Developer Forum

Hi guys,

Long ago, I learned that it was a bad idea to throw exceptions inside a
constructor. For instance:

Public Class OutCountMovementArgs

Private innerDirection As OutCountDirection
Private innerStartDate As Date
Private innerEndDate As Date

Public Sub New(ByVal direction As OutCountDirection, _
ByVal startDate As Date, _
ByVal endDate As Date)

If startDate < #10/1/2004# Or startDate > Today Then
Throw New DateOutOfRangeException("startDate")
End If

If endDate < #10/1/2004# Or endDate > Today Then
Throw New DateOutOfRangeException("endDate")
End If

If startDate > endDate Then
Throw New DateRangeOutOfOrderException
End If

innerDirection = direction
innerStartDate = startDate
innerEndDate = endDate

End Sub

Public ReadOnly Property Direction As OutCountDirection
Get
Return innerDirection
End Get
End Property

Public ReadOnly Property StartDate As Date
Get
Return innerStartDate
End Get
End Property

Public ReadOnly Property EndDate As Date
Get
Return innerEndDate
End Get
End Property

End Class

Is this still considered a bad idea? My gut tells me that it is, as a
lot of users tend to write variable initialization code outside of
Try-Catch blocks, like this:

Dim myStartDate As Date
Dim myEndDate As Date = Today
Dim args As New OutCountArgs(OutCountDirection.Into, myStartDate,
myEndDate)

Try
.
.
.
Catch...
End Try

The example above will fail (because myStartDate defaults to 1/1/1900),
resulting in a DateOutOfRangeException which isn't caught by the
Try...Catch handler, and surprises the developer. I, myself, try to
avoid that, but occasionally get caught by it myself. To code the class
the other way makes for very wordy code because you have to to
explicitly initialize every property (although that tends to be very
clear).

In general, how do you handle this? My inclination is to err on the
side of caution. I'm coding in a vaccuum here (only developer in the
entire company now for 2 years), so I have no one with whom to discuss
these kinds of issues, and I'm a little out of touch with the
community.

Any input is greatly appreciated!

Indicating an error from a constructor is one of the main reasons that
modern langauges have exceptions in the first place. It's not a bad idea to
throw from a constructor if it's not possible to construct a valid object
(due to invalid parameters or any other factors).

[ example snipped for brevity ]


It shouldn't surprise a .NET developer. The better structure is this:

Try
Dim myStartDate As Date
Dim myEndDate As Date = Today
Dim args As New OutCountArgs(OutCountDirection.Into, myStartDate,
myEndDate)
.
.
.
Catch...
End Try


... it also tends to be very fragile and hard to maintain if the class is
used in more than one place - it's a 30 year step backward in program
design.


You're handling it correctly, IMO.

-cd

Thanks very much for the advice! I wouldn't have thought of it that
way, but now that I think about it, it makes sense.

Thanks again!

and besides, what does it really matter since you have some serious job
security...

--

________________________
Warm regards,
Alvin Bruney [MVP ASP.NET]

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Professional VSTO.NET - Wrox/Wiley
The O.W.C. Black Book with .NET
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