Methods: write once, use many times

Why methods?

You have already called methods that .NET gives you: Console.WriteLine, int.Parse and Math.Round. A method is a named block of code that does one job. When you write your own, you can reuse the steps by calling the name. If the GST rate changes, you fix one method instead of twenty copies.

The parts of a method

static decimal AddGst(decimal price, decimal rate)
{
    decimal gst = price * rate;
    return price + gst;
}

Calling a method

decimal total = AddGst(1000m, 0.18m);
Console.WriteLine(total);                 // 1180.00
Console.WriteLine(AddGst(500m, 0.05m));   // 525.00

static decimal AddGst(decimal price, decimal rate)
{
    return price + price * rate;
}

Values you pass when calling (1000m, 0.18m) are called arguments. They are copied into the parameters (price, rate) in the same order.

void methods

PrintLine('=', 20);
Console.WriteLine("  BILL");
PrintLine('=', 20);

static void PrintLine(char symbol, int length)
{
    Console.WriteLine(new string(symbol, length));
}

Expression-bodied methods

static int Square(int n) => n * n;
static bool IsAdult(int age) => age >= 18;
static void Greet(string name) => Console.WriteLine($"Hi {name}");

Every path must return

static string Result(int marks)
{
    if (marks >= 40)
        return "Pass";
    // error CS0161: not all code paths return a value
}

Local variables and scope

A variable declared inside a method is local: it exists only while the method runs and only inside its braces. Two methods can both have a variable named total without any clash. This is called scope: the region of code where a name can be used.

Returning more than one value with a tuple

var (each, left) = SplitBill(1000, 3);
Console.WriteLine($"Each pays {each}, {left} rupee left");

static (int Each, int Left) SplitBill(int total, int people)
    => (total / people, total % people);

Recursion, briefly

static long Factorial(int n) => n <= 1 ? 1 : n * Factorial(n - 1);
Console.WriteLine(Factorial(5));   // 120

Common mistakes and fixes

• Printing instead of returning: a method named CalculateTotal that only prints cannot be used in further sums. Return the value and let the caller print.
• Forgetting to use the return value: AddGst(price, rate); on its own line throws the answer away. Store it: price = AddGst(price, rate);
• Methods that do five jobs: split them. A good method fits on one screen and its name says exactly what it does.
• Wrong argument order: AddGst(0.18m, 1000m) compiles but gives the wrong answer. Named arguments (next topic) help.