Logical Operators

Logical operators allow you to combine multiple conditions. They're essential for making complex decisions in your programs.

The Three Logical Operators

AND Operator (&&)

Returns true only when both conditions are true:

csharp
1bool a = true;
2bool b = true;
3bool c = false;
4
5Console.WriteLine(a && b);  // true (both true)
6Console.WriteLine(a && c);  // false (one is false)
7Console.WriteLine(c && c);  // false (both false)

Truth Table for AND

Practical AND Examples

csharp
1int age = 25;
2bool hasLicense = true;
3
4// Must be 18+ AND have a license to drive
5bool canDrive = age >= 18 && hasLicense;
6Console.WriteLine($"Can drive: {canDrive}");  // true
7
8// Check if value is in range
9int score = 85;
10bool isValidScore = score >= 0 && score <= 100;
11Console.WriteLine($"Valid score: {isValidScore}");  // true

OR Operator (||)

Returns true when at least one condition is true:

csharp
1bool a = true;
2bool b = false;
3
4Console.WriteLine(a || b);  // true (at least one is true)
5Console.WriteLine(b || b);  // false (both are false)

Truth Table for OR

Practical OR Examples

csharp
1string day = "Saturday";
2
3// Weekend is Saturday OR Sunday
4bool isWeekend = day == "Saturday" || day == "Sunday";
5Console.WriteLine($"Weekend: {isWeekend}");  // true
6
7// Discount for seniors OR students
8int age = 70;
9bool isStudent = false;
10
11bool getsDiscount = age >= 65 || isStudent;
12Console.WriteLine($"Gets discount: {getsDiscount}");  // true

NOT Operator (!)

Reverses a boolean value:

csharp
1bool isActive = true;
2
3Console.WriteLine(!isActive);  // false
4Console.WriteLine(!false);     // true

Practical NOT Examples

csharp
1bool isLoggedIn = false;
2
3// Check if NOT logged in
4if (!isLoggedIn)
5{
6    Console.WriteLine("Please log in");
7}
8
9// Check if string is NOT empty
10string name = "Alice";
11bool hasName = !string.IsNullOrEmpty(name);
12Console.WriteLine($"Has name: {hasName}");  // true

Combining Operators

You can combine multiple logical operators:

csharp
1int age = 25;
2bool hasID = true;
3bool isVIP = false;
4
5// Complex condition
6bool canEnter = (age >= 21 && hasID) || isVIP;
7Console.WriteLine($"Can enter: {canEnter}");  // true
8
9// Another example
10int score = 75;
11int attempts = 2;
12
13bool passed = score >= 70 && attempts <= 3;
14Console.WriteLine($"Passed: {passed}");  // true

Operator Precedence

Logical operators have precedence (evaluated in this order):

1. ! (NOT) - highest
2. && (AND)
3. || (OR) - lowest

csharp
1bool a = true;
2bool b = false;
3bool c = true;
4
5// Without parentheses
6bool result1 = a || b && c;  // true (AND evaluated first)
7// Equivalent to: a || (b && c) = true || false = true
8
9// With parentheses to change order
10bool result2 = (a || b) && c;  // true
11// (true || false) && true = true && true = true

Use parentheses to make your intentions clear:

csharp
1// Unclear
2bool unclear = a || b && c || !d;
3
4// Clear
5bool clear = a || (b && c) || (!d);

Short-Circuit Evaluation

C# uses short-circuit evaluation for && and ||:

csharp
1// AND: if first is false, second is not evaluated
2bool result = false && SomeFunction();  // SomeFunction() never runs
3
4// OR: if first is true, second is not evaluated
5bool result = true || SomeFunction();   // SomeFunction() never runs

Why It Matters

csharp
1string name = null;
2
3// Safe: if name is null, Length is never accessed
4bool isValid = name != null && name.Length > 0;
5
6// Unsafe: would throw NullReferenceException
7// bool isValid = name.Length > 0 && name != null;

Short-circuit evaluation allows safe null checks:

csharp
1// Check for null first, then access properties
2if (user != null && user.IsActive)
3{
4    Console.WriteLine("User is active");
5}

Practical Examples

csharp
1// Login validation
2string username = "admin";
3string password = "secret123";
4
5bool validUsername = username.Length >= 3;
6bool validPassword = password.Length >= 8;
7bool canLogin = validUsername && validPassword;
8
9Console.WriteLine($"Can login: {canLogin}");  // true
10
11// Shipping eligibility
12decimal orderTotal = 75.00m;
13bool isPremiumMember = false;
14string country = "USA";
15
16bool freeShipping = (orderTotal >= 50 && country == "USA") || isPremiumMember;
17Console.WriteLine($"Free shipping: {freeShipping}");  // true
18
19// Grade calculation
20int examScore = 85;
21int homeworkScore = 90;
22bool attendedClass = true;
23
24bool passedCourse = attendedClass && (examScore >= 60 || homeworkScore >= 80);
25Console.WriteLine($"Passed: {passedCourse}");  // true

Summary

In this lesson, you learned:

• && (AND): true when both conditions are true
• || (OR): true when at least one condition is true
• ! (NOT): reverses the boolean value
• Operator precedence: ! > && > ||
• Short-circuit evaluation skips unnecessary checks
• Use parentheses to clarify complex conditions

Next, we'll learn how to read user input for interactive programs.