When Object gets created...

I have the following code to instantiate an object:

    Private Sub butCreateCar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butCreateCar.Click

        MyCar = New Car 'instatiate Car class as MyCar

        SetMyCarProperties() 'associate user data with MyCar object
'???????????
        DisalbeControlsLevel(True)

    End Sub

Question 1: If one of the properties for MyCar is not set yet (it can not pass validation included with Set), does the object MyCar gets created? I hope it doesn't.

Question 2: How can I check if MyCar is created at line marked by '???????????

This I need to have to execute "DisalbeControlsLevel(True)" if in fact the object is created. Something like:

        If Not IsNothing(MyCar) Then
            DisalbeControlsLevel(True)
        End If

Thank you.
LVL 34
Mike EghtebasDatabase and Application DeveloperAsked:
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Mike TomlinsonMiddle School Assistant TeacherCommented:
(1) Yes...it was created in the line before.

(2) Instead of checking for creation, you should check for "validity".  Create a property that indicates if the instance is valid or not and set it from your SetMyCarProperties() method:
If MyCar.IsValid Then

Else

End If

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(3) If you really want to go with the "Nothing" paradigm then you need to make the constructor PRIVATE so you can no longer use the "New" keyword with your class.  Instead, provide a SHARED function that accepts the parameters and either returns a valid instance of Car, or returns Nothing to indicate failure.
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Mike EghtebasDatabase and Application DeveloperAuthor Commented:
I guess I need to include IsValid property with my Car glass in this case, is this what I need to do?

Also, I have three properties I am checking, ideally it will be much better to know the first bad data entry so I can send the focus to that text box on the form asking user to correct it.

Like:

        If MyCar.IsValid Then
            DisalbeControlsLevel(True)
        Else
            IF <txtMileage> then
               txtMileage.Focus()
           ElseIF <txtMPG> then
               txtMPG.Focus()
          Else
               txtGasLevel.Focus()
          End If
        End If

Mike
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CodeCruiserCommented:
You can add further properties like IsMileageValid, IsMPGValid etc and then use those in if conditions.
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Aruiz04Commented:
I would change the constructor of the MyCar to receive parameters, that way you validate your properties data before even declaring the MyCar Object.

Public MyCar(parameter1 as something, parameter1 as something, parameter3 as something)
End
'so when you check for the right parameters then you create it as
Dim theCar as new MyCar(par1, par2, par3)

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