Using vs2008 c#

i have this code snipet note: SUBMITTED_DT = DateTime? type

if ((DateTime.Now - billingEvent.SUBMITTED_DT).Days > 180)
{
throw new
BusinessException(Messages.BillingEvent_Save_TooOld);
}


this causes an error on build saying .Days is not supported, and
asking if i am missing an assembly reference.


if i change the code to (ie add SUBMITTED_DT.Value)

if ((DateTime.Now - billingEvent.SUBMITTED_DT.Value).Days > 180)
{
throw new
BusinessException(Messages.BillingEvent_Save_TooOld);
}

then this builds ok, i know its something to do with using DateTime?
but i dont realy understand why.

Can anyone give me a Captain Dummy explanation of why .Value is
required ?


thanks

Peter

I see. Ok thanks for that, that makes sense

cheers

Peter


>On Thu, 23 Oct 2008 19:54:35 -0700, <Peted> wrote:
>
>> [...]
>> then this builds ok, i know its something to do with using DateTime?
>> but i dont realy understand why.
>>
>> Can anyone give me a Captain Dummy explanation of why .Value is
>> required ?
>
>Well, I think the "Captain Dummy explanation" is simply "that's just the
>way it works".
>
>The slightly more complicated explanation is that through the magic of
>implicit conversions, subtracting a DateTime? from a DateTime results in a
>TimeSpan?, which is actually an instance of Nullable<TimeSpan>, which does
>not have a Days property (nor any other property that TimeSpan has).
>
>You have to get back to the non-nullable TimeSpan type in order to have a
>type with that property, and one of the ways you can do that is by
>short-circuiting the implicit conversions by explicitly getting the
>non-nullable DateTime Value property from your DateTime? instance. That
>removes the nullable type that was causing the expression type itself to
>be nullable, and thus the result is a plain non-nullable TimeSpan, with
>the Days property you expect.
>
>There are a variety of ways to write the code so that it works, but they
>all involve getting the expression back to the non-nullable TimeSpan
>type. For example, in addition to how you solved it, you could have
>written:
>
> if (((TimeSpan)(DateTime.Now - billingEvent.SUBMITTED_DT)).Days > 180)
>
>or
>
> if ((DateTime.Now - billingEvent.SUBMITTED_DT).Value.Days > 180)
>
>Pete

I thought that explanation was fine, actually, it does make sense to
me

thanks

Peter



>On Thu, 23 Oct 2008 22:14:43 -0700, <Peted> wrote:
>
>> I see. Ok thanks for that, that makes sense
>
>I'm glad it makes sense to you. Reading it again, I'm not sure it makes
>sense to me. The word "nullable" shows up so often, the whole thing seems
>a bit like a blur to me.
>
>But, if you're happy...I'm happy.