Logical operators

A logical operator is an operator that combines multiple boolean values (tests that evaluate to true or false) into a single boolean value.

The key thing is that they work with boolean values (true or false) and produce another boolean value as a result.

We can logically think of this as:

• If true AND true the the result is true
• If true OR false the the result is false
• If NOT true the result is false
• etc.

Cheatsheet

Operator	Name	Returns	Example	Result
&&	Logical AND	true if both are true	true && false	false
||	Logical OR	true if at least one is true	true || false	true
!	Logical NOT	opposite of the value	!true	false

Logical AND (&&)

The logical AND operator returns:

• true if both values are true
• false if at least one value is false

We can write this as a truth table:

A	B	A && B
true	true	true
true	false	false
false	true	false
false	false	false

An example of using the logical AND operator in C# is:

bool isLoggedIn = true;
bool isAdmin = false;
bool canAccess = isLoggedIn && isAdmin; // false

Or, for an example which returns true:

int age = 20;
bool hasLicence = true;
bool hasCar = true;
bool isOldEnough = age >= 17;
bool canDrive = hasLicence && hasCar && isOldEnough; // true

This also shows an example of chainingg multiple logical operators together.

Logical OR (||)

The logical OR operator returns:

• true if at least one value is true
• false if both values are false

We can write this as a truth table:

A	B	A || B
true	true	true
true	false	true
false	true	true
false	false	false

In other words, the only time it returns false is when both values are false.

An example of using the logical OR operator in C#:

bool isWeekend = false;
bool isHoliday = true;
bool canRelax = isWeekend || isHoliday; // true

Logical NOT (!)

The logical NOT operator returns the opposite of the boolean value.

• If the value is true, it returns false
• If the value is false, it returns true

We can write this as a truth table:

A	!A
true	false
false	true

In other words, it inverts the boolean value.

Here’s an example:

bool isRaining = true;
bool goodTimeForAPicnic = !isRaining; // false

Or, another example:

bool isWeekend = false;
bool isWeekday = !isWeekend; // true

Combining logical operators

Logical operators can be combined to create more complex boolean expressions.

For example:

bool isWeekend = true;
bool isHoliday = false;
bool isSunny = true;
bool canGoToBeach = (isWeekend || isHoliday) && isSunny; // true

Or, using the logical NOT operator:

bool isRaining = false;
bool isWindy = true;
bool isBadWeather = !(isRaining || isWindy);

Operator precedence

In the last example, you might wonder why the parentheses were necessary.

That’s because the logical NOT operator (!) has a higher precedence than the logical AND (&&) and logical OR (||) operators, and so, without the brackets, we would just have been negating the value of isRaining only.

Here’s the order of precedence for logical operators (from highest to lowest):

• Logical NOT (!)
• Logical AND (&&)
• Logical OR (||)

The ones higher on the list will be evaluated first.