Or versus OrElse

What's the difference between or and OrElse?

if temp is dbnull.value or temp = 0

produces the error:

Operator '=' is not defined for type 'DBNull' and type 'Integer'.

while this one works like a charm!?

if temp is dbnull.value OrElse temp = 0

Solution 1:

OrElse is a short-circuiting operator, Or is not.

By the definition of the boolean 'or' operator, if the first term is True then the whole is definitely true - so we don't need to evaluate the second term.

OrElse knows this, so doesn't try and evaluate temp = 0 once it's established that temp Is DBNull.Value

Or doesn't know this, and will always attempt to evaluate both terms. When temp Is DBNull.Value, it can't be compared to zero, so it falls over.

You should use... well, whichever one makes sense.

Solution 2:

This is the same behaviour as with C#, where everyone uses the Coditional Or (||) and the Conditional And (&&), where you also have the normal Or (|) and normal And (&). So comparing C# to VB.Net is:

| => Or

|| => OrElse

& => And

&& => AndAlso

The condifitonal boolean operators are very usefull preventing nested if constructions. But sometimes the normal boolean operators are needed to ensure hitting both code paths.

Solution 3:

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.