How Does ComboBox Determine What To Display?

21 Apr 2010 3:55 AM

eBob.com

I implemented as class like this ...

Public Class ComboxChoice
    Public ChoiceText As String
    Public LastUsed As Date
    Public UsedCount As Integer
    'to be serializable we have to have a parameterless constructor
    Sub New()
    End Sub
    Sub New(ByRef Choice As String, ByRef LastUsed As Date, _
            ByVal UsedCount As Integer)
        Me.ChoiceText = Choice
        Me.LastUsed = LastUsed
        Me.UsedCount = UsedCount
    End Sub

    Public Shadows ReadOnly Property ToString() As String
        Get
            Return Me.ChoiceText
        End Get
    End Property

    'Public Overrides Function tostring() As String
    '    Return ChoiceText
    'End Function
End Class

.... and I don't know what I was thinking when I made ToString a property
rather than a function. BUT this ...

MsgBox(New ComboxChoice("1st choice", DateTime.Now, 1).ToString)

.... gave the expected result, i.e. "1st choice", while this ...

cbx1.Items.AddRange(New Object() {New ComboxChoice("1st choice",
DateTime.Now, 1), _
                                          New ComboxChoice("2nd choice",
DateTime.Now, 1)})

.... (where cbx1 is a ComboBox) does not - i.e. the items in the combobox do
not appear as "1st choice" and "2nd choice". I understand that ComboBox is
seeing the items as having a type of Object while in the MsgBox statement
ToString is clearly a member of ComboxChoice. But what kind of invocation
is ComboBox doing which manages to get the Object .ToString function rather
than my objects ToString property? Isn't "Shadows" supposed to completely
hide any ToString member in Object?

Thanks, Bob

21 Apr 2010 5:00 AM

Tom Shelton

On 2010-04-21, eBob.com <eBob.***@totallybogus.com> wrote:
Show quoteHide quote

> I implemented as class like this ...
>
> Public Class ComboxChoice
>     Public ChoiceText As String
>     Public LastUsed As Date
>     Public UsedCount As Integer
>     'to be serializable we have to have a parameterless constructor
>     Sub New()
>     End Sub
>     Sub New(ByRef Choice As String, ByRef LastUsed As Date, _
>             ByVal UsedCount As Integer)
>         Me.ChoiceText = Choice
>         Me.LastUsed = LastUsed
>         Me.UsedCount = UsedCount
>     End Sub
>
>     Public Shadows ReadOnly Property ToString() As String
>         Get
>             Return Me.ChoiceText
>         End Get
>     End Property
>
>     'Public Overrides Function tostring() As String
>     '    Return ChoiceText
>     'End Function
> End Class
>
> ... and I don't know what I was thinking when I made ToString a property
> rather than a function. BUT this ...
>
> MsgBox(New ComboxChoice("1st choice", DateTime.Now, 1).ToString)
>
> ... gave the expected result, i.e. "1st choice", while this ...
>
> cbx1.Items.AddRange(New Object() {New ComboxChoice("1st choice",
> DateTime.Now, 1), _
>                                           New ComboxChoice("2nd choice",
> DateTime.Now, 1)})
>
> ... (where cbx1 is a ComboBox) does not - i.e. the items in the combobox do
> not appear as "1st choice" and "2nd choice". I understand that ComboBox is
> seeing the items as having a type of Object while in the MsgBox statement
> ToString is clearly a member of ComboxChoice. But what kind of invocation
> is ComboBox doing which manages to get the Object .ToString function rather
> than my objects ToString property? Isn't "Shadows" supposed to completely
> hide any ToString member in Object?
>
> Thanks, Bob
>

The default behavior is to call Object.ToString(). You have not overriden
..ToString - but have shadowed it's implementation. So, the default object
implmentation will be used. Shadows only hides the parent implementation from
your descendants.

--
Tom Shelton

21 Apr 2010 5:05 PM

eBob.com

Show quote Hide quote

"Tom Shelton" <tom_shel***@comcastXXXXXXX.net> wrote in message
news:OCWdD%23Q4KHA.4804@TK2MSFTNGP04.phx.gbl...
> On 2010-04-21, eBob.com <eBob.***@totallybogus.com> wrote:
>> I implemented as class like this ...
>>
>> Public Class ComboxChoice
>>     Public ChoiceText As String
>>     Public LastUsed As Date
>>     Public UsedCount As Integer
>>     'to be serializable we have to have a parameterless constructor
>>     Sub New()
>>     End Sub
>>     Sub New(ByRef Choice As String, ByRef LastUsed As Date, _
>>             ByVal UsedCount As Integer)
>>         Me.ChoiceText = Choice
>>         Me.LastUsed = LastUsed
>>         Me.UsedCount = UsedCount
>>     End Sub
>>
>>     Public Shadows ReadOnly Property ToString() As String
>>         Get
>>             Return Me.ChoiceText
>>         End Get
>>     End Property
>>
>>     'Public Overrides Function tostring() As String
>>     '    Return ChoiceText
>>     'End Function
>> End Class
>>
>> ... and I don't know what I was thinking when I made ToString a property
>> rather than a function. BUT this ...
>>
>> MsgBox(New ComboxChoice("1st choice", DateTime.Now, 1).ToString)
>>
>> ... gave the expected result, i.e. "1st choice", while this ...
>>
>> cbx1.Items.AddRange(New Object() {New ComboxChoice("1st choice",
>> DateTime.Now, 1), _
>>                                           New ComboxChoice("2nd choice",
>> DateTime.Now, 1)})
>>
>> ... (where cbx1 is a ComboBox) does not - i.e. the items in the combobox
>> do
>> not appear as "1st choice" and "2nd choice". I understand that ComboBox
>> is
>> seeing the items as having a type of Object while in the MsgBox statement
>> ToString is clearly a member of ComboxChoice. But what kind of
>> invocation
>> is ComboBox doing which manages to get the Object .ToString function
>> rather
>> than my objects ToString property? Isn't "Shadows" supposed to
>> completely
>> hide any ToString member in Object?
>>
>> Thanks, Bob
>>
>
> The default behavior is to call Object.ToString(). You have not overriden
> .ToString - but have shadowed it's implementation. So, the default object
> implmentation will be used. Shadows only hides the parent implementation
> from
> your descendants.
>
> --
> Tom Shelton

Thank you Tom, I was confused about the meaning of "Shadows". That
confusion came in part from a compiler/VS message! If my definition of the
ToString property omits "Shadows" then I get the message: "property
'ToString' conflicts with function 'ToString' in the base class 'Object' and
should be declared 'Shadows'". I would say that 'Shadows' in this case
didn't really resolve the conflict, or at least permitted an ambiguity.

Thanks again for your help, Bob