VB.NET - IIF(,,) - Both "sides" are evaluated. What situations should I watch out for?

I recently learned of the IIF(A,B,C) function. I'm a long time VB/VB.NET Coder who recently spent a lot of time coming up to speed in SQL coding.

One (obvious) common thing to do in SQL is something like the following:

select (case where @var = 0 then MyTable.Val1 else MyTable.Val2 end) from MyTable

IIF(A,B,C) will allow me to do this in VB.NET... all on one line.

However, I have read that both B and C are evaluated no matter what A evaluates to.

I can think of some obvious situations where this is a bad thing such as:

Dim X as integer = IIF(SomeBoolean = true, ExpensiveFunction1(), ExpensiveFunction2())

As I will be including this in my repertoire, are there any more subtle situations where I may get myself into trouble using IIF?

It's a pretty big departure in some situations from using the old fashioned:

Dim X as integer
if SomeBoolean = true then
  X = ExpensiveFunction1()
else
  X = ExpensiveFunction2()
end if

I'm hoping to save myself some annoying performance issues and/or bugs in the future.

Update 2016

For the past few years, a new VB.NET feature exists which eliminates the need to use the IIF() function.

if(Something = true, ExecuteA(), ExecuteB())

Only ExecuteA() OR ExecuteB() are executed. Finally, inline IF with shortcircuiting.

So, if you are using later versions of VB.NET (as of 2016), use this instead if you can.

Here is the most common gotcha.

Z = iif(y=0, 0, x/y)  'Throws a divide by zero exception when y is 0

Don't use it to avoid division by zero errors.

Another possible logic bug is when one side of the iif or the other calls a method that modifies the system state or has output parameters.

Z = iif(FunctionA(InputOutputParam), FunctionB(InputOutputParam))
'InputOutputParam is indeterminate or at least ambiguous here.

There really is no good reason to use IIF in my experience. Mostly it is just used to abbreviate code and given the problems it can cause, it just isn't worth it. Plus, I think it makes the code harder to read.

The other thing that bites is that it returns a boxed value (ie an Object data type) which you have to cast back to the desired type.

[IIF, not IFF]

The most common case we've seen is that one side or the other evaluates to Nothing.

Your code might be expecting to use IIF as guard to keep from getting a NullReferenceException, like this:

IIF(something Is Nothing, "nothing", something.Value)

But that won't work, because both sides are always evaluated. This happens a lot in code written by people who come from a C/C++/C#/Java background, since in those languages the ternary operator ?: does short-circuit evaluation.

And the fact that the VS 2005 IIF() documentation states that IIF is just like ?: doesn't help:

> The IIf function provides a counterpart for the ternary Conditional Operator: ? : in Visual C++.

Nowhere on that reference page does it state that both sides are evaluated.

According to MSDN, the "If" operator introduced in VB2008 will short-circuit instead, which would be ideal for your expensive computation case:

http://msdn.microsoft.com/en-us/library/bb513985.aspx

Well, you should also make sure you don't have any functions in in iif that modifies any data based on the condition. We use If for a rather lot of that. It just pays to remember that about iif.

I was forced into using an iif (for compactness of code) where I had a chunk of code that was copying values out of many arrays into a spreadsheet, but "nothing" entries in the array causes the code to exit the subroutine (not crash), so I wrapped the line in an iif to check if the array cell contained nothing - if it did, then pass back "" otherwise, pass back the array cell (converting to a string 1st). So as said above, another reason not to use iif.

I'm still in mourning due to the lack of the NZ function that I used all the time in MS Access.