Re: enumerations as parameters, VB, and the value 0
From: Jay B. Harlow [MVP - Outlook] (Jay_Harlow_MVP_at_msn.com)
Date: 08/18/04
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Date: Wed, 18 Aug 2004 08:45:26 -0500
Chris,
> I guess you could argue that the language definition itself _makes_ 0
> a more valid value than 2, but that is just a hair-puller for me.
Unfortunately with VS.NET 2002 & VS.NET 2003 you will be losing some hair.
:-|
As that is how it is currently defined to work! I stated I have not tried
VS.NET 2005 (aka Whidbey, due out in 2005) to see if things are improved or
not. I really hope they will be!
> On top of that, I tried
>
> dt = New Foo(CInt(0), Bar.Bar0)
Try
Dim zero As Integer = 0
dt = New Foo(zero, Bar.Bar0)
As that is the only way I know of to get VS.NET 2002 & VS.NET 2003 to work.
Yes MS knows there is a problem! As I told them a year ago when I came
across this quirk and remembered them when you asked your original question.
> Enum Bar
> Bar0 = -1
> Bar1 = -2
> Bar2 = -4
> End Enum
Remember that
Dim myBar As Bar
Will have the value of literal 0 per the CTS. Also remember that the CLR
does not validate values within an Enum, if you need validation of the
values you can use Enum.IsDefined.
If Not [Enum].IsDefined(GetType(Bar), myBar) Then
Throw New ArgumentOutOfRangeException("myBar", myBar, "Invalid Enum
Bar value!")
End If
Hope this helps
Jay
"Plausible Indirection" <cg22165@yahoo.com> wrote in message
news:7b9ebe1f.0408170912.31a101a6@posting.google.com...
> I've been busy for a few days and I came back to a lot more discussion
> on this than I expected at first. Thanks to all!
>
> Ok, there seems to be some sloppiness in the language spec.
> Basically, it does not enforce the restriction of binding of a value
> to an enumeration type to be one of the values defined for that type.
> Because of this lack of enforcement, it creates an implicit cast from
> 0 to enum that would otherwise not always be possible. It is quite
> possible to define an enumeration that does not contain a valid value
> for 0.
>
> On top of that, I tried
>
> dt = New Foo(CInt(0), Bar.Bar0)
>
> I would think that the conversion function would remove any
> possibility that the 0 could be interpreted as an enum, but the
> behavior remains the same. This just doesn't make sense to me.
>
> I also tried
>
> Enum Bar
> Bar0 = -1
> Bar1 = -2
> Bar2 = -4
> End Enum
>
> and again there was no behavior change. This really seems like a
> weakness in the language because, it would be a whole lot simpler, and
> IMHO, better, to define one rule for interpreting numeric literals as
> a type rather than multiple rules based on what the value happens to
> be. Ok, I'd allow promotion along the integer types, but still, in
> this case, the literal 0 is no more a valid value for the enumeration
> than the literal 2.
>
> I guess you could argue that the language definition itself _makes_ 0
> a more valid value than 2, but that is just a hair-puller for me.
>
> -Chris
>
>
> "Jay B. Harlow [MVP - Outlook]" <Jay_Harlow_MVP@msn.com> wrote in message
news:<OwpU#W5gEHA.2916@TK2MSFTNGP12.phx.gbl>...
> > Code,
> > A widening conversion is one where there is no loss of data. Integer
allows
> > all values from Int32.MinValue to In32.MaxValue, while Enum only allows
(not
> > enforced) the values that are defined on it (a sub set of Integer).
> >
> > A narrowing conversion is one where there may be a loss of data.
> >
> > A widening conversion is an implicit cast operation, while a narrowing
> > conversion is an explicit cast operation.
> >
> > As Jon pointed out literal 0 is special in that an implicit cast is
allowed,
> > while the literal 2 an explicit cast is required. Mostly because we know
> > that 0 is valid for the Enum (as it is its "default" value) while 2 may
or
> > may not have been defined in the Enum.
> >
> > Hope this helps
> > Jay
> >
> > "cody" <no_spam_deutronium@gmx.net> wrote in message
> > news:%23ef43v2gEHA.2916@TK2MSFTNGP12.phx.gbl...
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