Side Effects of VBA IIf

Side Effects of VBA IIf 

Another issue with IIf arises because it is a library function: unlike the C-derived conditional operator, both truepart and the falsepart will be evaluated regardless of which one is actually returned. Consider the following example:

value = 10
result = IIf(value = 10, TrueFunction, FalseFunction)

Although TrueFunction is the function intended to be called, IIf will cause both TrueFunction and FalseFunction to be executed.

Also consider this one:

a = 10
b = 0
result = IIf(b <> 0, a / b, 0)

While the programmer intends to avoid raising an error by performing a division by zero, whenever b is zero the error will actually happen. This is because the code in the snippet is to be read as

a = 10
b = 0
_temp1 = a / b ' Error if b = 0
_temp2 = 0
_temp3 = b <> 0
result = IIf(_temp3, _temp1 , _temp2)

This issue makes the IIf() call less useful than the conditional operator. To solve this issue, Microsoft developers had considered converting IIf to an intrinsic function; had this happened, the compiler would have been able to perform type inference and short-circuiting by replacing the function call with inline code.

References-

support.microsoft.com