Performance difference between IIf() and If

Question

In Visual Basic, is there a performance difference when using the IIf function instead of the If statement?

Reactionary

Answer 1

VB has the following If statement which the question refers to, I think:

' Usage 1
Dim result = If(a > 5, "World", "Hello")
' Usage 2
Dim foo = If(result, "Alternative")

The first is basically C#'s ternary conditional operator and the second is its coalesce operator (return result unless it’s Nothing, in which case return "Alternative"). If has thus replaced IIf and the latter is obsolete.

Like in C#, VB's conditional If operator short-circuits, so you can now safely write the following, which is not possible using the IIf function:

Dim len = If(text Is Nothing, 0, text.Length)

Staphylorrhaphy

You didn't answer the performance question and also don't mention the side-effects of IIf. – Intelligencer

@Intelligencer The last paragraph of my answer is about the side-effects. The performance isn’t really relevant when one of the options is obsolete. For what it’s worth, the native operator If is more efficient than the IIf function by far. – Staphylorrhaphy

Am I missing something? all you demonstrate is the If function... You don't demonstrate how the syntax or usage differs between IIf and If – Publicspirited

@mmcrae That's correct and on purpose: as I've said, IIf is obsolete, so there's little sense in discussing it. That said, my last paragraph does discuss the relevant difference. That's all you need to know, really. – Staphylorrhaphy

I appreciate you want to support good standards and ensure newbies don't pick up bad practices, but someone may have a perfectly legitimate issue that has caused them to want to understand the difference better... Such as maintaining legacy code, being stuck with some other tool that uses it, etc. And That's all you need to know, really doesn't sound like a conducive attitude for a website devoted to knowledge sharing ;) – Publicspirited

@mmcrae Oh, I didn't mean to be dismissive, I just really meant that the internals of IIf are boringly trivial. It simply contains a conventional If statement, which implies that the last piece of code in my answer wouldn't run. – Staphylorrhaphy

BTW: Avoid this type of thing... Dim name = IIf(myObject Is Nothing, "(not specified)", myObject.Name), as you will get a null reference exception if myObject is null. Both branches are evaluted, even if the result of only one branch will be returned. I personally also try to avoid anything written in VB, but I don't always have the choice. – Beriberi

Answer 2

IIf() runs both the true and false code. For simple things like numeric assignment, this isn't a big deal. But for code that requires any sort of processing, you're wasting cycles running the condition that doesn't match, and possibly causing side effects.

Code illustration:

Module Module1
    Sub Main()
        Dim test As Boolean = False
        Dim result As String = IIf(test, Foo(), Bar())
    End Sub

    Public Function Foo() As String
        Console.WriteLine("Foo!")
        Return "Foo"
    End Function

    Public Function Bar() As String
        Console.WriteLine("Bar!")
        Return "Bar"
    End Function
End Module

Outputs:

Foo!
Bar!

Jello

Answer 3

Also, another big issue with the IIf is that it will actually call any functions that are in the arguments [1], so if you have a situation like the following:

string results = IIf(Not oraData.IsDBNull(ndx), oraData.GetString(ndx), string.Empty)

It will actually throw an exception, which is not how most people think the function works the first time that they see it. This can also lead to some very hard to fix bugs in an application as well.

[1] IIf Function - http://msdn.microsoft.com/en-us/library/27ydhh0d(VS.71).aspx

Tamathatamaulipas

This is why if came to replace the old IIF, I used to have issues with IIF but IF saved adding many lines of code. – Heuristic

Answer 4

According to this guy, IIf can take up to 6x as long as If/Then. YMMV.

Hanseatic

In my empyrical calculations (10000000 cycles), I evaluated IIf being about 12x slower! – Grugru

Answer 5

Better use If instead of IIf to use the type inference mechanism correctly (Option Infer On)

In this example, Keywords is recognized as a string when I use If :

Dim Keywords = If(String.IsNullOrEmpty(SelectedKeywords), "N/A", SelectedKeywords)

Otherwise, it is recognized as an Object :

Dim Keywords = IIf(String.IsNullOrEmpty(SelectedKeywords), "N/A", SelectedKeywords)

Ragucci

Answer 6

On top of that, readability should probably be more highly preferred than performance in this case. Even if IIF was more efficient, it's just plain less readable to the target audience (I assume if you're working in Visual Basic, you want other programmers to be able to read your code easily, which is VB's biggest boon... and which is lost with concepts like IIF in my opinion).

Also, "IIF is a function, versus IF being part of the languages' syntax"... which implies to me that, indeed, If would be faster... if for nothing else than that the If statement can be boiled down directly to a small set of opcodes rather than having to go to another space in memory to perform the logic found in said function. It's a trite difference, perhaps, but worth noting.

Octa

Answer 7

I believe that the main difference between If and IIf is:

• If(test [boolean], statement1, statement2) it means that according to the test value either satement1 or statement2 will executed (just one statement will execute)

• Dim obj = IIF(test [boolean] , statement1, statement2) it means that the both statements will execute but according to test value one of them will return a value to (obj).

so if one of the statements will throw an exception it will throw it in (IIf) anyway but in (If) it will throw it just in case the condition will return its value.

Handal

Answer 8

...as to why it can take as long as 6x, quoth the wiki:

Because IIf is a library function, it will always require the overhead of a function call, whereas a conditional operator will more likely produce inline code.

Essentially IIf is the equivalent of a ternary operator in C++/C#, so it gives you some nice 1 line if/else type statements if you'd like it to. You can also give it a function to evaluate if you desire.

Fluviomarine

Answer 9

Those functions are different! Perhaps you only need to use IF statement. IIF will always be slower, because it will do both functions plus it will do standard IF statement.

If you are wondering why there is IIF function, maybe this will be explanation:

Sub main()
    counter = 0
    bln = True
    s = iif(bln, f1, f2)
End Sub

Function f1 As String
    counter = counter + 1
    Return "YES"
End Function

Function f2 As String
    counter = counter + 1
    Return "NO"
End Function

So the counter will be 2 after this, but s will be "YES" only. I know this counter stuff is useless, but sometimes there are functions that you will need both to run, doesn't matter if IF is true or false, and just assign value from one of them to your variable.

Fulgurant

if you need both to run, you should explicitly call them both and then do a standard If. Using IIf() in this way is just going to confuse people reading you code. – Expletive

I didn't mean for the comment to come across as a criticism; I was just pointing out that using it in this way is not a good practice. – Expletive