Method Overloading: Same Name, Different Jobs

C# picks the right method based on parameters you pass.

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

Console.WriteLine(Add(2, 3)); // 5 - calls int version
Console.WriteLine(Add(2.5, 3.1)); // 5.6 - calls double
Console.WriteLine(Add(1, 2, 3)); // 6 - calls 3-param

Optional Parameters & Named Arguments

static void SendEmail(string to, string subject = "No Subject", bool urgent = false) {
    Console.WriteLine($"To: {to}, Subject: {subject}, Urgent: {urgent}");
}
SendEmail("a@b.com"); // subject="No Subject", urgent=false
SendEmail("a@b.com", "Hello"); // urgent=false
SendEmail("a@b.com", urgent: true); // Named: skip subject, set urgent

Ref vs Out: Modify Outside Variables

static void Increment(ref int x) { x++; } // ref: must be initialized
int num = 5;
Increment(ref num); // num is now 6

static bool TryParse(string s, out int result) {
    return int.TryParse(s, out result); // out: must assign before return
}
if (int.TryParse("123", out int val)) {
    Console.WriteLine(val); // 123
}

Expression-Bodied Methods: Short Syntax

static int Square(int x) => x * x; // Same as { return x * x; }
static bool IsEven(int x) => x % 2 == 0;
static void Print(string msg) => Console.WriteLine(msg);

Real Example: Razorpay Payment Method

static bool ProcessPayment(decimal amount, string cardNumber, out string txnId) {
    txnId = Guid.NewGuid().ToString();
    
    if (amount <= 0) throw new ArgumentException("Amount must be positive");
    if (string.IsNullOrEmpty(cardNumber)) throw new ArgumentNullException("Card required");
    
    // Call payment gateway API here
    Console.WriteLine($"Charged ₹{amount}. TxnID: {txnId}");
    return true;
}

if (ProcessPayment(500m, "1234-5678", out string id)) {
    Console.WriteLine($"Success! ID: {id}");
}