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

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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 ..

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