using VS 2003, VB.net...

Why is this valid:
dim x as datetime = nothing

but this is invalid:

public sub DoSomething (Optional byval x as datetime = nothing)
....code to do something
end sub

Nothing shows are error message that you cannot set a system object to
nothing.

Is there a work around for the sub problem? I must set it equal to
something since it is optional.

Thanks!
Bob Day

Hi Bob,

Just to expand on your statement of the problem (which I consider
to be a bug*).

There is a problem setting an optional DateTime parameter to its
default value as illustrated with the following:

Sub Foo (ByRef Optional dtParam As DateTime = Nothing) 'Error
Dim dtLocal As DateTime = Nothing 'This is fine.
End Sub

The pre-compiler reports an error by underlining the Nothin in the
parameter list. The error given is:
Conversion from 'System.Object' to 'Date' cannot occur in a
constant expression.

The Visual Basic Language Reference says a lot about Nothing: :-)
The Nothing keyword represents the default value of any data
type. Assigning Nothing to a variable sets it to the default value for
its declared type. If that type contains variable members, they are
all set to their default values.

===============================
The workaround is to use a set date. The earliest DateTime is
12:00:00 midnight, January 1, 0001 C.E. (Common Era). Given that
you've found a bug, you could use #1/4/2003#.

Regards,
Fergus

=========================
*You wait months for a bug and nothing and all of a sudden - two in
one day!!

Hello,

"Bob Day" <(E-Mail Removed)> schrieb:
> using VS 2003, VB.net...
>
> Why is this valid:
> dim x as datetime = nothing
>
> but this is invalid:
>
> public sub DoSomething (Optional byval x as datetime = nothing)
> ....code to do something
> end sub
>
> Nothing shows are error message that you cannot set a system object to
> nothing.

DateTime is a value type, so assigning Nothing doesn't make sense. The
default value will be assigned.

\\\
Dim d As Date = Nothing
MsgBox(IsNothing(d)) ' Returns False.
MsgBox(d.ToLongDateString())
///

HTH,
Herfried K. Wagner
--
MVP · VB Classic, VB .NET
http://www.mvps.org/dotnet

Hi Herfried,

The pre-compiler gives an error.
If I may refer you to my response to the same query ? ...

Regards,
Fergus

Hello,

"Fergus Cooney" <filter-(E-Mail Removed)> schrieb:
> The pre-compiler gives an error.

If Nothing is assigned in the parameter.

> If I may refer you to my response to the same query ? ...

I know.

Regards,
Herfried K. Wagner
--
MVP · VB Classic, VB .NET
http://www.mvps.org/dotnet

Hello,

"Fergus Cooney" <filter-(E-Mail Removed)> schrieb:
> Just to expand on your statement of the problem (which I consider
> to be a bug*).
>
> There is a problem setting an optional DateTime parameter to its
> default value as illustrated with the following:
>
> Sub Foo (ByRef Optional dtParam As DateTime = Nothing) 'Error
> Dim dtLocal As DateTime = Nothing 'This is fine.
> End Sub
>
> The pre-compiler reports an error by underlining the Nothin in the
> parameter list. The error given is:
> Conversion from 'System.Object' to 'Date' cannot occur in a
> constant expression.
>
> The Visual Basic Language Reference says a lot about Nothing: :-)
> The Nothing keyword represents the default value of any data
> type. Assigning Nothing to a variable sets it to the default value for
> its declared type. If that type contains variable members, they are
> all set to their default values.

Have a look at the code below:

\\\
Const d As DateTime = Nothing
///

This code will cause the same error as assigning Nothing as the default
value of an optional parameter. I think the reason is that the compiler
won't do the conversion for you when compiling the application.

> *You wait months for a bug and nothing and all of a sudden - two in
> one day!!

I am not sure if this is a bug.

HTH,
Herfried K. Wagner
--
MVP · VB Classic, VB .NET
http://www.mvps.org/dotnet

I have read all your replies, and just to make things less clear, consider

Dim x As DateTime = DateTime.MinValue
Dim z As String = x.ToString

These should produce the same value, they do not. Hold your cursor over X
and you get only the time (and Locals in Debug shows the same). Hold your
cursor over X and you get the full date and time (and Locals window in debug
shows the same).

Why?

Bob Day



"Bob Day" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> using VS 2003, VB.net...
>
> Why is this valid:
> dim x as datetime = nothing
>
> but this is invalid:
>
> public sub DoSomething (Optional byval x as datetime = nothing)
> ....code to do something
> end sub
>
> Nothing shows are error message that you cannot set a system object to
> nothing.
>
> Is there a work around for the sub problem? I must set it equal to
> something since it is optional.
>
> Thanks!
> Bob Day
>
>

"Bob Day" <(E-Mail Removed)> schrieb
> I have read all your replies, and just to make things less clear,
> consider
>
> Dim x As DateTime = DateTime.MinValue
> Dim z As String = x.ToString
>
> These should produce the same value, they do not. Hold your cursor
> over X and you get only the time (and Locals in Debug shows the
> same). Hold your cursor over X and you get the full date and time
> (and Locals window in
debug
> shows the same).
>
> Why?

Because the tooltip in the IDE performs a different date-to-string
conversion from that you do.


--
Armin