if, else & Conditions

Programs become useful when they can make decisions: give a discount only to students, allow cash withdrawal only in multiples of Rs 100, show "seat" or "seats" depending on the count. In C#, the if statement runs code only when a condition is true.

In this lesson you will write if, else if and else chains, combine conditions, use the short ?: operator for simple choices, and learn the mistakes that make conditions silently go wrong.

if and else

An if statement has a condition in brackets, which must be a bool, followed by a block. An optional else block runs when the condition is false.

int balance = 5000;
int amount = 2500;

if (amount <= balance)
{
    balance -= amount;
    Console.WriteLine($"Please collect Rs {amount}.");
}
else
{
    Console.WriteLine("Insufficient balance.");
}

Console.WriteLine($"Balance: Rs {balance}");

Please collect Rs 2500.
Balance: Rs 2500

The condition must really be a bool. In C, if (x) works with a number; in C# it is a compile error. Write the comparison you mean, for example if (x != 0).

else if chains

When there are more than two possibilities, chain conditions with else if. C# checks them from top to bottom and runs only the first block whose condition is true. Everything after it is skipped.

int marks = 67;

if (marks >= 90)
{
    Console.WriteLine("Grade A+");
}
else if (marks >= 75)
{
    Console.WriteLine("Grade A");
}
else if (marks >= 60)
{
    Console.WriteLine("Grade B");
}
else if (marks >= 40)
{
    Console.WriteLine("Grade C");
}
else
{
    Console.WriteLine("Needs improvement");
}

Grade B

Because the checks run in order, the second condition does not need to say marks >= 75 && marks < 90: if marks were 90 or more, the first block would already have run. This is why the order of an else if chain matters. If you put marks >= 40 first, every passing student would get Grade C.

Combining conditions

Use && (and), || (or) and ! (not) to build bigger conditions. Brackets make mixed conditions easier to read and remove any doubt about the order.

int age = 62;
bool isWoman = true;
bool isStudent = false;
decimal fare = 1240m;

decimal discount;
if (age >= 60 && isWoman)
{
    discount = 0.50m;
}
else if (age >= 60 || isStudent)
{
    discount = 0.25m;
}
else
{
    discount = 0m;
}

decimal toPay = fare - fare * discount;
Console.WriteLine($"Discount: {discount * 100:F0}%");
Console.WriteLine($"To pay: Rs {toPay:F2}");

int year = 2028;
bool isLeap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
Console.WriteLine($"{year} is a leap year: {isLeap}");

Discount: 50%
To pay: Rs 620.00
2028 is a leap year: True

The concession rates are invented for the example. Notice that discount is declared without a value but assigned in every branch. The compiler checks this: if one branch forgot to assign it, using discount later would be error CS0165.

The leap-year rule shows how brackets group a condition: a year is a leap year if it is divisible by 4 but not by 100, or if it is divisible by 400. .NET also has DateTime.IsLeapYear(year) ready-made.

The conditional operator ?:

For a simple choice between two values, the conditional operator (often called the ternary operator) is shorter than a full if. It reads: condition ? value if true : value if false.

int seatsLeft = 1;
string word = seatsLeft == 1 ? "seat" : "seats";
Console.WriteLine($"{seatsLeft} {word} left in the Vande Bharat");

int temperature = 43;
Console.WriteLine(temperature > 40 ? "Heatwave alert for Jaipur" : "Normal day");

int runs = 98;
string message = runs >= 100 ? "Century!" : $"{100 - runs} runs to a century";
Console.WriteLine(message);

1 seat left in the Vande Bharat
Heatwave alert for Jaipur
2 runs to a century

Use ?: when both sides are short values. If either side needs several statements, or if you are tempted to nest one ?: inside another, write a normal if instead.

Checking for null with is

Some values may be null, meaning "no value", such as the result of Console.ReadLine(). The clearest way to test for that is the is null and is not null patterns:

string? coupon = null;

if (coupon is null)
{
    Console.WriteLine("No coupon applied.");
}

coupon = "DIWALI10";
if (coupon is not null && coupon.StartsWith("DIWALI"))
{
    Console.WriteLine("Festival discount: 10% off");
}

No coupon applied.
Festival discount: 10% off

The order inside && matters here. Because && short-circuits, coupon.StartsWith runs only when coupon is not null, so the program can never crash with a NullReferenceException at that line. x == null also works and you will see it in older code, but is null cannot be changed by a class's custom == operator, so it is always a true null check.

Common mistakes

• A semicolon after the condition. if (marks >= 40); ends the if immediately, so the block below always runs. The compiler warns with CS0642: Possible mistaken empty statement.
• Using = instead of ==. if (age = 18) does not compile in C#, which saves you from a classic C bug.
• Wrong order in an else if chain. Put the most specific or strictest condition first.
• Comparing double values with ==. 0.1 + 0.2 == 0.3 is false. Compare with a small tolerance, or use decimal.
• Case-sensitive text checks. city == "pune" is false for "Pune". Use string.Equals(city, "pune", StringComparison.OrdinalIgnoreCase).

Key points

• if (condition) { ... } else { ... } runs one of two blocks; the condition must be a bool.
• In an else if chain only the first true branch runs, so order matters.
• Combine conditions with &&, || and !; && and || short-circuit.
• condition ? a : b picks between two values in one expression.
• Use is null and is not null to check for null safely.
• Always use braces, and watch out for a stray semicolon after if (...).

int units = 260;
int bill;

if (units <= 100)
{
    bill = units * 3;
}
else if (units <= 200)
{
    bill = 100 * 3 + (units - 100) * 5;
}
else
{
    bill = 100 * 3 + 100 * 5 + (units - 200) * 7;
}

if (units > 300)
{
    bill += 150;
}

Console.WriteLine($"Units: {units}, Bill: Rs {bill}");

Units: 260, Bill: Rs 1220