Or versus OrElse

Question

What\'s the difference between or and OrElse?

if temp is dbnull.value or temp = 0

produces the error:

Answer 1

OrElse is short circuited, this means that only one side of the expression will be tested if the first side is a match.

Just like AndAlso will only test one side of the expression if the first half is a fail.

Answer 2

The reason the compilation fails in the example is the order of operations.

The expression parser is trying to evaluate "dbnull.value or temp" first.

if temp is (dbnull.value or temp) = 0

The error is here, because you can't do a bitwise OR between an integer (temp) and dbnull.value.

OrElse fixes this, not because it's short-circuited, but because it's lower on the order of operations, and so "temp is dbnull.value" and "3=0" are being evaluated first, rather than the parser trying to compare dbNull and temp.

So the evaluation with OrElse works like you're expecting: (assume temp=3)

if temp is dbnull.value OrElse temp = 0 then
if 3 is dbnull.value OrElse 3 = 0 then
if false OrElse 3=0 then
if false OrElse false then
if false then

This was actually on an entry exam at a software company I used to work for, and it was a common problem I used to encounter in VB6. So it's a good idea to parenthesize your sub-expressions when using boolean operators:

This would have compiled properly:

if (temp is dbnull.value) Or (temp = 0) then 

Although, as everyone has already pointed out, OrElse and AndAlso are really the correct operators to use in this context.