Lesson 7 — Decision Making

Conditional Statements

C# conditionals work like Java's with one powerful addition: the switch expression, which is concise, exhaustive, and eliminates fall-through bugs.

CSHARP — if/else if/else 
int mark = 74;

if (mark >= 80)
    Console.WriteLine("Distinction");
else if (mark >= 60)
    Console.WriteLine("Merit");
else if (mark >= 50)
    Console.WriteLine("Pass");
else
    Console.WriteLine("Not yet competent");

Switch Expression (C# 8+)

The switch expression is concise, exhaustive (the compiler warns if a case is missing), and has no fall-through. It returns a value directly.

CSHARP — Switch expression 
// Switch expression — cleaner than switch statement
string result = mark switch {
    >= 80 => "Distinction",
    >= 60 => "Merit",
    >= 50 => "Pass",
    _     => "Not yet competent"  // _ is the catch-all
};
Console.WriteLine(result);

// Traditional switch statement (still used with legacy code):
string day = "Wednesday";
switch (day) {
    case "Monday":    Console.WriteLine("Java class");    break;
    case "Wednesday": Console.WriteLine("Career dev");    break;
    case "Friday":    Console.WriteLine("Presentations"); break;
    default:          Console.WriteLine("Self-study");    break;
}

Pattern Matching in Conditions

CSHARP — Pattern matching 
// is pattern — type check and cast in one step:
object obj = "Hello from Your IT Tutor";
if (obj is string s && s.Length > 5)
    Console.WriteLine($"String of length {s.Length}: {s}");

// Switch with type patterns:
static string Describe(object o) => o switch {
    int n    when n > 0 => $"Positive int: {n}",
    int n               => $"Non-positive int: {n}",
    string s            => $"String: {s}",
    null                => "null",
    _                   => $"Other: {o.GetType().Name}"
};

Practice Task

Your Turn

Write a program with a hardcoded learner mark and a boolean for on-time submission. Use a switch expression to assign a grade label. Use a nested if to print an extra message if they passed but submitted late. Use a traditional switch for the day-of-week schedule.

Common Mistakes

  • Forgetting the _ catch-all in a switch expression — the compiler will warn about unhandled cases.
  • Missing break in traditional switch — fall-through is silent.
  • Semicolon after if(...) — creates an empty body.
  • Ordering conditions from least to most restrictive — a 90% would match >= 50 first and show 'Pass'.

Professional Tip

The switch expression is one of C#'s best modern features. It eliminates three entire classes of bugs: forgetting break, missing cases, and ordering issues. Use it over switch statements for new code.

Mini Quiz

What is the catch-all symbol in a C# switch expression?