What is the benefit of com-interfaces?

[Edwin Knoppert]

For most classes i would not have a 2nd or more interface.
The 2nd can imo never be instantiated in the first class and therefore the instance variables are not shared.

I would like to have a com interface in an existing interface but so far i don't see how and use (odd) references.

I guess com-interfaces are really implemented like this to offer different versions while underlaying functionality may differ only little (or a lot but that's not the purpose).

To be clear, a 2nd interface is always created in a new object right?
Thus no sharing existing data right?

[José Roca]

You can have as many IUnknown interfaces as you wish in the same class, but only one dual interface.

[Edwin Knoppert]

Two interfaces declared in the same class will (still) never share the instance data right?

[José Roca]

Not right.

[Edwin Knoppert]

A ha!

    Local oInterface1 As Interface1
    oInterface1 = Class "Class1"
    Local oInterface2 As Interface2
    
    oInterface1.DateValue = "1234"
    oInterface2 = oInterface1

    MsgBox oInterface2.DateValueInt2

It seems to work fine, am i doing it right?

If so, this would really apply to my other topic:

http://www.powerbasic.com/support/pbforums/showthread.php?t=39098

(Partly though..)

[José Roca]

It seems to work fine, am i doing it right?

Yes. When you have more than one interface in a class, you just have to call QueryInterface to navigate between them. oInterface2 = oInterface1
implicitily calls QueryInterface.

If so, this would really apply to my other topic

No. Circular references should be avoided at all cost.

[Edwin Knoppert]

Yes i know and will but i mean tracking a 'parent' (or it's data) is much easier this way.
This code makes com programming much more easier and interesting.

Thanks,

[José Roca]

If you need them to be compatible with Automation compilers such VB6 and .NET, you can use INHERIT IAutomation. You can have has many interfaces as you wish in the class and still be usable with VB. However, scripting languages such VBScript need dispatch interfaces.

[Edwin Knoppert]

Meaning: only one interface for those isn't?

While it's good to know, i'll be using the IUnknown interface for most of my work.
In a 'dual' case i may need to write me a wrapper class or so.
(don't see any use for me anyway at this time)

[Edwin Knoppert]

I have worked it out but i have a question.
Will obtaining the interface like i did below be a problem (if i use it a lot)?
Frankly i think this is the better thing to do, the object returned will be terminated right away.
My debug shows the destruction of the class and therefore no circular reference.
Iow, looks good!

Class ParentClass

    Instance m_Value As String

    Interface IParentClass: Inherit IUnknown

        Property Get Items As iItems
            [B]Property = Me[/B]
        End Property

        Property Get TheDate As String
            Property = Date$
        End Property

    End Interface

    Interface iItems: Inherit IUnknown

        Property Get Parent As IParentClass
            [B]Property = Me[/B]
        End Property

        Property Get Value() As String
            Property = m_Value
        End Property

        Property Set Value( ByVal Value As String )
            m_Value = Value
        End Property

    End Interface

End Class

Example:

    Local o As IParentClass
    o = Class "ParentClass"
    'Local i As iItems
    'i = o.Items
    'i.Value = Time$
    o.Items.Value = Time$
    
    MsgBox o.Items.Value & ", " & o.Items.Parent.TheDate

[Edwin Knoppert]

Just added a parent property as well
Still don't know how to deal with multiple items being maintained by iItems.
(Items not shown here)

[José Roca]

No problem, although it adds some overhead. In each call, the compiler has to create a temporary object variable, call QueryInterface and later release it.

[Edwin Knoppert]

Yes, but maybe this is the way to go.

Still messing with the items themselves.
It must be able to use any additional class thus outside this parentclass anyway.
I was wondering how this was done with a .NET class like

ParentClass p = new ParentClass()
Items it = p.Items
Item i = it[0]

it = null
p = null
So.. what about i ?

[José Roca]

PB does not currently support collections.

[Edwin Knoppert]

I know, the question was about something else.

However, i have placed a piece of code on my board.
It shows the relationship issue and maintaining an items collection.
At this time Array ... calls don't seem to apply to interfaces so i used old fashion redim and copy and such.

http://www.hellobasic.com/cgi-bin/forum/YaBB.pl?board=VDExamples;action=display;num=1228306509

[Paul Squires]

Hi Edwin,

Here is code that I wrote when I was playing with the parent/child relationship for PB objects. It is a simple one Company with many Customers relationship. It is a little different than yours in that it creates the child array in the CREATE method and assumes that it will be one based. Also, I return a reference to the child object when it is added to the parent and then assign the child properties, versus creating a child object, filling in its properties and then assigning it to the parent. Also, the code is not as complete as yours because I haven't included anything to remove one or all of the child items (that should be easy enough to do though).

A Collections object would be nice....

#Compile Exe

Class clsCustomer
   Instance m_sCustID   As String
   Instance m_sCustName As String
   Instance m_nCustBal  As Currency
   
   Interface CustomerInterface: Inherit IUnknown     
    
      Property Get CustID() As String
         Property = m_sCustID
      End Property
      Property Set CustID( ByVal sCustID As String ) 
         m_sCustID = sCustID
      End Property

      Property Get CustName() As String
         Property = m_sCustName
      End Property
      Property Set CustName( ByVal sCustName As String ) 
         m_sCustName = sCustName
      End Property

      Property Get CustBal() As Currency
         Property = m_nCustBal
      End Property
      Property Set CustBal( ByVal nCustBal As Currency ) 
         m_nCustBal = nCustBal
      End Property

   End Interface
End Class


Class clsCompany
   Instance m_pCustomers()   As CustomerInterface
   Instance m_nCustomerCount As Long
   Instance m_sCompanyName   As String
   Instance m_sCompanyID     As String
   
   Class Method Create()
      ' Initialize the customer array
      Dim m_pCustomers(0) As Instance CustomerInterface
   End Method

   
   Interface CompanyInterface: Inherit IUnknown     
      
      Property Get CompanyName() As String
         Property = m_sCompanyName
      End Property
      Property Set CompanyName( ByVal sCompanyName As String ) 
         m_sCompanyName = sCompanyName
      End Property

      Property Get CompanyID() As String
         Property = m_sCompanyID 
      End Property
      Property Set CompanyID( ByVal sCompanyID As String ) 
         m_sCompanyID = sCompanyID
      End Property

      Property Get CustomerCount() As Long
         Property = m_nCustomerCount
      End Property

      Method AddCustomer() As CustomerInterface
         Incr m_nCustomerCount
         ReDim Preserve m_pCustomers( m_nCustomerCount ) As Instance CustomerInterface
         m_pCustomers(m_nCustomerCount) = Class "clsCustomer"
         ' Return a reference to the new customer object
         Method = m_pCustomers(m_nCustomerCount)  
      End Method
      
      Method GetCustomer( ByVal nPosition As Long ) As CustomerInterface
         Method = m_pCustomers( nPosition )
      End Method

   End Interface

   
End Class



'//
'//  SAMPLE PROGRAM
'//
Function PBMain() As Long
   
   Local cCompany As CompanyInterface
   Local cCust    As CustomerInterface
   Local y        As Long

   ' Create a new company
   cCompany = Class "clsCompany"
   
   ' Add info to our new company
   cCompany.CompanyName = "PlanetSquires Software"
   cCompany.CompanyID   = "PLANETSQ"
   

   ' Add two new customers to the company
   cCust = cCompany.AddCustomer()
   cCust.CustID   = "1000"
   cCust.CustName = "Paul Squires"
   cCust.CustBal  = 1000.99
   
   cCust = cCompany.AddCustomer()
   cCust.CustID   = "2000"
   cCust.CustName = "Tammy Squires"
   cCust.CustBal  = 2000.99
   
   
   ' Display our Company information
   ? "Company Name: " & cCompany.CompanyName & $CrLf & _
     "Company ID:   " & cCompany.CompanyID

   
   ' Display our Customer information
   For y = 1 To cCompany.CustomerCount
      cCust = cCompany.GetCustomer(y)
      
      If IsObject(cCust) Then
         ? cCust.CustName & $CrLf & _
           cCust.CustID   & $CrLf & _
           Format$(cCust.CustBal, "$#.00")
      End If
   Next
   
   
   ? "done"
   
End Function

[José Roca]

You can build collections using the Dictionary object.

[Paul Squires]

Thanks José - that's a good point. Simply add the dictionary to the class to handle all children objects. I think that I'll do that.

[Michael Mattias]

MS Data Shaping OLE provider (with ADO) was designed to support hierachical data relationships.

No, I never tried it, but just this week I received my O'Reilly book on ADO and found that whilst browsing it.