Logical and Bitwise Operators in Visual Basic

Logical operators compare Boolean expressions and return a Boolean result. The And, Or, AndAlso, OrElse, and Xor operators are binary because they take two operands, while the Not operator is unary because it takes a single operand. Some of these operators can also perform bitwise logical operations on integral values.

Unary Logical Operator

The Not Operator (Visual Basic) performs logical negation on a Boolean expression. It yields the logical opposite of its operand. If the expression evaluates to True, then Not returns False; if the expression evaluates to False, then Not returns True. The following example illustrates this.

Dim x, y As Boolean
x = Not 23 > 14
y = Not 23 > 67
' The preceding statements set x to False and y to True.

Binary Logical Operators

The And Operator (Visual Basic) performs logical conjunction on two Boolean expressions. If both expressions evaluate to True, then And returns True. If at least one of the expressions evaluates to False, then And returns False.

The Or Operator (Visual Basic) performs logical disjunction or inclusion on two Boolean expressions. If either expression evaluates to True, or both evaluate to True, then Or returns True. If neither expression evaluates to True, Or returns False.

The Xor Operator (Visual Basic) performs logical exclusion on two Boolean expressions. If exactly one expression evaluates to True, but not both, Xor returns True. If both expressions evaluate to True or both evaluate to False, Xor returns False.

The following example illustrates the And, Or, and Xor operators.

Dim a, b, c, d, e, f, g As Boolean

a = 23 > 14 And 11 > 8
b = 14 > 23 And 11 > 8
' The preceding statements set a to True and b to False.

c = 23 > 14 Or 8 > 11
d = 23 > 67 Or 8 > 11
' The preceding statements set c to True and d to False.

e = 23 > 67 Xor 11 > 8
f = 23 > 14 Xor 11 > 8
g = 14 > 23 Xor 8 > 11
' The preceding statements set e to True, f to False, and g to False.

Short-Circuiting Logical Operations

The AndAlso Operator (Visual Basic) is very similar to the And operator, in that it also performs logical conjunction on two Boolean expressions. The key difference between the two 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 because it cannot alter the final result, and AndAlso returns False.

Similarly, the OrElse Operator (Visual Basic) 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 because it cannot alter the final result, and OrElse returns True.

Short-Circuiting Trade-Offs

Short-circuiting can improve performance by not evaluating an expression that cannot alter the result of the logical operation. However, if that expression performs additional actions, short-circuiting skips those actions. For example, if the expression includes a call to a Function procedure, that procedure is not called if the expression is short-circuited, and any additional code contained in the Function does not run. Therefore, the function might run only occasionally, and might not be tested correctly. Or the program logic might depend on the code in the Function.

The following example illustrates the difference between And, Or, and their short-circuiting counterparts.

Dim amount As Integer = 12
Dim highestAllowed As Integer = 45
Dim grandTotal As Integer

If amount > highestAllowed And checkIfValid(amount) Then
    ' The preceding statement calls checkIfValid().
End If
If amount > highestAllowed AndAlso checkIfValid(amount) Then
    ' The preceding statement does not call checkIfValid().
End If
If amount < highestAllowed Or checkIfValid(amount) Then
    ' The preceding statement calls checkIfValid().
End If
If amount < highestAllowed OrElse checkIfValid(amount) Then
    ' The preceding statement does not call checkIfValid().
End If

Function checkIfValid(ByVal checkValue As Integer) As Boolean
    If checkValue > 15 Then
        MsgBox(CStr(checkValue) & " is not a valid value.")
        ' The MsgBox warning is not displayed if the call to
        ' checkIfValid() is part of a short-circuited expression.
        Return False
    Else
        grandTotal += checkValue
        ' The grandTotal value is not updated if the call to
        ' checkIfValid() is part of a short-circuited expression.
        Return True
    End If
End Function

In the preceding example, note that some important code inside checkIfValid() does not run when the call is short-circuited. The first If statement calls checkIfValid() even though 12 > 45 returns False, because And does not short-circuit. The second If statement does not call checkIfValid(), because when 12 > 45 returns False, AndAlso short-circuits the second expression. The third If statement calls checkIfValid() even though 12 < 45 returns True, because Or does not short-circuit. The fourth If statement does not call checkIfValid(), because when 12 < 45 returns True, OrElse short-circuits the second expression.

Bitwise Operations

Bitwise operations evaluate two integral values in binary (base 2) form. They compare the bits at corresponding positions and then assign values based on the comparison. The following example illustrates the And operator.

Dim x As Integer
x = 3 And 5

The preceding example sets the value of x to 1. This happens for the following reasons:

• The values are treated as binary:

  3 in binary form = 011

  5 in binary form = 101

• The And operator compares the binary representations, one binary position (bit) at a time. If both bits at a given position are 1, then a 1 is placed in that position in the result. If either bit is 0, then a 0 is placed in that position in the result. In the preceding example this works out as follows:

  011 (3 in binary form)

  101 (5 in binary form)

  001 (The result, in binary form)

• The result is treated as decimal. The value 001 is the binary representation of 1, so x = 1.

The bitwise Or operation is similar, except that a 1 is assigned to the result bit if either or both of the compared bits is 1. Xor assigns a 1 to the result bit if exactly one of the compared bits (not both) is 1. Not takes a single operand and inverts all the bits, including the sign bit, and assigns that value to the result. This means that for signed positive numbers, Not always returns a negative value, and for negative numbers, Not always returns a positive or zero value.

The AndAlso and OrElse operators do not support bitwise operations.

See Also

Reference

Logical/Bitwise Operators (Visual Basic)

Concepts

Boolean Expressions (Visual Basic)

Arithmetic Operators in Visual Basic

Comparison Operators in Visual Basic

Concatenation Operators in Visual Basic

Efficient Combination of Operators (Visual Basic)