C# Fundamentals

What is C#?

C# is a modern, statically typed, object-oriented language by Microsoft, running on the .NET runtime. Used for web APIs (ASP.NET Core), desktop (WPF/MAUI), cloud services, and games (Unity).

Types & Variables

int    age    = 30;
double price  = 9.99;
bool   active = true;
string name   = "Alice";
var    x      = 42;        // compiler infers int

Nullable Types

string? text  = null;            // nullable reference type
int?    count = null;            // nullable value type
string  s     = text  ?? "n/a"; // null-coalescing
string? upper = text?.ToUpper(); // null-conditional

Control Flow

if (x > 0) { } else if (x == 0) { } else { }

for (int i = 0; i < 10; i++) { }
foreach (var item in list) { }
while (running) { }

switch (role)
{
    case "admin": GrantAccess(); break;
    default:      Deny();        break;
}

Classes & Inheritance

public class Animal
{
    public string Name { get; set; }
    public Animal(string name) => Name = name;
    public virtual string Speak() => "...";
}

public class Dog : Animal
{
    public Dog(string name) : base(name) { }
    public override string Speak() => "Woof!";
}

LINQ

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

var evens  = nums.Where(n => n % 2 == 0);
var mapped = nums.Select(n => n * 2);
var sum    = nums.Sum();                        // 15
var first  = nums.FirstOrDefault(n => n > 3);  // 4

Async / Await

public async Task<string> FetchAsync(string url)
{
    using var client = new HttpClient();
    return await client.GetStringAsync(url);
}

Use async Task — never async void except for event handlers. await suspends the method without blocking the thread.

Key Syntax

Feature	Example
String interpolation	$"Hello, {name}!"
Pattern matching	if (obj is Dog d) { }
Record	record Point(int X, int Y);
Expression body	int Square(int x) => x * x;
Ternary	x > 0 ? "pos" : "neg"