C#: Language Fundamentals

C# is a strongly-typed, object-oriented language developed by Microsoft. It runs on the .NET CLR and is the primary language for ASP.NET Core, Unity, and Xamarin/MAUI. Modern C# (10–13) continues to add expressive features like records, pattern matching, and primary constructors.

Types & Variables

// Value types (stored on stack, copied on assignment)
int count = 42;
double pi = 3.14159;
bool isActive = true;
char grade = 'A';
decimal price = 19.99m;   // decimal for money — exact, no float errors

// Reference types (stored on heap, shared reference)
string name = "Alice";
int[] scores = { 95, 82, 78 };
object anything = "can be any type";

// Type inference
var list = new List<string>();   // inferred as List<string>
var user = new User("Alice", 30);

// Constants and readonly
const double Pi = 3.14159;           // compile-time constant
static readonly int MaxSize = 100;   // runtime constant (can be computed)

// Nullable value types
int? nullableInt = null;
double? maybeDouble = GetValue();    // returns null if unavailable
int result = nullableInt ?? 0;       // null coalescing
int len = nullableInt?.ToString().Length ?? 0;  // null conditional

// String
string msg = $"Hello, {name}! You are {count} years old.";  // interpolation
string raw = @"C:\Users\Alice\Documents";  // verbatim — no escape needed
string multiline = """
    This is a raw string literal (C# 11)
    No escaping needed for "quotes" or \backslashes
    """;

Control Flow

// if / else
if (score >= 90)
    grade = 'A';
else if (score >= 80)
    grade = 'B';
else
    grade = 'C';

// Switch statement (classic)
switch (day)
{
    case DayOfWeek.Monday:
    case DayOfWeek.Tuesday:
        Console.WriteLine("Early week");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

// Switch expression (C# 8+) — expression form, returns value
string label = status switch
{
    "active"   => "Active User",
    "inactive" => "Inactive User",
    null       => "Unknown",
    _          => $"Unknown status: {status}",
};

// For / foreach
for (int i = 0; i < 10; i++) { }
foreach (var item in collection) { }

// While / do-while
while (condition) { }
do { } while (condition);

// Pattern matching in if
if (obj is string s && s.Length > 0)
    Console.WriteLine(s.ToUpper());

// throw expressions (C# 7+)
var value = input ?? throw new ArgumentNullException(nameof(input));

Methods & Parameters

// Basic method
public static int Add(int a, int b) => a + b;  // expression body

// Optional parameters
public string Greet(string name, string greeting = "Hello") =>
    $"{greeting}, {name}!";

// Named arguments
Greet(greeting: "Hi", name: "Bob");

// Params — variable arguments
public int Sum(params int[] numbers) => numbers.Sum();
Sum(1, 2, 3, 4, 5);  // no array needed at call site

// Out parameters
bool success = int.TryParse("42", out int value);
if (success) Console.WriteLine(value);

// Ref and in parameters
void Increment(ref int count) => count++;
void Process(in LargeStruct data) { }  // in = readonly ref (no copy, no modify)

// Tuples
(string Name, int Age) GetPerson() => ("Alice", 30);
var (name, age) = GetPerson();  // deconstruct

// Local functions
int Factorial(int n)
{
    return n <= 1 ? 1 : n * Inner(n - 1);

    int Inner(int x) => x <= 1 ? 1 : x * Inner(x - 1);  // local function
}