Mark Hurd Mark Hurd - 6 months ago 14
Vb.net Question

Why Round(Fix(x))?

In the Reflector extract for

Microsoft.VisualBasic
it references
Microsoft.VisualBasic.Conversion.Fix
in 3 places not including the
Fix(Object)
overload.

In each case it applies
Math.Round
to the result. (Especially in
DateAdd
and
DateDiff
; the third use in
Choose
does subtract
1
, and redundantly cast to
Double
again before applying
Round
.)

When can
Math.Round(Conversion.Fix(x)) <> Conversion.Fix(x)
for
Double x
?

(I'd check the Reference Source myself but I can't find a download that does include
Microsoft.VisualBasic
.)
Reference Source now available online.

Answer

All of these references are then cast to Integer or Long:

CInt and CLng explicitly call Math.Round before their corresponding IL conv.ovf.i4/8 cast when casting from Single and Double.

This enforces the Banker's Rounding that is a VB.NET known "quirk".

conv.ovf.i4 alone truncates towards zero, which happens to be the same as Fix (for the numbers that fit in an Integer, or a Long for conv.ovf.i8).

(When I originally typed in the question I thought I had checked that this was not the cause.)

For a while, Microsoft made the Reference Source available, so I was able to confirm they were all enclosed in CInt or CLng and the Math.Round is added by the compiler.

The VB.NET "source" currently available at the above link now only provides the "reference assembly" which has no code :-(