Hello. I have two snippets of code here that are very similar. One works,
but the other doesn't. Can someone explain why?

Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
date to 1/1/0001 12:00:00 AM.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click

Dim x As Date

x = Nothing

Debug.WriteLine(x)

End Sub

Snippet 2: Optional date parameter contains default value of nothing. .NET
2003 refuses to compile. Gives error of "Conversion from 'System.Object' to
'Date' cannot occur in a constant expression."

Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)

Debug.WriteLine(x)

End Sub

Why am I getting this error? It doesn't make sense to me. If x is a
different type (such as Integer), .NET doesn't complain at all. So why is
Date treated differently?

Thanks in advance for any feedback!

Scott

Scott,

I don't know about other people but I never use optional anymore. Try
overloading the method.

Private Sub ThisDoesntWork()
ThisDoesntWork(Nothing)
End Sub
Private Sub ThisDoesntWork(ByVal x As Date)
Debug.WriteLine(x)
End Sub

Hope this helps.
Chris.


"Scott Hembrough" wrote:

> Hello. I have two snippets of code here that are very similar. One works,
> but the other doesn't. Can someone explain why?
>
> Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
> date to 1/1/0001 12:00:00 AM.
>
> Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
> System.EventArgs) Handles Button1.Click
>
> Dim x As Date
>
> x = Nothing
>
> Debug.WriteLine(x)
>
> End Sub
>
> Snippet 2: Optional date parameter contains default value of nothing. .NET
> 2003 refuses to compile. Gives error of "Conversion from 'System.Object' to
> 'Date' cannot occur in a constant expression."
>
> Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
>
> Debug.WriteLine(x)
>
> End Sub
>
> Why am I getting this error? It doesn't make sense to me. If x is a
> different type (such as Integer), .NET doesn't complain at all. So why is
> Date treated differently?
>
> Thanks in advance for any feedback!
>
> Scott
>
>
>

Chris,

I assume it is a hypothetical question. As far as I can see has this method
not any sence.

>Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)

Because when the Date is not set it is Nothing and when it is set it is
Something

Why it gives the error is as well described on this page.

http://msdn.microsoft.com/library/de...tionalargs.asp

Cor

"Chris Podmore" <(E-Mail Removed)>


> Scott,
>
> I don't know about other people but I never use optional anymore. Try
> overloading the method.
>
> Private Sub ThisDoesntWork()
> ThisDoesntWork(Nothing)
> End Sub
> Private Sub ThisDoesntWork(ByVal x As Date)
> Debug.WriteLine(x)
> End Sub
>
> Hope this helps.
> Chris.
>
>
> "Scott Hembrough" wrote:
>
>> Hello. I have two snippets of code here that are very similar. One
>> works,
>> but the other doesn't. Can someone explain why?
>>
>> Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
>> date to 1/1/0001 12:00:00 AM.
>>
>> Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
>> System.EventArgs) Handles Button1.Click
>>
>> Dim x As Date
>>
>> x = Nothing
>>
>> Debug.WriteLine(x)
>>
>> End Sub
>>
>> Snippet 2: Optional date parameter contains default value of nothing.
>> .NET
>> 2003 refuses to compile. Gives error of "Conversion from 'System.Object'
>> to
>> 'Date' cannot occur in a constant expression."
>>
>> Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
>>
>> Debug.WriteLine(x)
>>
>> End Sub
>>
>> Why am I getting this error? It doesn't make sense to me. If x is a
>> different type (such as Integer), .NET doesn't complain at all. So why
>> is
>> Date treated differently?
>>
>> Thanks in advance for any feedback!
>>
>> Scott
>>
>>
>>

Cor,

I assumed that Scott had posted some sample code rather than his actual code
which he was having problems with.

Chris.

"Cor Ligthert" wrote:

> Chris,
>
> I assume it is a hypothetical question. As far as I can see has this method
> not any sence.
>
> >Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
>
> Because when the Date is not set it is Nothing and when it is set it is
> Something
>
> Why it gives the error is as well described on this page.
>
> http://msdn.microsoft.com/library/de...tionalargs.asp
>
> Cor
>
> "Chris Podmore" <(E-Mail Removed)>
>
>
> > Scott,
> >
> > I don't know about other people but I never use optional anymore. Try
> > overloading the method.
> >
> > Private Sub ThisDoesntWork()
> > ThisDoesntWork(Nothing)
> > End Sub
> > Private Sub ThisDoesntWork(ByVal x As Date)
> > Debug.WriteLine(x)
> > End Sub
> >
> > Hope this helps.
> > Chris.
> >
> >
> > "Scott Hembrough" wrote:
> >
> >> Hello. I have two snippets of code here that are very similar. One
> >> works,
> >> but the other doesn't. Can someone explain why?
> >>
> >> Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
> >> date to 1/1/0001 12:00:00 AM.
> >>
> >> Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
> >> System.EventArgs) Handles Button1.Click
> >>
> >> Dim x As Date
> >>
> >> x = Nothing
> >>
> >> Debug.WriteLine(x)
> >>
> >> End Sub
> >>
> >> Snippet 2: Optional date parameter contains default value of nothing.
> >> .NET
> >> 2003 refuses to compile. Gives error of "Conversion from 'System.Object'
> >> to
> >> 'Date' cannot occur in a constant expression."
> >>
> >> Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
> >>
> >> Debug.WriteLine(x)
> >>
> >> End Sub
> >>
> >> Why am I getting this error? It doesn't make sense to me. If x is a
> >> different type (such as Integer), .NET doesn't complain at all. So why
> >> is
> >> Date treated differently?
> >>
> >> Thanks in advance for any feedback!
> >>
> >> Scott
> >>
> >>
> >>
>
>
>

Yes, that is correct. It's just sample code. I was posting a very simple
example that didn't have any extra code that would detract from the actual
question.

As for ways to deal with this, I realize function overloading would be a
better solution (as suggested in another post). I just happened to run
across it as I was converting some old VB6 code. We have a very large
project, and right now we're trying to get it to run in .NET with minimal
changes. Rearchitecting the code will come later.

Our original function had an optional date parameter declared as a variant.
Within the function, we used IsMissing to determine if anything had been
passed in. Since IsMissing is no longer an option, I was defaulting the
parameter to nothing and checking for IsNothing within the function. This
would get us by until we can go back and take advantage of features such as
function overloading (which would make this a moot point anyway).

So, back to the original question: Does anyone know why this is happening?
Is this a bug? Or is there a more obvious explanation that I'm just
overlooking?

"Chris Podmore" <(E-Mail Removed)> wrote in message
news:3257E15C-6CE8-48D0-9303-(E-Mail Removed)...
> Cor,
>
> I assumed that Scott had posted some sample code rather than his actual
code
> which he was having problems with.
>
> Chris.

Scott,

Did you see this text on the link I showed?

If the optional argument is a reference type such as a String, you can use
Nothing as the default value, provided this is not an expected value for the
argument.

Cor

OK. I'd overlooked one thing that Cor wrote when he responded to me. The
link that he provided said that "If the optional argument is a reference
type such as a String, you can use Nothing as the default value, provided
this is not an expected value for the argument."

Is date considered to be a value type or a reference type? I didn't see it
in the list of value types I was looking at in the help file. So, if it's
reference, then it seems like the MS article is saying that I *should* be
able to set this to nothing.

Also, integer *is* a value type and I can set an optional integer parameter
to nothing with no problems. So it seems like I should be able to do this
with a date.

Lots of questions, but I'm just trying to understand how all this works as
we delve into .NET. Again, thanks for all the help.

"Scott Hembrough" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Yes, that is correct. It's just sample code. I was posting a very simple
> example that didn't have any extra code that would detract from the actual
> question.
>
> As for ways to deal with this, I realize function overloading would be a
> better solution (as suggested in another post). I just happened to run
> across it as I was converting some old VB6 code. We have a very large
> project, and right now we're trying to get it to run in .NET with minimal
> changes. Rearchitecting the code will come later.
>
> Our original function had an optional date parameter declared as a
variant.
> Within the function, we used IsMissing to determine if anything had been
> passed in. Since IsMissing is no longer an option, I was defaulting the
> parameter to nothing and checking for IsNothing within the function. This
> would get us by until we can go back and take advantage of features such
as
> function overloading (which would make this a moot point anyway).
>
> So, back to the original question: Does anyone know why this is
happening?
> Is this a bug? Or is there a more obvious explanation that I'm just
> overlooking?
>
> "Chris Podmore" <(E-Mail Removed)> wrote in message
> news:3257E15C-6CE8-48D0-9303-(E-Mail Removed)...
> > Cor,
> >
> > I assumed that Scott had posted some sample code rather than his actual
> code
> > which he was having problems with.
> >
> > Chris.
>
>

I found this link which indicates that Date is indeed a value type:
http://msdn.microsoft.com/library/de...uereftypes.asp

So, as Cor pointed out earlier, if you have a reference type you can set it
to nothing without any problems in an optional parameter. Since Date is a
value type, I guess this is why I'm having an issue. However, if that's the
case, then why is this allowed?

Private Sub Test(Optional ByVal x As Integer = Nothing)

Integer is also a value type, so I could think it should be throwing an
error as well (assuming behavior should be consistent across all value
types).

"Scott Hembrough" <(E-Mail Removed)> wrote in message
news:%236%23HF%(E-Mail Removed)...
> OK. I'd overlooked one thing that Cor wrote when he responded to me. The
> link that he provided said that "If the optional argument is a reference
> type such as a String, you can use Nothing as the default value, provided
> this is not an expected value for the argument."
>
> Is date considered to be a value type or a reference type? I didn't see
it
> in the list of value types I was looking at in the help file. So, if it's
> reference, then it seems like the MS article is saying that I *should* be
> able to set this to nothing.
>
> Also, integer *is* a value type and I can set an optional integer
parameter
> to nothing with no problems. So it seems like I should be able to do this
> with a date.
>
> Lots of questions, but I'm just trying to understand how all this works as
> we delve into .NET. Again, thanks for all the help.
>
> "Scott Hembrough" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > Yes, that is correct. It's just sample code. I was posting a very
simple
> > example that didn't have any extra code that would detract from the
actual
> > question.
> >
> > As for ways to deal with this, I realize function overloading would be a
> > better solution (as suggested in another post). I just happened to run
> > across it as I was converting some old VB6 code. We have a very large
> > project, and right now we're trying to get it to run in .NET with
minimal
> > changes. Rearchitecting the code will come later.
> >
> > Our original function had an optional date parameter declared as a
> variant.
> > Within the function, we used IsMissing to determine if anything had been
> > passed in. Since IsMissing is no longer an option, I was defaulting the
> > parameter to nothing and checking for IsNothing within the function.
This
> > would get us by until we can go back and take advantage of features such
> as
> > function overloading (which would make this a moot point anyway).
> >
> > So, back to the original question: Does anyone know why this is
> happening?
> > Is this a bug? Or is there a more obvious explanation that I'm just
> > overlooking?
> >
> > "Chris Podmore" <(E-Mail Removed)> wrote in
message
> > news:3257E15C-6CE8-48D0-9303-(E-Mail Removed)...
> > > Cor,
> > >
> > > I assumed that Scott had posted some sample code rather than his
actual
> > code
> > > which he was having problems with.
> > >
> > > Chris.
> >
> >
>
>

Scott,

I think that it is a flaw (usable bug) in the implementation. I saw that
other values do it as well.

However much need is not for it as I said before

Test(nothing) gives the same result withouth optional, and is in my opinion
nicer code.

I never use such a possible flaw because you will not be awared in future if
this is will be changed.

I have seen bad samples where people did that and because of that the
applications where not any more upgradable to newer compiller versions,
without almost completly writting them new.

Just my thought,

Cor

"Scott Hembrough" <(E-Mail Removed)>

>I found this link which indicates that Date is indeed a value type:
> http://msdn.microsoft.com/library/de...uereftypes.asp
>
> So, as Cor pointed out earlier, if you have a reference type you can set
> it
> to nothing without any problems in an optional parameter. Since Date is a
> value type, I guess this is why I'm having an issue. However, if that's
> the
> case, then why is this allowed?
>
> Private Sub Test(Optional ByVal x As Integer = Nothing)
>
> Integer is also a value type, so I could think it should be throwing an
> error as well (assuming behavior should be consistent across all value
> types).
>
> "Scott Hembrough" <(E-Mail Removed)> wrote in message
> news:%236%23HF%(E-Mail Removed)...
>> OK. I'd overlooked one thing that Cor wrote when he responded to me.
>> The
>> link that he provided said that "If the optional argument is a reference
>> type such as a String, you can use Nothing as the default value, provided
>> this is not an expected value for the argument."
>>
>> Is date considered to be a value type or a reference type? I didn't see
> it
>> in the list of value types I was looking at in the help file. So, if
>> it's
>> reference, then it seems like the MS article is saying that I *should* be
>> able to set this to nothing.
>>
>> Also, integer *is* a value type and I can set an optional integer
> parameter
>> to nothing with no problems. So it seems like I should be able to do
>> this
>> with a date.
>>
>> Lots of questions, but I'm just trying to understand how all this works
>> as
>> we delve into .NET. Again, thanks for all the help.
>>
>> "Scott Hembrough" <(E-Mail Removed)> wrote in message
>> news:(E-Mail Removed)...
>> > Yes, that is correct. It's just sample code. I was posting a very
> simple
>> > example that didn't have any extra code that would detract from the
> actual
>> > question.
>> >
>> > As for ways to deal with this, I realize function overloading would be
>> > a
>> > better solution (as suggested in another post). I just happened to run
>> > across it as I was converting some old VB6 code. We have a very large
>> > project, and right now we're trying to get it to run in .NET with
> minimal
>> > changes. Rearchitecting the code will come later.
>> >
>> > Our original function had an optional date parameter declared as a
>> variant.
>> > Within the function, we used IsMissing to determine if anything had
>> > been
>> > passed in. Since IsMissing is no longer an option, I was defaulting
>> > the
>> > parameter to nothing and checking for IsNothing within the function.
> This
>> > would get us by until we can go back and take advantage of features
>> > such
>> as
>> > function overloading (which would make this a moot point anyway).
>> >
>> > So, back to the original question: Does anyone know why this is
>> happening?
>> > Is this a bug? Or is there a more obvious explanation that I'm just
>> > overlooking?
>> >
>> > "Chris Podmore" <(E-Mail Removed)> wrote in
> message
>> > news:3257E15C-6CE8-48D0-9303-(E-Mail Removed)...
>> > > Cor,
>> > >
>> > > I assumed that Scott had posted some sample code rather than his
> actual
>> > code
>> > > which he was having problems with.
>> > >
>> > > Chris.
>> >
>> >
>>
>>
>
>
>

"Scott Hembrough" <(E-Mail Removed)> schrieb:
> Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
> date to 1/1/0001 12:00:00 AM.

'DateTime' (= 'Date') is a value type. By setting a value type to
'Nothing', its value will be set to the type's default value. For numeric
data types, that's 0, for dates it's the date you mention above.

If you need to deal with real nullable dates, you can use
'System.Data.SqlTypes' instead of 'System.DateTime'. In .NET 2.0 (2005)
there will be a 'System.Nullable' generic type that can be used to make
other types nullable ('Nullable(Of Date)').

> Dim x As Date
>
> x = Nothing

The line above can be removed, because 'x' is already initialized with
'Nothing'.

--
Herfried K. Wagner [MVP]
<URL:http://dotnet.mvps.org/>