(OrElse and Or) and (AndAlso and And) - When to use?

vb.net andalso
vb or
andalso meaning
c# orelse

What is the difference between (OrElse and Or) and (AndAlso and And)? Is there any difference in their performances, let say the correctness benefit?? Is there any situation that I shoudn't use OrElse and AndAlso?

Or/And will always evaluate both1 the expressions and then return a result. They are not short-circuiting.

OrElse/AndAlso are short-circuiting. The right expression is only evaluated if the outcome cannot be determined from the evaluation of the left expression alone. (That means: OrElse will only evaluate the right expression if the left expression is false, and AndAlso will only evaluate the right expression if the left expression is true.)

Assuming that no side effects occur in the expressions and the expressions are not dependent (and any execution overhead is ignored), then they are the same.

However, in many cases it is that the expressions are dependent. For instance, we want to do something when a List is not-Nothing and has more than one element:

If list IsNot Nothing AndAlso list.Length > 0 Then .. 'list has stuff

This can also be used to avoid an "expensive" computation (or side-effects, ick!):

If Not Validate(x) OrElse Not ExpensiveValidate(x) Then .. 'not valid

Personally, I find that AndAlso and OrElse are the correct operators to use in all but the 1% - or less, hopefully! - of the cases where a side-effect is desired.

Happy coding.


1 An Exception thrown in the first expression will prevent the second expression from being evaluated, but this should hardly be surprising ..

vb.net - (OrElse and Or) and (AndAlso and And), Or/And will always evaluate both1 the expressions and then return a result. They are not short-circuiting. OrElse/AndAlso are short-circuiting. When you use AndAlso, VB.NET knows that the expression can't succeed once it is determined that the first part of the condition—a is not greater than Value1—is false. So VB.NET stops evaluating the expression right there. A similar example could be constructed using OrElse.

Besides the short-circuiting mentioned in the other answers, Or/And are usable as bitwise operators where OrElse/AndAlso are not. Bitwise operations include combining values of Flags enums, such as the FileAttributes enumeration where you might indicate a file is both read only and hidden by FileAttributes.ReadOnly Or FileAttributes.Hidden

AndAlso and OrElse VB.NET Basic Logical Operators, When you use AndAlso, VB.NET knows that the expression can't succeed once it is determined that the first part of the condition—a is not greater  At last for VB.NET developers, they can use these operators by the way "AndAlso" and "OrElse". Actually VB.NET guys can use "AND" and "OR" operators. But they are basically from VB6 and supported still by VB.NET, like many other functionalities. But my advice is to use "AndAlso" and "OrElse". Because you already see some positive side of these

The difference is that OrElse and AndAlso will short-circuit based on the first condition, meaning that if the first condition doesn't pass, the second (or more) conditions will not be evaluated. This is particularly useful when one of the conditions might be more intensive than the other.

Example where Or is fine (both conditions evaluated):

If Name = "Fred" Or Name = "Sam" Then

It really doesn't matter which way around they are evaluated

The following AndAlso is useful because the second condition might fail

If Not SomeObject Is Nothing AndAlso CheckObjectExistsInDatabase(SomeObject) Then

This allows for the first condition to check whether the object has been set and only if it has been set will go and check the database (or some other task). If this had been a plain And keyword, both would be evaluated.

OrElse Operator, How is the OrElse operator different from the OR operator? ' The OrElse operator is the homologous of AndAlso. It lets us perform a boolean ' comparison evaluating the second condition only if the first one is False If testFunction(5) = True OrElse otherFunction(4) = True Then ' If testFunction(5) is True, otherFunction(4) is not called. ' Insert code to be executed. End If

@Gideon - glad someone pointed that out. Here is a simple test that shows the dramatic impact of AndAlso:

    Dim tm As New Stopwatch
    Const tries As Integer = 123456
    Dim z As Integer = 0
    Dim s() As String = New String() {"0", "one"}

    Debug.WriteLine("AndAlso")
    For x As Integer = 0 To s.Length - 1
        z = 0
        tm.Restart() 'restart the stopwatch
        For y As Integer = 0 To tries
            If s(x) = x.ToString AndAlso s(x) = y.ToString Then '<<<<<<<<<<
                z += 1
            End If
        Next
        tm.Stop()
        Debug.WriteLine(x.ToString.PadRight(3, " "c) & z.ToString.PadRight(10, " "c) & tm.Elapsed.ToString)
    Next

    Debug.WriteLine("And")
    For x As Integer = 0 To s.Length - 1
        z = 0
        tm.Restart() 'restart the stopwatch
        For y As Integer = 0 To tries
            If s(x) = x.ToString And s(x) = y.ToString Then '<<<<<<<<<<
                z += 1
            End If
        Next
        tm.Stop()
        Debug.WriteLine(x.ToString.PadRight(3, " "c) & z.ToString.PadRight(10, " "c) & tm.Elapsed.ToString)
    Next

Or versus OrElse, , if the first term is True then the whole is definitely true - so we don't need to evaluate the second term. Or doesn't know this, and will always attempt to evaluate both terms. The OrElse operator is defined only for the Boolean Data Type. Visual Basic converts each operand as necessary to Boolean before evaluating the expression. If you assign the result to a numeric type, Visual Basic converts it from Boolean to that type such that False becomes 0 and True becomes -1 . For more information, see Boolean Type Conversions.

What is the difference between And and AndAlso in VB.NET?, What is the difference between and and AndAlso in VB net? Using "Or" is simple and requires less typing. The computational time savings of using OrElse is negligible in most cases. Most importantly, using OrElse can hide errors in later clauses that may not be initially revealed until those conditions would eventually be met by the program logic.

AndAlso & OrElse Operators in C#, like you would use && . Checks if x is equal to 5, and if y is equal to 7, then continues if both are true. The AndAlso operator is defined only for the Boolean Data Type. Visual Basic converts each operand as necessary to Boolean before evaluating the expression. If you assign the result to a numeric type, Visual Basic converts it from Boolean to that type such that False becomes 0 and True becomes -1 .

Why And is needed when there is AndAlso, converts it from Boolean to that type such that False becomes 0 and True becomes -1 . Though slower in general, you might have a complex expression as the second one where the case is faster, and in special cases the second part might not even evaluate (see the example by MikeMuffinMan), so in some cased the andalso/orelse approach with a CASE is a must.

Comments
  • possible duplicate of Should I always use the AndAlso and OrElse operators?
  • Just started using VB.NET (first time in many years for VB). I came online only to ask this specific question. My thanks to @aer and the many folks who clarified with their great answers.