Courses/C#/Control Flow

Control Flow

Lesson 4 • Beginner Track

What You'll Learn

Write if, else-if, and else statements to make decisions in your code
Use the ternary operator for concise one-line conditional assignments
Build switch statements for handling multiple specific cases
Use modern switch expressions with pattern matching (C# 8+)
Apply relational patterns (< 5, >= 18) and combinators (and, or) in switch
Classify objects by type using pattern matching with the is and switch keywords

💡 Real-World Analogy

Control flow is like a railway switching station. A train (your program) arrives at a junction and the switch operator (your condition) sends it down one of several tracks. An if-else is a simple fork: left or right. A switch statement is a complex junction with many possible tracks. Pattern matching is like an automated system that reads the cargo type and weight to decide which platform to route it to.

📊 Control Flow Quick Reference

Structure	Use When	Example
if / else	2-3 conditions, boolean logic	Age check, validation
ternary ? :	Simple true/false assignment	Status = condition ? "A" : "B"
switch statement	Many discrete values	Day of week, menu options
switch expression	Return a value from patterns	Price tiers, classifications
pattern matching	Type checks, range checks	Object classification

1. If, Else-If, and Else

The if statement evaluates a boolean condition. If true, the code block runs. Use else if for additional checks and else as a catch-all. For simple true/false assignments, the ternary operator (condition ? a : b) is more concise.

If-Else Statements

Grade a score and check multiple conditions.

Try it Yourself »

using System;

class Program
{
    static void Main()
    {
        int score = 85;

        // If-else if-else chain
        if (score >= 90)
        {
            Console.WriteLine("Grade: A — Excellent!");
        }
        else if (score >= 80)
        {
            Console.WriteLine("Grade: B — Great job!");
        }
        else if (score >= 70)
        {
            Console.WriteLine("Grade: C — Good effort");
        }
        else if (score >= 60)
        {
            Console.WriteLin
...

2. Switch Statements & Expressions

Use switch when you have many specific values to check. The traditional switch statement uses case and break. Modern C# 8+ switch expressions are more concise — they return a value directly and support relational patterns like >= 5 and < 18.

Switch Statement & Expression

Compare traditional switch with modern switch expressions.

Try it Yourself »

using System;

class Program
{
    static void Main()
    {
        // Traditional switch statement
        int day = 3;
        switch (day)
        {
            case 1: Console.WriteLine("Monday"); break;
            case 2: Console.WriteLine("Tuesday"); break;
            case 3: Console.WriteLine("Wednesday"); break;
            case 4: Console.WriteLine("Thursday"); break;
            case 5: Console.WriteLine("Friday"); break;
            case 6:
            case 7: Console.WriteLine("Wee
...

3. Pattern Matching

Pattern matching is one of C#'s most powerful features. You can match on value ranges (relational patterns), types (type patterns), and even object properties (property patterns). This replaces complex if-else chains with clean, declarative code.

Pattern Matching

Use relational patterns and type matching to classify data.

Try it Yourself »

using System;

class Program
{
    static string GetTicketPrice(int age) => age switch
    {
        < 5 => "Free",
        >= 5 and < 13 => "£5 (Child)",
        >= 13 and < 18 => "£8 (Teen)",
        >= 18 and < 65 => "£12 (Adult)",
        >= 65 => "£6 (Senior)",
        _ => "Unknown"
    };

    static string Classify(object obj) => obj switch
    {
        int n when n > 0 => $"Positive integer: {n}",
        int n => $"Non-positive integer: {n}",
        string s => $"String of length {s.
...

Pro Tips

💡 Prefer switch expressions over if-else chains: When mapping input to output, switch expressions are cleaner and the compiler warns if you miss a case.
💡 Always include a default/discard (_) case: This catches unexpected values and prevents runtime errors.
💡 Use pattern matching with is: if (obj is string s && s.Length > 5) combines type checking and casting in one step.
💡 Avoid deeply nested ifs: Use early returns or guard clauses to reduce nesting and improve readability.

Common Mistakes

Forgetting break in switch: Unlike JavaScript, C# requires break in each case (unless cases are stacked intentionally). Missing it causes a compile error.
Using == for strings incorrectly: C# string comparison with == works correctly (unlike Java), but be aware of case sensitivity. Use StringComparison.OrdinalIgnoreCase when needed.
Order of else-if matters: Put the most specific conditions first. if (score >= 60) before if (score >= 90) means 90+ never triggers the A grade.
Empty if blocks: if (condition) {} compiles but does nothing — usually a bug. Add a comment if intentional.
Missing exhaustive patterns: Switch expressions without _ will throw SwitchExpressionException if no pattern matches.

🎉 Lesson Complete

✅ if / else if / else handles conditional branching with boolean expressions
✅ Ternary operator ? : for concise single-line conditional assignments
✅ switch statements handle multiple specific values with case and break
✅ Switch expressions (C# 8+) return values directly with => syntax
✅ Relational patterns: < 5, >= 18 and < 65 — range matching in switch
✅ Type patterns: obj is string s — combine type checking and casting
✅ Next lesson: Loops — for, while, do-while, and foreach

Control Flow

What You'll Learn

💡 Real-World Analogy

📊 Control Flow Quick Reference

1. If, Else-If, and Else

If-Else Statements

2. Switch Statements & Expressions

Switch Statement & Expression

3. Pattern Matching

Pattern Matching

Pro Tips

Common Mistakes

🎉 Lesson Complete

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