Ken Dopierala Jr.
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"Chad" <chad.dokmanovich@unisys.com> wrote in message
news:dcdpo7$odh$1@trsvr.tr.unisys.com...

When I said that "the Value property on my object cannot be made the default
property" I meant that in VB.NET, unlike VB6, you could not have a default
property unless the property accepts a parameter.
In the IntergerNullable class I created, the Value property accepted no
params. I believe you suggested that it should. I am confused as to what you
are trying to accomplish by adding a param.

Secondly, I see you are correct about the SQLDataReader.GetSqlInt32 method,
it does accept as its param, the index of the field that you want to get as
a SqlInt32 datatype. However, I feel my question remains.

Please consider the example below. I have a class Account, which has three
properties, AccountId, AccountMgrId and SupervisorId. The first is a
required integer property. The other properties I would like to allow a NULL
value or an integer value only. The AccountMgrId property is implemented to
use my custom class IntegerNullable, which is a class that I created to
"extend" the Integer so it can also accept NULL values. The problem I have
with it is that, because unlike the native INTEGER type, it is a true object
and it is awkward to refer to the value of an IntegerNullable type by
refering to its Value property rather than just the object itself. However,
I do not consider this horrible. In fact, I kind of think that Microsft
maybe should have implemented Integers as a real object for consistancy, but
I suspect there is alot to this subject. Anyway...

The SupervisorID I implemented in the Account class as a SqlInt32 datatype.
I can move a value from the SupervisorId database column to this property
using this command:

Account.SupervisorId = DataReader.GetSqlInt32(1)

but in the GUI layer, how do I move a value, say the Integer value "2" to
the Account.Supervisor property?

As you see below, when I attempt to do this, I get the error 'System.DBNull'
cannot be converted to 'System.Data.SqlTypes.SqlInt32'.

I get the same error if I try to use the CTYPE or the CINT functions:

Account.SupervisorId = CInt(2) 'Value of type 'Integer' cannot be converted
to 'System.Data.SqlTypes.SqlInt32'.
Account.SupervisorId = CType(2, Integer) 'Value of type 'Integer' cannot be
converted to 'System.Data.SqlTypes.SqlInt32'.

Cor wrote:

A value is placed on the stack and holds (a) value(s)
An object is placed on the managed heap. If its address to that is null,
than it Is Nothing or as you wrote (Null).

I hope that this gives an idea?

That wasn't enough help for me. Perhaps you can clarify this point for me?

Anyway, I think my IntegerNullable class will work fine, I just think it is
a little awkward.

You comments are welcomed.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

   Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load

       Dim Account As New Account(AccountId:=1)
       Dim i As Integer

       Account.AccountMgrId = New IntegerNullable(2)

       If Account.AccountMgrId.IsNull Then
           System.Diagnostics.Debug.WriteLine("The Account has no Manager")
       Else
           System.Diagnostics.Debug.WriteLine("The Manager's ID is " &
Account.AccountMgrId.Value)
       End If

       Dim DataReader As SqlDataReader

       Account.SupervisorId = DBNull.Value 'System.DBNull' cannot be
converted to 'System.Data.SqlTypes.SqlInt32'.

       Account.SupervisorId = 2 'System.DBNull' cannot be converted to
'System.Data.SqlTypes.SqlInt32'.

   End Sub

Public Class Account

   'An account must have an ID but it may or may not have an Account
Manager

   Private m_AccountID As Integer
   Private m_AccountMgrId As IntegerNullable
   Private m_SupervisorId As System.Data.SqlTypes.SqlInt32

   Public Sub New(ByVal AccountID As Integer)
       m_AccountID = AccountID
       m_AccountMgrId = New IntegerNullable
   End Sub

   Public Property AccountId() As Integer
       Get
           Return m_AccountID
       End Get
       Set(ByVal AccountId As Integer)
           m_AccountID = AccountId
       End Set
   End Property

   Public Property AccountMgrId() As IntegerNullable
       Get
           Return m_AccountMgrId
       End Get

       Set(ByVal AccountMgrId As IntegerNullable)
           m_AccountMgrId = AccountMgrId
       End Set

   End Property

   Public Property SupervisorId() As System.Data.SqlTypes.SqlInt32
       Get
           Return m_SupervisorId
       End Get
       Set(ByVal Value As System.Data.SqlTypes.SqlInt32)
           m_SupervisorId = Value
       End Set
   End Property
End Class

Option Strict On
Option Explicit On

Public Class IntegerNullable

   Inherits System.Object

   Implements System.Data.SqlTypes.INullable

   Private mintValue As Integer
   Private mblnIsNull As Boolean

   Public Sub New()
       mintValue = Nothing
       mblnIsNull = True
   End Sub

   Public Sub New(ByVal Value As Integer)
       mintValue = Value
       mblnIsNull = True
   End Sub

   Public Property Value() As Integer

       Get
           If mblnIsNull Then
               Throw New Exception("Value property is not accessible when
isNull is True. Use IsNull property to test before accessing Value
property")
           End If
           Return mintValue
       End Get

       Set(ByVal Value As Integer)
           mintValue = Value
           mblnIsNull = False
       End Set

   End Property

   Public ReadOnly Property IsNull() As Boolean Implements
System.Data.SqlTypes.INullable.IsNull
       Get
           Return mblnIsNull
       End Get
   End Property

   Public Sub Nullify()
       mblnIsNull = True
       mintValue = Nothing
   End Sub

End Class