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Logical Operators

The logical operators compare Boolean expressions and return a Boolean result. The And, Or, AndAlso, OrElse, and Xor operators take two operands, and the Not operator takes a single operand.

The Not operator performs logical negation on a Boolean expression. Put simply, it yields the opposite of the expression it evaluates. If the expression evaluates to True, Not yields False; if the expression evaluates to False, Not yields True. An example is shown below:

Dim x As Boolean
x = Not 23 > 12   ' x equals False.
x = Not 23 > 67   ' x equals True.

The And operator performs logical conjunction on two Boolean expressions. That is, if both expressions evaluate to True, then the And operator returns True. If either or both expressions evaluate to False, then And returns False.

The Or operator performs logical disjunction on two Boolean expressions. If either expression evaluates to True, Or returns True. If neither expression evaluates to True, Or returns False.

Xor performs logical exclusion on two expressions. If either expression evaluates to True, but not both, Xor returns True. If both expressions evaluate to True or both expressions evaluate to False, Xor returns False.

Examples of the And, Or, and Xor operators are shown below:

Dim x As Boolean
x = 23 > 12 And 12 >4   ' x = True
x = 12 > 23 And 12 > 4   ' x = False
x = 23 > 12 Or 4 > 12`   ' x = True
x = 23 > 45 Or 4 > 12   ' x = False
x = 23 > 45 Xor 12 > 4   ' x = True
x = 23 > 12 Xor 12 > 4   ' x = False
x = 12 > 23 Xor 4 > 12   ' x = False
Note   In addition to being logical operators, Not, Or, And, and Xor also perform bitwise arithmetic when used on numeric values. Information on this functionality can be found at Arithmetic Operators.

The AndAlso operator is very similar to the And operator, in that it also performs logical conjunction on two Boolean expressions. The key difference between AndAlso and And is that AndAlso exhibits short-circuiting behavior. If the first expression in an AndAlso expression evaluates to False, then the second expression is not evaluated and False is returned for the AndAlso expression.

Similarly, the OrElse operator performs short-circuiting logical disjunction on two Boolean expressions. If the first expression in an OrElse expression evaluates to True, then the second expression is not evaluated and True is returned for the OrElse expression. Below are some examples that illustrate the difference between And, Or, and their counterparts:

12 > 45 And MyFunction(4) ' MyFunction() is called.
12 > 45 AndAlso MyFunction(4)   ' MyFunction() is not called.
45 > 12 Or MyFunction(4)   ' MyFunction is called.
45 > 12 OrElse MyFunction(4)   ' MyFunction is not called 

In the first example, MyFunction() is called, even though 12 > 45 returns False, because And does not short circuit. In the second example, MyFunction is not called, because 12 > 45 returns False, so AndAlso short circuits the second expression. In the third example, MyFunction is called, even though 45 > 12 returns True, because Or does not short circuit. In the fourth example, MyFunction is not called because 45 > 12 returns True, so OrElse short circuits the second expression.

Note   Although And and Or support bitwise operations with numeric values, AndAlso and OrElse do not.

See Also

Logical/Bitwise Operators | Arithmetic Operators | Boolean Expressions | Efficient Combination of Operators

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