Making decisions with if / else

Programs that choose

So far our programs ran every line from top to bottom. Real programs make decisions: show an error if the password is wrong, apply a discount for students, skip empty lines. The if statement runs a block of code only when a condition is true.

int temperature = 32;
 
if (temperature > 30)
{
    Console.WriteLine("It's hot — drink water!");
}

The condition inside the parentheses must be a bool expression — a comparison, a bool variable, or a combination with &&, ||, !.

else: the other path

if (balance >= price)
{
    Console.WriteLine("Purchase approved");
}
else
{
    Console.WriteLine("Insufficient funds");
}

Exactly one of the two blocks runs.

else if: chains of rules

if (temperature > 30)
{
    Console.WriteLine("Hot");
}
else if (temperature > 20)
{
    Console.WriteLine("Warm");      // only checked if the first condition was false
}
else if (temperature > 10)
{
    Console.WriteLine("Mild");
}
else
{
    Console.WriteLine("Cold");
}

C# checks the conditions in order and runs the first block whose condition is true. Order matters: if you checked > 10 first, 35 °C would print "Mild".

Combining conditions

if (age >= 18 && hasTicket)                // both must be true
    Console.WriteLine("Welcome in");
 
if (day == "Saturday" || day == "Sunday")  // either is enough
    Console.WriteLine("Weekend!");
 
if (!isLoggedIn)                           // negation
    Console.WriteLine("Please log in");

Guard clauses: fail fast, stay flat

Nested ifs quickly become hard to read:

string Describe(int score)
{
    if (score >= 0)
    {
        if (score <= 100)
        {
            if (score >= 50) return "pass";
            else return "fail";
        }
    }
    return "invalid";
}

Handle the edge cases first with early returns, and the main logic stays at the top level:

string Describe(int score)
{
    if (score < 0 || score > 100) return "invalid";   // guard clause
 
    return score >= 50 ? "pass" : "fail";
}

The ternary operator

For a simple either/or value, use condition ? whenTrue : whenFalse:

string label = age >= 18 ? "adult" : "minor";
int shipping = total >= 50 ? 0 : 5;
Console.WriteLine($"You have {count} {(count == 1 ? "item" : "items")}");

Keep ternaries short — if you need to nest them, an if or a switch (next lesson) is clearer.

Try it

Change temperature and isRaining and see which message appears:

Key takeaways

• if runs a block when its condition is true; else handles the other case.
• In an else if chain, the first true condition wins — order your checks carefully.
• Guard clauses handle invalid input early and keep code flat.
• ?: picks between two values in one expression.