C# Cheat Sheet

Console.WriteLine("Hello, World!");

// No Main method needed (C# 9+)
Console.WriteLine("Hello!");

// Single line comment
/* Multi-line
   comment */
/// <summary>XML doc</summary>

Console.Write("Name: ");
string name = Console.ReadLine();

int.Parse("42")     // string to int
42.ToString()       // int to string
Convert.ToDouble("3.14")

string name = "John";

int age = 25;

double price = 19.99;

bool isActive = true;

var items = new List<string>();

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

List<string> names = new List<string>();

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

+ - * /    // Add, Sub, Mul, Div
%          // Modulus (remainder)
++ --      // Increment/Decrement

==  !=     // Equal, Not equal
>   <      // Greater, Less than
>=  <=     // Greater/Less or equal

&&   // AND (short-circuit)
||   // OR (short-circuit)
!    // NOT
&  |  ^  // Bitwise AND, OR, XOR

??   // Null-coalescing
??=  // Null-coalescing assignment
?.   // Null-conditional
!    // Null-forgiving

if (obj is string s) { ... }
var str = obj as string;  // null if fails

obj is int n and > 0
obj is { Name: "John" }
obj is [1, 2, ..]

array[0..3]    // Range slice
array[^1]      // Last element
array[1..^1]   // Skip first & last

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

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

while (count < 10)
{
    count++;
}

do
{
    count++;
} while (count < 10);

for (int i = 0; i < 10; i++)
{
    if (i == 5) break;
}

for (int i = 0; i < 10; i++)
{
    if (i == 5) continue;
    Console.WriteLine(i);
}

list.ForEach(item => Console.WriteLine(item));

Parallel.ForEach(list, item =>
{
    Process(item);
});

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

if (x > 0)
{
    Console.WriteLine("positive");
}
else if (x < 0)
{
    Console.WriteLine("negative");
}
else
{
    Console.WriteLine("zero");
}

string msg = (age >= 18) ? "adult" : "minor";

var result = day switch
{
    "Mon" => "Monday",
    "Tue" => "Tuesday",
    _ => "Other"
};

switch (day)
{
    case "Mon":
        break;
    default:
        break;
}

var result = obj switch
{
    int n when n > 0 => "positive",
    string s => s.Length,
    null => "null",
    _ => "other"
};

public string Greet(string name)
{
    return $"Hello, {name}!";
}

public static int Add(int a, int b) => a + b;

public int Square(int x) => x * x;

public void Log(string msg, int level = 0) { }

public bool TryParse(string s, out int result) { }

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

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

    public string Bark() => $"{Name} says woof!";
}

public record Person(string Name, int Age);

public interface IAnimal
{
    void Speak();
}

public class Puppy : Dog
{
    public Puppy(string name) : base(name) { }
}

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

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

var sorted = users.OrderBy(u => u.Age);

var user = users.FirstOrDefault(u => u.Id == 1);

bool hasAdmin = users.Any(u => u.IsAdmin);

var groups = users.GroupBy(u => u.Department);

public async Task<string> FetchDataAsync()
{
    var result = await httpClient.GetStringAsync(url);
    return result;
}

await Task.Run(() => HeavyComputation());

await Task.WhenAll(task1, task2, task3);

try
{
    var result = 10 / 0;
}
catch (DivideByZeroException ex)
{
    Console.WriteLine(ex.Message);
}

try { } catch { } finally { Cleanup(); }

throw new ArgumentException("Invalid input");