new object, return nothing

[Drstein99]

I have a custom class. I want to send the NEW command, and if the data doesnt validate - i want to return NOTHING

How can i call the NEW command on my class and return NOTHING and totally not new the object (if data is invalid)?

 

[PlausiblyDamp]

When you try to instantiate an object the New keyworld should always result in a valid object being returned - for this to just result in a null reference (I.e. the object is set to nothing) is extremely unusual behaviour. Within VB the only way for a Sub New to not return an object is for it to throw an exception, this however can be a performance hit and again is not standard behaviour.

A more OO way of acheiving this is to delegate the creation of class instances to a shared method - this may return Nothing.

i.e.

Public Class DemoClass

    Private _number As Integer

    Public ReadOnly Property Number() As Integer
        Get
            Return _number
        End Get
    End Property

    Declared protected so cannot be called directly
    Protected Sub New(ByVal i As Integer)

    End Sub

    Public Shared Function CreateDemo(ByVal number As Integer) As DemoClass
        for demonstration purposes we validate the number and only
        create a class for odd numbers
        If number Mod 2 <> 0 Then
            Dim tmp As New DemoClass(number)
            Return tmp
        End If
        Return Nothing
    End Function

End Class

and this could be called like

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

        c = DemoClass.CreateDemo(1)     works
        d = DemoClass.CreateDemo(2)     Returns nothing

    End Sub