Decisions: if, else, switch and switch expressions

Why decisions?

Real programs must react differently to different data. A train ticket costs less for a senior citizen. A payment fails if the balance is too low. A decision in code is a place where the program checks a condition and picks a path.

Conditions are bool values

A condition is an expression that gives true or false, a bool. You build it with comparison operators.

if, else if, else

int marks = 72;

if (marks >= 90)
{
    Console.WriteLine("Grade A");
}
else if (marks >= 60)
{
    Console.WriteLine("Grade B");
}
else
{
    Console.WriteLine("Grade C");
}

Combining conditions: && || !

int age = 65;
bool hasId = true;

if (age >= 60 && hasId)
    Console.WriteLine("Senior citizen discount");

bool isHoliday = false, isSunday = true;
if (isHoliday || isSunday)
    Console.WriteLine("Office closed");

if (!hasId)
    Console.WriteLine("Please show ID");

&& and || are short-circuit operators: if the left side already decides the answer, the right side is not checked. So if (name != null && name.Length > 3) is safe: when name is null, the length check never runs.

The ternary operator ? :

int age = 15;
string ticket = age < 12 ? "Child" : "Adult";
Console.WriteLine(ticket);

The switch statement

When you compare one value against many fixed options, a switch is cleaner than a long else-if chain. Each case ends with break (or return). Unlike C and Java, C# does not let one non-empty case fall through to the next.

string day = "Sun";
switch (day)
{
    case "Sat":
    case "Sun":
        Console.WriteLine("Weekend");
        break;
    case "Fri":
        Console.WriteLine("Almost weekend");
        break;
    default:
        Console.WriteLine("Weekday");
        break;
}

Switch expressions (C# 8 and later)

A switch expression gives back a value. Each line is called an arm: a pattern, an arrow =>, and the result. The underscore _ is the discard pattern, meaning "anything else". With relational patterns (C# 9) you can even compare ranges.

int marks = 83;
string grade = marks switch
{
    >= 90 => "A",
    >= 75 => "B",
    >= 40 => "C",
    _ => "F"
};
Console.WriteLine(grade);   // B

string day = "Sat";
string kind = day switch
{
    "Sat" or "Sun" => "Weekend",
    _ => "Weekday"
};

Common mistakes and fixes

• Wrong order in else-if chains: checking marks >= 40 before marks >= 90 makes everyone a C. Check the strictest condition first.
• Forgetting break in a switch case: error CS0163, control cannot fall through. Add break or return.
• Comparing strings with different case: "pune" == "Pune" is false. Use string.Equals(a, b, StringComparison.OrdinalIgnoreCase).
• Writing if (x > 5 && < 10): each side needs a full comparison: if (x > 5 && x < 10).