Q:

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.

A:

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 :-(

