[2005] leave instance set to Nothing on failure of constructor

[trisuglow]

How would I achieve setting an instance of one of my classes to Nothing if the constructor fails?

Public Class MyClass

    Public Sub New( Parm As Integer )
        If Parm < 1 Then 'make the instance Nothing
    End Sub

End Class

In the calling code doing Dim obj As New MyClass( 0 ) should leave obj set to Nothing.

[stanav]

Make the constructor private and create a shared function to return an instance of the class. Something like this:

Public Class Class1
    Private Sub New()

    End Sub

    Public Shared Function GetInstance(ByVal value As Integer) As Class1
        If value <= 0 Then
            Return Nothing
        Else
            Return New Class1()
        End If
    End Function
End Class

And use it like this

Dim myClass As Class1 = Class1.GetInstance(3)

[Kasracer]

Why would you need to do this? Even if your Object takes up a ton of memory, if the constructor failed then the logic that would load, process or create data would not function I would imagine.

Just have it throw an exception on failure.

[trisuglow]

I wanted to do this because elsewhere in the code I test to see if the instance is Nothing to decide whether or not I'm going to do anything with it.

Thanks stanav - I've implemented your solution and it works nicely.

[Shaggy Hiker]

In some articles I have read for C++, the case was made that under no circumstance should a constructor fail. I have totally forgotten the reason for that, but I've followed it. If I have a class that could fail to fully construct, I add a boolean member called ImValid, and just access that via a property which I test rather than testing for Nothing.

[trisuglow]

I guess that if your class is intended to be used by other programmers their expectation would be that instantiating an object would always be successful. So, Shaggy's approach is better than what I've done.

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