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I'm fairly new to .NET programming, and I recently stumbled across AndAlso. As I understand it, AndAlso evaluates the first expression, if it is true then it evaluates the second...and so on. And, on the other hand, always evaluates all terms in the expression.

If my understanding is correct, would AndAlso always be (marginally) more efficient. If this is the case, why use And at all?

JShell
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  • possible duplicate of [What is the difference between And and AndAlso in VB.net?](http://stackoverflow.com/questions/302047/what-is-the-difference-between-and-and-andalso-in-vb-net) – GSerg Jun 17 '15 at 19:14
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    I believe I understand the difference between the two. My question is more towards why And is ever preferable. – JShell Jun 17 '15 at 19:15
  • possible duplicate of [(OrElse and Or) and (AndAlso and And) - When to use?](http://stackoverflow.com/q/8409467/11683) – GSerg Jun 17 '15 at 19:15
  • Same problem...that link discusses code correctness, which I believe I understand. – JShell Jun 17 '15 at 19:18
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    It's pretty clear based on the links. AND is a bitwise operator and evaluate both sides, if you don't need to do that than use AndAlso. – the_lotus Jun 17 '15 at 19:30

3 Answers3

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And is bitwise. AndAlso is boolean.

Correctness wise, if we disregard short-circuting, And can always be used in all cases where AndAlso can be used (unless you get creative), but the other way round is not true.

That is, And is a "universal" operator that will "do the right thing" in both the bitwise and logical contexts, but is not optimized for either. AndAlso can only be used in the boolean context, but it is optimized for it.

Visual Basic did not use to have AndAlso, so And was the only choice. Now that VB has AndAlso, And should may be used for:

  • bitwise math (performing the bitwise AND over two numbers)
  • expressions with side effects (you want to disable short-circuiting and have both parts of the expression always evaluated). I would certainly not call this a recommended practice, because you might end up with very unobvious code, but it is probably okay if you have legacy code upgraded from VB6.
Community
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GSerg
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    Having an expression with side-effects in the second part of an if statement would be a big code smell. So `And` should really only be used for bitwise math. – Meta-Knight Jun 17 '15 at 19:45
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    @Meta-Knight I wasn't trying to say one should actually do it, rather, that it can be done and it distinguishes `And` from `AndAlso`. Oh wait, I actually did say "should..." – GSerg Jun 17 '15 at 19:49
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Maybe it's a little of a personal choice. but I tend to think And should never used.
Of course And is needed for bitwise operation but it's not really the question here (you don't have choice for bitwise conjunction between And and AndAlso).
I don't like the "side-effect" trick ; if you want side effects perform them and then test the result values

Dim result1 = SideEffect1(), result2 = SideEffect2()
If result1 AndAlso result2 Then ' ...

is IMHO better than

If SideEffect1() And SideEffect2() Then ' ...

The intent is clearer but sure both way are possible.

Sehnsucht
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This can be useful if you have an expression with side effects. For example, your second expression is a function that you always want to run, regardless of what evaluates before it.

aw04
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