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C# Cheatsheet

This page is a quick reference for C# patterns that commonly appear in DSA, competitive programming, and technical interviews.

Basic Syntax​

Hello World​

using System;

class Program
{
    static void Main()
    {
        Console.WriteLine("Hello World");
    }
}

Data Types​

int a = 10;
long b = 10000000000L;
double d = 3.14;
float f = 2.5f;
char c = 'A';
bool flag = true;
string s = "Hello";

Operators and Control Flow​

if (a > 0)
{
    Console.WriteLine("Positive");
}
else if (a == 0)
{
    Console.WriteLine("Zero");
}
else
{
    Console.WriteLine("Negative");
}

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

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

Arrays​

Array Declaration​

int[] arr = new int[5];

int[] nums = {1, 2, 3, 4, 5};

Console.WriteLine(arr.Length);

Multidimensional Array​

int[,] grid = new int[3,3];

grid[0,0] = 10;

Strings​

Common String Operations​

string s = "hello";

int len = s.Length;

string upper = s.ToUpper();

bool contains = s.Contains("ell");

string sub = s.Substring(1, 3);

string replaced = s.Replace("h", "H");

StringBuilder​

using System.Text;

StringBuilder sb = new StringBuilder();

sb.Append("Hello");
sb.Append(" World");

string result = sb.ToString();

Collections​

List​

using System.Collections.Generic;

List<int> list = new List<int>();

list.Add(10);
list.Add(20);

int first = list[0];

list.Remove(10);

int size = list.Count;

Dictionary​

Dictionary<string, int> map =
    new Dictionary<string, int>();

map["Alice"] = 95;

if (map.ContainsKey("Alice"))
{
    Console.WriteLine(map["Alice"]);
}

HashSet​

HashSet<int> set = new HashSet<int>();

set.Add(1);
set.Add(2);

bool exists = set.Contains(1);

set.Remove(2);

Queue​

Queue<int> q = new Queue<int>();

q.Enqueue(10);
q.Enqueue(20);

int front = q.Dequeue();

Stack​

Stack<int> st = new Stack<int>();

st.Push(1);
st.Push(2);

int top = st.Pop();

Sorting​

Array Sort​

int[] arr = {5, 2, 8, 1};

Array.Sort(arr);

Custom Sort​

Array.Sort(arr, (a, b) => b.CompareTo(a));

LINQ​

Filtering​

using System.Linq;

List<int> nums =
    new List<int>() {1,2,3,4,5};

var even =
    nums.Where(x => x % 2 == 0);

foreach(var x in even)
{
    Console.WriteLine(x);
}

Aggregation​

int sum = nums.Sum();

int max = nums.Max();

int min = nums.Min();

double avg = nums.Average();

Classes and OOP​

Class​

class Person
{
    public string Name { get; set; }

    public Person(string name)
    {
        Name = name;
    }

    public void Greet()
    {
        Console.WriteLine($"Hello {Name}");
    }
}

Person p = new Person("John");
p.Greet();

Inheritance​

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

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

Generics​

class Box<T>
{
    public T Value;

    public Box(T value)
    {
        Value = value;
    }
}

Box<int> box = new Box<int>(10);

Exception Handling​

try
{
    int x = int.Parse("123");
}
catch(Exception ex)
{
    Console.WriteLine(ex.Message);
}
finally
{
    Console.WriteLine("Done");
}

File Handling​

using System.IO;

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

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

Common DSA Patterns​

Fast Input​

string[] input = Console.ReadLine()?.Split() ?? Array.Empty<string>();

int n = input.Length > 0 ? int.Parse(input[0]) : 0;

Priority Queue (.NET 6+)​

PriorityQueue<int, int> pq = new(); // Min-heap by default

pq.Enqueue(5, 5);
pq.Enqueue(2, 2);

int value = pq.Dequeue(); // Returns 2
int index =
    Array.BinarySearch(arr, 5);

if(index >= 0)
{
    Console.WriteLine("Found");
}

References​