What is the difference between And and AndAlso in VB.NET?

Question

In VB.NET, what is the difference between And and AndAlso? Which should I use?

Anthonyanthophore

AndAlso is more like AND in most other languages I've come across. The unnecessary checking of secondary terms by VB And seems to be a unfortunate and unusual legacy - perhaps simply because the original implementation was suboptimal(?). BTW Sadly, VB has a similar issue with Or and OrElse ;) – Mesoderm

Answer 1

The And operator evaluates both sides, where AndAlso evaluates the right side if and only if the left side is true.

An example:

If mystring IsNot Nothing And mystring.Contains("Foo") Then
  ' bla bla
End If

The above throws an exception if mystring = Nothing

If mystring IsNot Nothing AndAlso mystring.Contains("Foo") Then
  ' bla bla
End If

This one does not throw an exception.

So if you come from the C# world, you should use AndAlso like you would use &&.

More info here: http://www.panopticoncentral.net/2003/08/18/the-ballad-of-andalso-and-orelse/

Embonpoint

"More info" link is broken. Here's the new URL: panopticoncentral.net/2003/08/18/… – Strange

just edit the answer Rory... no need to leave broken links for people to trip over :) – Cellist

@Cellist I was worried the suggested edit would be rejected as “not substantial” – a few past rejections have made me wary. But your edit passed, so I guess a plain link-fixing edit is safe. – Strange

Awesome! I've wondered if there was a way to have it short circuit off if half of the and worked and the second half didnt need to be evaluated the way I was used to in Java – Runlet

Yes always is evaluated from left to right, it very useful to evaluate if the object to query is null before ask from some properties of it. As Nico said, it the same as && in C#. – Defazio

Does anybody know why they haven't chosen "AndThen" instead? I think this would be more intuitive. – Frightful

I've always thought this was counter intuitive. AndAlso would imply it's going to check both and this should be the other way around! Hence why I have to come to SO to check things like this (yes I'm a c# guy). You should also never have to ask a question like "difference between And and AndAlso". Too many ands! My English teacher would have killed me – Profitsharing

Anytime I use OrElse I always think of South Park "Or else what?" "Exactly" – Profitsharing

@Frightful guessing that it would cause more confusion because Then is its own keyword. – Candiscandle

@RobinFrench I came from Java and Php to hit this. it also baffled me. but some one explained to me that in early versions of VB there was no way to short circuit at all. and when they added the ability lots of older code was having issues because a lot of old code was written in a way that the short circuiting actually did cause a short, so they had to leave the regular And, OR work like they classically did and and in new keywords. – Gauze

@RobinFrench 'You should also never have to ask a question like "difference between And and AndAlso"' - Strongly disagree. Im not saying that And/AndAlso is as clear as it could possibly be, but presumably, at some point, you had to ask the difference between "&" and "&&" in C#, as that isn't immediately obvious if you don't already know either. – Foreign

@tmighty, I don't know this for sure but, to me, AndThen implies performing actions rather than evaluating criteria. Also, AndThen matches up with OrElse better linguistically than AndThen does. – Heraclitean

That should have been: "Also, AndAlso matches up with OrElse better linguistically than AndThen does". – Heraclitean

Answer 2

The And operator will check all conditions in the statement before continuing, whereas the Andalso operator will stop if it knows the condition is false. For example:

if x = 5 And y = 7

Checks if x is equal to 5, and if y is equal to 7, then continues if both are true.

if x = 5 AndAlso y = 7

Checks if x is equal to 5. If it's not, it doesn't check if y is 7, because it knows that the condition is false already. (This is called short-circuiting.)

Generally people use the short-circuiting method if there's a reason to explicitly not check the second part if the first part is not true, such as if it would throw an exception if checked. For example:

If Not Object Is Nothing AndAlso Object.Load()

If that used And instead of AndAlso, it would still try to Object.Load() even if it were nothing, which would throw an exception.

Blotch

If the right side has a side effect you need, just move it to the left side rather than using "And". You only really need "And" if both sides have side effects. And if you have that many side effects going on you're probably doing something else wrong. – Apportion

First, Andalso is not primarily used to 'save on runtime', that's preposterous. Second, having the second argument perform some useful 'side effect' is ugly-ass bad practice. – Guanidine

You can't argue that it doesn't 'save on runtime' though. And I have found situations where side effects are in fact, not ugly-ass bad practice. – Blotch

"Generally people use the short-circuiting method if there's a reason to explicitly not check the second part" Erm, wut? I would hope people use short-circuiting unless they have a reason not to! No one should use the old and/or unless they have a reason - of which I think legitimate ones are few & far between. Surely there's a reason most other languages short-circuit by default: it retains the sense of the result while not evaluating potentially costly expressions when they contribute nothing. Stashing side-effects in conditions should be, well, side-eyed. But that's just my opinion... – Iain

Answer 3

Interestingly none of the answers mentioned that And and Or in VB.NET are bit operators whereas OrElse and AndAlso are strictly Boolean operators.

Dim a = 3 OR 5 ' Will set a to the value 7, 011 or 101 = 111
Dim a = 3 And 5 ' Will set a to the value 1, 011 and 101 = 001
Dim b = 3 OrElse 5 ' Will set b to the value true and not evaluate the 5
Dim b = 3 AndAlso 5 ' Will set b to the value true after evaluating the 5
Dim c = 0 AndAlso 5 ' Will set c to the value false and not evaluate the 5

Note: a non zero integer is considered true; Dim e = not 0 will set e to -1 demonstrating Not is also a bit operator.

|| and && (the C# versions of OrElse and AndAlso) return the last evaluated expression which would be 3 and 5 respectively. This lets you use the idiom v || 5 in C# to give 5 as the value of the expression when v is null or (0 and an integer) and the value of v otherwise. The difference in semantics can catch a C# programmer dabbling in VB.NET off guard as this "default value idiom" doesn't work in VB.NET.

So, to answer the question: Use Or and And for bit operations (integer or Boolean). Use OrElse and AndAlso to "short circuit" an operation to save time, or test the validity of an evaluation prior to evaluating it. If valid(evaluation) andalso evaluation then or if not (unsafe(evaluation) orelse (not evaluation)) then

Bonus: What is the value of the following?

Dim e = Not 0 And 3

Canadian

I think you've confused || and && with ??. While there are languages where || will return the non-falsey value, C# is not one of them, and returns a bool (except for lifted nullable operators, where you get a Nullable<bool> result) – Epiclesis

This is why I hate VB – Rubato

Answer 4

If Bool1 And Bool2 Then

Evaluates both Bool1 and Bool2

If Bool1 AndAlso Bool2 Then

Evaluates Bool2 if and only if Bool1 is true.

Clifford

Answer 5

Just for all those people who say side effects are evil: a place where having two side effects in one condition is good would be reading two file objects in tandem.

While File1.Seek_Next_Row() And File2.Seek_Next_Row()
    Str1 = File1.GetRow()
    Str2 = File2.GetRow()
End While

Using the And ensures that a row is consumed every time the condition is checked. Whereas AndAlso might read the last line of File1 and leave File2 without a consumed line.

Of course the code above wouldn't work, but I use side effects like this all the time and wouldn't consider it "bad" or "evil" code as some would lead you to believe. It's easy to read and efficient.

Mutant

Answer 6

AndAlso is much like And, except it works like && in C#, C++, etc.

The difference is that if the first clause (the one before AndAlso) is true, the second clause is never evaluated - the compound logical expression is "short circuited".

This is sometimes very useful, e.g. in an expression such as:

If Not IsNull(myObj) AndAlso myObj.SomeProperty = 3 Then
   ...
End If

Using the old And in the above expression would throw a NullReferenceException if myObj were null.

Guanidine

I think you mean when the first clause is false, the second clause will not be evaluated? In example above Not IsNull(myObj) is FALSE, then it shouldn't evaluate the 2nd clause. Otherwise, it will throw a NullReferenceException. – Gilberto

Answer 7

Also see Stack Overflow question: Should I always use the AndAlso and OrElse operators?.

Also: A comment for those who mentioned using And if the right side of the expression has a side-effect you need:

If the right side has a side effect you need, just move it to the left side rather than using "And". You only really need "And" if both sides have side effects. And if you have that many side effects going on you're probably doing something else wrong. In general, you really should prefer AndAlso.

Apportion

Answer 8

A simple way to think about it is using even plainer English

If Bool1 And Bool2 Then
If [both are true] Then


If Bool1 AndAlso Bool2 Then
If [first is true then evaluate the second] Then

Patron

English is more confusing than code because it doesn’t have a spec! – Candiscandle

Answer 9

In addition to the answers above, AndAlso provides a conditioning process known as short circuiting. Many programming languages have this functionality built in like vb.net does, and can provide substantial performance increases in long condition statements by cutting out evaluations that are unneccessary.

Another similar condition is the OrElse condition which would only check the right condition if the left condition is false, thus cutting out unneccessary condition checks after a true condition is found.

I would advise you to always use short circuiting processes and structure your conditional statements in ways that can benefit the most by this. For example, test your most efficient and fastest conditions first so that you only run your long conditions when you absolutely have to and short circuit the other times.

Matazzoni

Also worth mentioning is IIF (Immediate If) because it does not short-circuit and can create conditions quite unlike the programmer's intent, since both the true and false components are evaluated. Earlier, someone stated that they use this side-effect behavior intentionally, but -- I do not view that as an effective method, since the Immediate If has no purposeful intent in performing the evaluation in that manner that I can detect. – Mucilaginous

"In addition to the answers above, AndAlso provides a conditioning process known as short circuiting." Every answer above this one focusses on short-circuiting. :P But good description and recommendations. – Iain

Answer 10

For majority of us OrElse and AndAlso will do the trick except for a few confusing exceptions (less than 1% where we may have to use Or and And).

Try not to get carried away by people showing off their boolean logics and making it look like a rocket science.

It's quite simple and straight forward and occasionally your system may not work as expected because it doesn't like your logic in the first place. And yet your brain keeps telling you that his logic is 100% tested and proven and it should work. At that very moment stop trusting your brain and ask him to think again or (not OrElse or maybe OrElse) you force yourself to look for another job that doesn't require much logic.

Tonjatonjes

Answer 11

Use And and Or for logical bit operations, e.g. x% = y% Or 3

AndAlso and OrElse are for If statements:

If x > 3 AndAlso x <= 5 Then

is same as

If (x > 3) And (x <= 5) Then

The way I see it...

Also it's worth noting that it's recommended (by me) that you enclose equations with logical operators in If statements, so they don't get misinterpreted by the compiler, for example:

If x = (y And 3) AndAlso ...

Hammertoe

Answer 12

To understand with words not cods:

Use Case:
With “And” the compiler will check all conditions so if you are checking that an object could be “Nothing” and then you are checking one of it’s properties you will have a run time error.
But with AndAlso with the first “false” in the conditions it will checking the next one so you will not have an error.

Inclinable

Answer 13

The And / AndAlso Or / OrElse are actually quite useful. Consider this code, where the function DoSomethingWihtAandB may not set A and/or B if it returns False:

Dim A,B,C,D As Object
If DoSomethingWithAandB(A,B)=True And A=1 And B=2 Then C=3

It will crash if DoSomethingWithAandB returns False, because it will continue to evaluate after the And and A and B will equal Nothing

Dim A,B,C,D As Object
If DoSomethingWithAandB(A,B)=True AndAlso A=1 AndAlso B=2 Then C=3

This won't crash if DoSomethingWithAandB returns False because it will stop evaluating at DoSomethingWithAandB(A,B)=True, because False is returned. The AndAlso prevents any further evaluation of the conditions (as the first condition failed). OrElse works the same way. The first True evaluated in the logic chain will stop any further evaluation.

Stuck