C# Cheatsheet

// Using a top-level statement (.NET 6+)
using System;

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

// Variable declaration
string message = "Modern C# is concise.";
// `var` infers the type from the value
var inferredMessage = "This is also a string.";

// Basic Types
int anInt = 42;
double aDouble = 3.14;
bool isCSharpCool = true;
char aChar = 'C';

// String interpolation
Console.WriteLine($"The integer is {anInt}.");

using System;
using System.Collections.Generic;

// Array (fixed size)
int[] numbers = new int[] { 1, 2, 3 };
Console.WriteLine($"First number: {numbers[0]}");

// List<T> (dynamic size, most common)
List<string> names = new List<string> { "Alice", "Bob" };
names.Add("Charlie");

// Dictionary<TKey, TValue> (key-value pairs)
Dictionary<string, int> ages = new Dictionary<string, int>();
ages["Alice"] = 30;
ages["Bob"] = 35;

// HashSet<T> (unique elements)
HashSet<string> uniqueNames = new HashSet<string> { "Alice", "Alice" };
Console.WriteLine($"Count of unique names: {uniqueNames.Count}"); // 1

int score = 85;
if (score >= 90) {
    Console.WriteLine("A");
} else if (score >= 80) {
    Console.WriteLine("B");
} else {
    Console.WriteLine("C or lower");
}

// foreach loop (for iterating over collections)
var fruits = new List<string> { "Apple", "Banana" };
foreach (var fruit in fruits) {
    Console.WriteLine(fruit);
}

// Switch expression (.NET Core 3.0+)
var grade = score switch {
    >= 90 => "A",
    >= 80 => "B",
    _ => "C or lower" // _ is the discard/default case
};

// Interface
public interface IAnimal {
    void Speak();
}

// Class with properties and a constructor
public class Dog : IAnimal {
    // Auto-implemented property
    public string Name { get; set; }

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

    // Virtual method can be overridden by derived classes
    public virtual void Speak() {
        Console.WriteLine("Woof!");
    }
}

// Inheritance
public class GoldenRetriever : Dog {
    public GoldenRetriever(string name) : base(name) { } // Call base constructor

    // Override the virtual method
    public override void Speak() {
        Console.WriteLine($"{Name} says: Gentle Woof!");
    }
}

using System;
using System.IO;

try {
    string text = File.ReadAllText("nonexistent.txt");
} catch (FileNotFoundException ex) {
    Console.WriteLine($"Error: {ex.Message}");
} catch (Exception ex) {
    Console.WriteLine($"An unexpected error occurred: {ex.Message}");
} finally {
    Console.WriteLine("This block always executes.");
}

// Throwing an exception
// throw new ArgumentException("Invalid argument provided.");

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

var numbers = new List<int> { 1, 2, 3, 4, 5, 6 };

// Method Syntax (most common)
var evenSquares = numbers
    .Where(n => n % 2 == 0)
    .Select(n => n * n)
    .ToList(); // Result: [4, 16, 36]

// Query Syntax
var oddNumbers = from n in numbers
                 where n % 2 != 0
                 select n;

using System;
using System.Net.Http;
using System.Threading.Tasks;

public class AsyncExample {
    public static async Task FetchDataAsync() {
        using var client = new HttpClient();
        try {
            // await pauses execution until the task is complete
            string result = await client.GetStringAsync("https://api.example.com");
            Console.WriteLine(result);
        } catch (HttpRequestException e) {
            Console.WriteLine($"Request error: {e.Message}");
        }
    }
}

# Create a new console application
dotnet new console -n MyConsoleApp

# Create a new web API project
dotnet new webapi -n MyWebApi

# Restore dependencies
dotnet restore

# Build the project
dotnet build

# Run the project
dotnet run

# Run tests
dotnet test