq;

List<int> nums = new List<int> { 10, 20, 30 };
nums.Add(40);
Console.WriteLine(nums[0]);          // 10
Console.WriteLine(nums.Count);       // 4

// LINQ: filter + transform in one line
var evens = nums.Where(x => x % 2 == 0).ToList();
int total = nums.Sum();

Classes

class Dog {
    public string Name { get; set; }
    public int Age { get; set; }

    public Dog(string name, int age) {
        Name = name; Age = age;
    }

    public void Bark() {
        Console.WriteLine(Name + " says woof!");
    }
}

// var d = new Dog("Rex", 3); d.Bark();

🚀 Practice problems

Full runnable programs, compiled and run in a real C# (.NET/Mono) sandbox — same workspace as the other cheat sheets.

Sum an array EASY

using System;

class Program {
    static void Main() {
        int[] nums = { 4, 8, 15, 16, 23, 42 };
        int sum = 0;
        foreach (int n in nums) sum += n;
        Console.WriteLine(sum);
    }
}

Reverse a string MEDIUM

using System;
using System.Linq;

class Program {
    static string Reverse(string s) {
        return new string(s.Reverse().ToArray());
    }
    static void Main() {
        Console.WriteLine(Reverse("csharp"));
    }
}

FizzBuzz MEDIUM

using System;

class Program {
    static void Main() {
        for (int i = 1; i <= 15; i++) {
            if (i % 15 == 0) Console.WriteLine("FizzBuzz");
            else if (i % 3 == 0) Console.WriteLine("Fizz");
            else if (i % 5 == 0) Console.WriteLine("Buzz");
            else Console.WriteLine(i);
        }
    }
}

Word frequency with a Dictionary HARD

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        string text = "the cat sat on the mat the cat ran";
        var counts = new Dictionary<string, int>();
        foreach (string word in text.Split(' ')) {
            if (!counts.ContainsKey(word)) counts[word] = 0;
            counts[word]++;
        }
        foreach (var kv in counts)
            Console.WriteLine(kv.Key + ": " + kv.Value);
    }
}