Operators

By the end of this lesson you'll be able to do maths in C#, compare values, combine conditions with AND/OR, write compact decisions with the ternary operator, and handle missing (null) values safely — the toolkit every decision in your programs is built from.

Part of the free C# course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn

💡 Real-World Analogy

Operators are the buttons on a calculator . Arithmetic operators (+, -, ×, ÷) do maths. Comparison operators are a referee making yes/no calls — "Is player A's score higher than player B's?" Logical operators are how you combine conditions on a checklist: "Do I have BOTH a passport AND a visa?" ( && ) or "Do I have EITHER cash OR a card?" ( || ). The ternary operator is the quick "if yes do this, otherwise do that" sticky note. Master these and you can express almost any decision a program needs to make.

📊 Operator Precedence (who goes first)

Like maths, C# evaluates some operators before others. Higher priority happens first; () always wins, so when in doubt, add brackets.

Priority

Operators

Meaning

1 (Highest)

() ?. !

Parentheses, null-conditional, not

2

* / %

Multiply, divide, remainder

3

+ -

Add, subtract

4

< > <= >=

Comparison

5

== !=

Equality

6

&&

Logical AND

7

||

Logical OR

8

?:

Ternary

9 (Lowest)

= += -= *= ??=

Assignment

1. Arithmetic Operators

Arithmetic operators do the maths: + , - , * , / , and % (remainder). The one that trips up every beginner is integer division : when both numbers are whole, 10 / 3 gives 3 — the decimal part is thrown away, not rounded. The % operator gives you what's left over (handy for "is this even?"). For tidy updates, ++ / -- change a value by one, and compound operators like += mean "add to what's already there". Read this worked example, run it, then you'll write your own.

Your turn. The program below tallies a shopping basket — fill in the four blanks marked ___ using the hints, then run it and check the expected output.

2. Comparison & Logical Operators

A comparison asks a yes/no question and gives back a bool — true or false . You have == (equal), != (not equal), and < > <= >= . Logical operators glue those answers together: && (AND) is true only when both sides are true, || (OR) is true when at least one side is, and ! (NOT) flips a value. These are the foundation of every if statement you'll write next lesson.

Now you build a real rule. A ride needs the person to be old enough and tall enough — both must pass. Fill in the three blanks:

3. Ternary & Null-Safe Operators

The ternary operator condition ? a : b is a one-line if/else that produces a value : "if the condition is true use a , otherwise b ". C# also has null-safe operators for values that might be missing: ?? supplies a fallback when something is null , ??= assigns only if the variable is currently null, and ?. reads a member without crashing on null. (A null value means "nothing here yet" — reading from it directly throws a NullReferenceException , the most common runtime crash in C#.)

🔎 Deep Dive: short-circuiting & precedence

Short-circuit evaluation means C# stops the moment the answer is certain. With && , if the left side is false the whole thing is false , so the right side is never run . With || , if the left side is true the right side is skipped. This is not just an optimisation — you rely on it to stay safe, e.g. order != null && order.Total > 0 only touches order.Total once you know order isn't null.

Precedence decides who runs first when you don't add brackets. * and / beat + and - (just like maths), comparisons beat && , and && beats || . When a line gets busy, brackets make your intent obvious and remove all doubt.

Putting It Together: a Ticket Pricer

Here's a small but real program that uses arithmetic ( % ), comparison, logical OR, and a chained ternary together. Read it line by line — you understand every operator in it now.

A chained ternary reads like a ladder: the first matching condition wins. age < 13 ? 6m : age < 18 || isStudent ? 8m : 12m checks youngest first, then teen-or-student, then falls through to the standard price.

Pro Tips

• 💡 Use parentheses for clarity: even when precedence is correct, (a > 5) && (b < 10) reads more clearly than a > 5 && b < 10 .
• 💡 Prefer ?? over an if-null check: string name = input ?? "default"; is cleaner than a 4-line if/else.
• 💡 Short-circuit evaluation: && stops at the first false and || stops at the first true . Put the cheap or most-likely-to-decide check first.
• 💡 Use % for "every Nth": i % 2 == 0 is even; i % 5 == 0 is every fifth item.
• 💡 Chain ?. for nested data: user?.Address?.City returns null safely instead of crashing.

Common Errors (and the fix)

• "CS0029: Cannot implicitly convert type 'int' to 'bool'": you wrote = (assign) where you meant == (compare). if (x = 5) is wrong; use if (x == 5) .
• Integer division surprise: 7 / 2 gives 3 , not 3.5 . Make one side a decimal: (double)7 / 2 or 7.0 / 2 → 3.5 .
• Confusing ++ placement: x++ uses the old value then adds 1; ++x adds 1 first . They print different things inside an expression.
• "CS0019: Operator ' < ' cannot be applied": you tried to chain comparisons like maths — 1 < x < 10 doesn't compile. Write x > 1 && x < 10 .
• "CS8602: Dereference of a possibly null reference": you read .Length on something that might be null. Use text?.Length or give a fallback with ?? — don't silence it with ! .

📋 Quick Reference

Operator

Does

Example

Result

%

Remainder

7 % 2

1

/

Integer divide

7 / 2

+=

Add & assign

n += 5

n grows by 5

==

Equal?

3 == 3

true

AND (both)

true && false

false

OR (either)

true || false

x > 0 ? "+" : "-"

"+" or "-"

??

Null fallback

name ?? "Guest"

name or "Guest"

Frequently Asked Questions

Q: Why did 10 / 3 give me 3 instead of 3.33 ?

Both numbers are int , so C# does integer division and drops the remainder. Make one side a decimal type — 10.0 / 3 or (double)10 / 3 → 3.333... .

One = assigns a value ( x = 5 puts 5 into x). Two == compares and returns a bool ( x == 5 asks "is x equal to 5?"). Using = in an if is a classic bug — C# usually catches it with a compile error.

Q: When should I use the ternary operator instead of an if/else?

Use the ternary when you're choosing a value in one short expression, like age >= 18 ? "Adult" : "Minor" . If the branches do several things or get long, a full if/else stays more readable.

Q: What does % actually do, and why is it useful?

It gives the remainder after division. 7 % 2 is 1 . The classic trick is n % 2 == 0 to test if a number is even, or i % 10 == 0 to do something every tenth time.

Mini-Challenge: Even or Odd?

No blanks this time — just a brief and a blank canvas (with an outline to keep you on track). Combine the remainder operator % with a ternary to decide the answer, run it, and check your output against the examples in the comments.

🎉 Lesson Complete

• ✅ Arithmetic: + , - , * , / , % — and integer division truncates
• ✅ Increment/decrement: x++ (post) vs ++x (pre) return different values
• ✅ Compound assignment: += , -= , *= update a variable in place
• ✅ Comparison operators return bool : == , != , > , < , >= , <=
• ✅ Logical: && (AND), || (OR), ! (NOT) — with short-circuiting
• ✅ Ternary condition ? a : b ; null-safe ?? , ??= , ?.
• ✅ Next lesson: Control Flow — turn these conditions into if , switch , and pattern matching

Practice quiz

Continue this course

• Previous: Variables & Data Types
• Next: Control Flow