VB.NET How To Tell If New Class Instance Was Ended Early? - vb.net

Question

I'm trying to detect if the Sub New() in my class has ended early due to missing fields. Below is a sample of my code:
Class Account
Public Sub New(ByVal Firstname As String, ByVal LastName As String, ByVal Username As String, ByVal Email As String, ByVal Password As String)
' Check For Blank Fields
If Firstname = "" Or LastName = "" Or Username = "" Or Email = "" Or Password = "" Then
MessageBox.Show("Please Enter All Information Requested")
Exit Sub
End If
' Set Public Variables Of Class
Firstname = Firstname
LastName = LastName
Username = Username
Email = Email
Password = Password
End Sub
Public Shared Sub OtherUse()
End Sub
End Class
' Create New Instance
Dim Process As New Account(txtFirstName.Text, txtLastName.Text, txtUsername.Text, txtEmail.Text, txtPassword.Text)
' HERE - How Can I Catch The Early Exit From The Instance Due To Potential Missing Fields?
' Use Instance For Other Use
Process.OtherUse()
How would I catch the Exit Sub from the class in the parent form to prevent the further processing of Process.OtherUse()?

Answer 1

You're approaching this problem the wrong way. Validate the input first, and then once the input is valid, create a new Account using New.
Another option would be to initialize data in New without checking if it's valid or not, then have an IsValid method in that class that you would call from another class to know whether or not the messagebox should be shown.
One way or another, the Account class shouldn't be responsible for a UI concern like showing a MessageBox on the screen. And the constructor should only be responsible for constructing the object, not validating the input, because you can't "abort" a constructor. You have a reference to a new object even though you call Exit Sub.

Answer 2

A fourth alternative, in addition to the three mentioned by Meta-Knight:
Have the constructor throw an exception when the parameters aren’t valid.
As an aside, your use of Or, while working, is a logical error: And and Or are bitwise arithmetic operations. You want a logical operation, OrElse (there’s also AndAlso). These may seem similar to Or and And and in this particular case, both happen to work. But they are actually quite different semantically, bitwise operations are just wrong here (it’s a pity that this code even compiles. The compiler shouldn’t allow this).