Spot the error (ie I can't)

  • Thread starter Thread starter Steven Spencer \(Spinalogic\)
  • Start date Start date
S

Steven Spencer \(Spinalogic\)

Dim dlg As New dlgChangeDate

Dim fromdate As DateTime = dlg.GetaDate(System.DateTime.Now, "Please Enter
the Start of the Holiday")

If (Not fromdate.CompareTo(New DateTime(1, 1, 1, 1, 1, 1, 1)) = 0) Then

While IsDateBetweenShifts(fromdate, cboHolidayDoctors.SelectedValue)

MsgBox("The date you have entered does not occur between the
shifts specified on that day.")

fromdate = dlg.GetaDate(System.DateTime.Now, "Please Enter the
Start of the Holiday")

If IsNothing(fromdate) Then

Exit While

End If

End While

End if

My dialog just gets a date time. It returns nothing for a failure.

I tried making the if statement " if not IsNothing(fromDate) then" and this
still ran, so I tried the godawful combination that I have now.

Now I have checked in the debugger, the statement "(Not
fromdate.CompareTo(New DateTime(1, 1, 1, 1, 1, 1, 1)) = 0) " is false. This
means that the if statement shouldn't run.

HOWEVER the statement ALWAYS enters the if statement block. Before you ask,
I have written an else statement and NOT clicked cancel and this results in
the statement returning true, and THEN entering the if block (not the else
block). Its as if its discarding the if statement and just entering anyway.

For the life of me I cannot spot this error, and I'm on the managed MSDN so
I'd really appreciate some help here from MVP/MSFT.

Cheers.
 
More info.

I disabled Compiler optomizations (just in case it was optimizing the if
statement to permanently true) and this still results in the exact same
behaviour.
 
If todate.CompareTo(New DateTime(1, 1, 1, 1, 1, 1, 1)) = 0 Then Exit Sub

I've tried this line, verified in the debugger that todate.CompareTo(New
DateTime(1, 1, 1, 1, 1, 1, 1)) = 0 is true, yet this does not execute exit
sub

when it's false, it still doesn't run the exit sub (correctly) but there
isn't an inversion happening.
 
Spotted!!!!!!!

The first issue is that assigning Nothing to a variable of type DateTime is
the same as assigning 01/01/0001 12:00:00 AM to it, i.e., Midnight (being
the start of the day) on the 1st of January in the year 1.

Dim _d As DateTime = Nothing
Dim _d As DateTime = New DateTime(1, 1, 1)
Dim _d As DateTime = New DateTime(1, 1, 1, 0, 0, 0, 0)
Dim _d As DateTime = DateTime.MinValue

to name just a few ways of initialising a variable of type DateTime and all
thiose shown result in _d having the same value.

The second issue is that you are comparing fromdate to 01/01/0001
01:01:01.001 AM. When the dialog 'fails', this test results in a value of -1
(fromdate (01/01/0001 12:00:00 AM is less than 01/01/0001 01:01:01.001 AM)
and therfore your boolean expression equates to True and the code inside the
If ... End If is executed as you are observing.

I would be inclined to recode it so that it is something like:

Dim dlg As New dlgChangeDate

Dim fromdate As DateTime = dlg.GetaDate(System.DateTime.Now, "Please Enter
the Start of the Holiday")

If fromdate <> DateTime.MinValue Then
While IsDateBetweenShifts(fromdate, cboHolidayDoctors.SelectedValue)
MessageBox.Show("The date you have entered does not occur between the
shifts specified on that day.", "Invalid Value")
fromdate = dlg.GetaDate(System.DateTime.Now, "Please Enter the Start
of the Holiday")
If fromdate <> DateTime.MinValue Then Continue
End While
End If
 
Seeing as you're on Framework 1.1 the line:

If fromdate <> DateTime.MinValue Then Continue

should read:

If fromdate = DateTime.MinValue Then Exit While

It would have been nice if you had included that information in your
original post.
 
I'm actually a java programmer by trade.

I HATE VB.net

C# is sooo much nicer.

But yes, it was a goddamn reference type thing.
It SAID it was 12:00am in the debugger which is the value I replicated.. I
however had failed to identify the problem as I used the IsNothing(blah)
function on the datetime. Which returned false.

I changed the code to read fromdate = nothing and then this worked
correctly.

I do hope to all god that the only reason the reference type/value type
stuff is there is for VB6.0 compatibility otherwise whoever did it at MS
needs to be shot. And my employer doesn't know how to read C# and we have
WAY to much code to change right now. Thanks for your help though.

++karma
 
I'm actually a java programmer by trade.

I HATE VB.net

C# is sooo much nicer.

But yes, it was a goddamn reference type thing.
It SAID it was 12:00am in the debugger which is the value I replicated.. I
however had failed to identify the problem as I used the IsNothing(blah)
function on the datetime. Which returned false.

I changed the code to read fromdate = nothing and then this worked
correctly.

I do hope to all god that the only reason the reference type/value type
stuff is there is for VB6.0 compatibility otherwise whoever did it at MS
needs to be shot. And my employer doesn't know how to read C# and we have
WAY to much code to change right now. Thanks for your help though.
Click to expand...

Sorry to disappoint you, but the reference type vs value type stuff is not a
..NET thing and has nothing to do with VB.NET. The same thing would happen in
C#.

They have added "Nullable Types" in .NET 2.0:

Dim myNullableDate As Nullable(Of DateTime) = Nothing

The syntax is cleaner in C#:
DateTime? myNullableDate = null;
 
When the hell is someone going to make an object oriented language that is
actually FULLY OO.

Same stuff in C#. Well you learn something every day.

:'(
 
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