Can a constructor return Nothing??

[Eidolon]

Is there any way that you can write your constructor so that it returns
nothing?

For example:
Take a Customer object which has a constructor whose constructor has the
signiture:

Public Sub New(CustomerID as String)

What i would like to do is to write it so that the the actual code of the
constructor goes something like the following pseudo-code:

<pcode>
Dim dt as datatable = getDataTable(SELECT * FROM CUSTOMER WHERE CUSTID =
CustomerID)
If dt.Rows.Count = 0 then
return nothing
Else
InitializeMe(dt.Rows(0))
End if
</pcode>

This way from my client code i can do something like:

<code>
Dim tmpCust As Customer = New Customer(MyParameter)
If IsNothing(tmpCust) Then
ShowNoCustMsg
Else
ConitnueOn()
End If
</code>

Thanks in advance.

[Marina]

No, I don't believe it can.

Here are a couple of other options:
1) Have a private constructor only. Create a shared method on your class
that creates the object. So users would call it:
Customer.CreateCustomer(MyParameter). This CreateCustomer method could then
create the Customer object, and make sure that your conditions are met. If
the conditions are met, it can return the object, otherwise, it will return
Nothing.
2) set a property on Customer, saying that it is not a valid customer. The
if statement users would use would be: If Not tmpCust.Valid Then
3) Throw an exception saying it's not a valid customer. The user would need
to place the call in a try/catch block.
"Eidolon" <ei**************@yahoo.com> wrote in message
news:Od**************@TK2MSFTNGP10.phx.gbl...

Is there any way that you can write your constructor so that it returns
nothing?

For example:
Take a Customer object which has a constructor whose constructor has the
signiture:

Public Sub New(CustomerID as String)

What i would like to do is to write it so that the the actual code of the
constructor goes something like the following pseudo-code:

<pcode>
Dim dt as datatable = getDataTable(SELECT * FROM CUSTOMER WHERE CUSTID = CustomerID)
If dt.Rows.Count = 0 then
return nothing
Else
InitializeMe(dt.Rows(0))
End if
</pcode>

This way from my client code i can do something like:

<code>
Dim tmpCust As Customer = New Customer(MyParameter)
If IsNothing(tmpCust) Then
ShowNoCustMsg
Else
ConitnueOn()
End If
</code>

Thanks in advance.

[Mitchell S. Honnert]

I'm not positive, but I don't believe that a constructor can "return
nothing". However, I do have an alternative suggestion...

Why not use a custom exception? The Customer constructor would look like
so...

Public Sub New(customerID as String)
<pcode>
Dim dt as datatable = getDataTable(SELECT * FROM CUSTOMER WHERE CUSTID =
CustomerID)
If dt.Rows.Count = 0 then
Throw New CustomerDoesNotExistException(customerID)
Else
InitializeMe(dt.Rows(0))
End if
</pcode>

The calling code would look like so...

<code>
Try
Dim tmpCust As Customer = New Customer(MyParameter)
ConitnueOn()
Catch exc as CustomerDoesNotExistException
ShowNoCustMsg(exc.CustomerID)
End Try
</code>

IMHO, the code using the exception looks "cleaner" than the trick with the
nothing.

HTH

- Mitchell S. Honnert
"Eidolon" <ei**************@yahoo.com> wrote in message
news:Od**************@TK2MSFTNGP10.phx.gbl...

Is there any way that you can write your constructor so that it returns
nothing?

For example:
Take a Customer object which has a constructor whose constructor has the
signiture:

Public Sub New(CustomerID as String)

What i would like to do is to write it so that the the actual code of the
constructor goes something like the following pseudo-code:

<pcode>
Dim dt as datatable = getDataTable(SELECT * FROM CUSTOMER WHERE CUSTID = CustomerID)
If dt.Rows.Count = 0 then
return nothing
Else
InitializeMe(dt.Rows(0))
End if
</pcode>

This way from my client code i can do something like:

<code>
Dim tmpCust As Customer = New Customer(MyParameter)
If IsNothing(tmpCust) Then
ShowNoCustMsg
Else
ConitnueOn()
End If
</code>

Thanks in advance.

[Chris Dunaway]

On Tue, 11 May 2004 09:06:41 -0400, in microsoft.public.dotnet.languages.vb
you wrote:

Is there any way that you can write your constructor so that it returns
nothing?

The only way I can think of to accomplish that would be to make the
constructor private and create a shared method to create the instance of
the class

Public Class Customer

Private Sub New()
'Constructor code here
End Sub

Shared Function CreateCustomer()
'Code
If SomeCondition Then
Return New Customer
Else
Return Nothing
End If
End Function

End Class

Public Sub Main()
Dim cust As Customer = Customer.CreateCustomer()
If (cust Is Nothing) Then
ShowNoCustMsg
Else
ConitnueOn()
End If
End Sub

--
Chris

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