C# if, else and switch

int score = 72;

if (score >= 90)
{
    Console.WriteLine("Grade A");
}
else if (score >= 70)
{
    Console.WriteLine("Grade B");
}
else
{
    Console.WriteLine("Keep practising");
}

• The condition in brackets must be a bool.
• C# checks each condition from top to bottom and runs only the first block whose condition is true.
• Braces are optional for one-line bodies, but using them always prevents mistakes when lines are added later.

The conditional operator chooses between two values: condition ? valueIfTrue : valueIfFalse.

string label = stock > 0 ? "In stock" : "Sold out";

In methods, a guard clause handles invalid cases early and returns, so the main logic isn't buried in nested if blocks:

if (basket.Count == 0) return "Your basket is empty";
if (!user.IsVerified) return "Please verify your email";
// main logic here, without nesting

A switch statement compares one value against many cases:

switch (day)
{
    case "Sat":
    case "Sun":
        Console.WriteLine("Weekend");
        break;
    default:
        Console.WriteLine("Weekday");
        break;
}

Each section ends with break, and C# doesn't let execution fall through into the next section.

A switch expression returns a value and is often clearer:

string message = status switch
{
    "paid" => "Thanks for your order",
    "shipped" => "On its way",
    _ => "Processing"
};

_ is the discard pattern that matches anything, like default.

Patterns make range and combination checks readable:

string group = age switch
{
    < 13 => "Child",
    >= 13 and <= 19 => "Teen",
    >= 65 => "Senior",
    _ => "Adult"
};

if (input is not null and not "") { }
if (code is "GB" or "IE") { }

A when clause adds an extra condition to a case: case > 100 when isMember:. Patterns also work with types and object properties, which you'll meet in the pattern matching lesson.

Sometimes a decision depends on more than one value. Instead of nesting if statements, put the values in a tuple and switch on them together:

static string Discount(bool isMember, decimal total) => (isMember, total) switch
{
    (true, >= 50m) => "20% off",
    (true, _) => "10% off",
    (false, >= 100m) => "5% off",
    _ => "No discount"
};

Console.WriteLine(Discount(true, 60m));    // 20% off
Console.WriteLine(Discount(false, 120m));  // 5% off

Each arm reads like a row in a table of rules, which makes it easy to compare with a written specification. _ inside the brackets means "any value in this position", so (true, _) matches every member whose total didn't reach the first arm.

A switch statement without a default section simply does nothing when no case matches. A switch expression is different, because it must produce a value. If you leave a gap, the compiler warns you with CS8509, "the switch expression does not handle all possible values", and even names an example value it doesn't cover:

static string Size(int n) => n switch
{
    < 10 => "small",
    >= 10 and < 100 => "medium"
};   // warning CS8509: 100 is not covered

If you ignore the warning and call Size(500), the program throws a SwitchExpressionException at run time. Fix it by adding a final _ => arm, either with a sensible value or with throw new ArgumentOutOfRangeException(nameof(n)) when the value really should never happen. Treat this warning as an error: it points straight at a bug waiting to happen.