Control flow and functions in C# 14

Control flow in C#

Control flow determines the order in which statements execute. C# offers the classic structures plus expressive modern updates.

if / else

The most basic structure for making decisions:

int temperature = 35;

if (temperature > 40)
{
    Console.WriteLine("Extreme heat");
}
else if (temperature > 30)
{
    Console.WriteLine("Strong heat");   // ← executes
}
else if (temperature > 20)
{
    Console.WriteLine("Pleasant temperature");
}
else
{
    Console.WriteLine("Cold");
}

// Single-line if (no braces) — only when obvious
if (temperature > 0) Console.WriteLine("Above zero");

Conditional expression (ternary)

bool isAdult = age >= 18;
string message = isAdult ? "Welcome" : "Access denied";

// Nested ternary (use sparingly)
string grade = points >= 90 ? "A" : points >= 70 ? "B" : points >= 50 ? "C" : "F";

switch statement

To compare a value against multiple options:

string dayName = "Monday";

switch (dayName)
{
    case "Monday":
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
    case "Friday":
        Console.WriteLine("Weekday");
        break;
    case "Saturday":
    case "Sunday":
        Console.WriteLine("Weekend");
        break;
    default:
        Console.WriteLine("Invalid day");
        break;
}

switch expressions (C# 8+, improved in C# 14)

The modern, more expressive version. Returns a value directly:

// Over enums — the compiler verifies exhaustiveness
enum Status { Active, Inactive, Suspended, Deleted }

string DescribeStatus(Status status) => status switch
{
    Status.Active    => "User can access",
    Status.Inactive  => "Account temporarily deactivated",
    Status.Suspended => "Account suspended for violation",
    Status.Deleted   => "Account permanently deleted"
    // No default: the compiler warns if an enum case is missing
};

// With property patterns
record Order(decimal Total, string Country, bool IsPremium);

string CalculateShipping(Order o) => o switch
{
    { Total: > 100, Country: "US" }  => "Free shipping",
    { IsPremium: true }              => "Free shipping (premium)",
    { Country: "US", Total: > 50 }   => "Discounted shipping",
    _                                => "Standard shipping"
};

Loops

for — when you know the number of iterations

// Multiplication table of 5
for (int i = 1; i <= 10; i++)
{
    Console.WriteLine($"5 × {i} = {5 * i}");
}

// Reverse iteration
for (int i = 10; i >= 1; i--)
{
    Console.Write($"{i} ");
}
Console.WriteLine();

// for with multiple variables
for (int i = 0, j = 10; i < j; i++, j--)
{
    Console.WriteLine($"i={i}, j={j}");
}

foreach — for collections

string[] languages = { "C#", "Go", "Rust", "Python", "TypeScript" };

foreach (string lang in languages)
{
    Console.WriteLine($"  → {lang}");
}

// foreach with index using LINQ
foreach (var (lang, idx) in languages.Select((l, i) => (l, i)))
{
    Console.WriteLine($"{idx + 1}. {lang}");
}

while — while a condition holds

int attempts  = 0;
int maxTries  = 3;
bool success  = false;

while (attempts < maxTries && !success)
{
    Console.Write("Enter the password: ");
    string pwd = Console.ReadLine() ?? "";
    attempts++;

    if (pwd == "dotnet10")
    {
        success = true;
        Console.WriteLine("Access granted!");
    }
    else
    {
        Console.WriteLine($"Wrong password. Attempts: {attempts}/{maxTries}");
    }
}

do-while — executes at least once

int number;
do
{
    Console.Write("Enter a positive number: ");
} while (!int.TryParse(Console.ReadLine(), out number) || number <= 0);

Console.WriteLine($"Valid number: {number}");

break, continue, and return

// break — exits the loop
for (int i = 0; i < 100; i++)
{
    if (i == 5) break;
    Console.Write($"{i} "); // 0 1 2 3 4
}

// continue — skips to the next iteration
for (int i = 0; i < 10; i++)
{
    if (i % 2 == 0) continue; // skip even numbers
    Console.Write($"{i} "); // 1 3 5 7 9
}

Methods (functions)

In C#, functions are called methods and always belong to a class or struct. The Program.cs entry point uses top-level statements, but internally it is also a method.

// Method with return type
static int Add(int a, int b)
{
    return a + b;
}

// Expression-bodied method (single expression)
static int Multiply(int a, int b) => a * b;

// void method (no return)
static void Greet(string name)
{
    Console.WriteLine($"Hello, {name}!");
}

// Optional parameters (must come last)
static string Format(string text, bool uppercase = false, char separator = ' ')
{
    string result = text.Trim();
    if (uppercase) result = result.ToUpper();
    return result.Replace(' ', separator);
}

Console.WriteLine(Format("hello world"));           // "hello world"
Console.WriteLine(Format("hello world", true));     // "HELLO WORLD"
Console.WriteLine(Format("hello world", true, '-')); // "HELLO-WORLD"

Named arguments

// Named arguments improve readability
string result = Format(
    text:      "hello world",
    separator: '_',
    uppercase: true
);
Console.WriteLine(result); // "HELLO_WORLD"

Local functions

Local functions are methods defined inside another method. They are only visible within the parent method:

static string ProcessOrder(string[] items, decimal discount)
{
    // Local function — only visible here
    decimal CalculateTotal()
    {
        decimal sum = 0;
        foreach (string item in items)
            sum += GetPrice(item);
        return sum * (1 - discount);
    }

    decimal GetPrice(string item) => item switch
    {
        "laptop"   => 999.99m,
        "mouse"    => 29.99m,
        "keyboard" => 79.99m,
        _          => 0m
    };

    decimal total = CalculateTotal();
    return $"Total: ${total:F2}";
}

Practice

1. FizzBuzz: Use a for loop from 1 to 100. If the number is divisible by 3, print "Fizz"; by 5, "Buzz"; by both, "FizzBuzz"; otherwise the number.
2. Calculator with switch expression: Create a method Calculate(double a, double b, string operation) that uses a switch expression for +, -, *, / and returns the result.
3. params: Implement a method Average(params double[] values) that returns the average of the received values.

In the next lesson we will learn how to define classes and objects — the core of object-oriented programming in C#.