How to evaluate for interfaces ?

[Steven Pringels 3]

Hi,

The following will illustrate what I'm after...

assume one has a class (methods and properties are left out for easier understanding)

 class cField

	interface iField
	inherit Dual
	end interface	

 end class

and another class inheriting the ifield interface

 class cCheckbox

     interface iCheckbox
     inherit cField, iField
     end interface
 end class

There would be a bunch of other classes that mimic the usage of controls
in a window. but for illustration purposes the checkbox will suffice.

Now, I have a form class with a method AddField and GetField which
would look like this

 class cForm
     interface iForm
     inherit Dual

     method Addfield(ByVal clsField as Dispatch)

	... collection handling is here... 
	actually just an dispatch array holding the 
        clsfield object

     end method 

     method GetField(byVal nId as long) as Dispatch

	... lookup field here by evaluating the id which
        ... is a property of the iField interface.

        myDispatch = m_FieldCol(n)

        method = myDispatch

     end method 


     end method

 end Class

Now, I wish I could evaluate for certain interfaces.

  Local clsField		as Dispatch

  clsField = MyForm.GetField(Id)

  select case <INTERFACE QUERY ON OBJECT HERE>
  case "IFIELD" ' Base class
\ Case "ICHECKBOX" ' A checkbox... what can we do with
				   a checkbox that we
				   can't do with the baseclass.

       ' We need to use object Call here since we're referencing
       ' a dispatch interface instead of the direct interface.

       Object Call clsField.SetTrue()

  End Select

So, all this stuff just to ask, can we get the interface name of an
object by some kind of trick ?

Cheers
Steven

[Steven Pringels 3]

Ok, thinking about this I could set a property that
would indicate the type of interface I'm dealing with
using an instance variable.

but I was hoping for a more elegant solution.

[Michael Mattias]

>..could set a property that would indicate the type of interface I'm dealing with....
>but I was hoping for a more elegant solution

Some of us find simplicity elegant.

[Fred Harris]

I think what you are looking for is named RTTI (Run-Time Type Information) in OOP mumbo-jumbo. One way of getting it is to set up an enumeration a variable of which type can be an instance variable in your ultimate base class. When you create each object of various derived classes set a simple enum constant to some value that can later be tested.

http://en.wikipedia.org/wiki/RTTI

[Steven Pringels 3]

Hi Fred,

Yes, kind of however instance variables are not accessible from inherited classes so I would have to use a get/set property.

[Steven Pringels 3]

Oh, and BTW, ....

I wish one could do

Object.method or Object.property with dispatch interfaces too... really...
it bothers me a bit I have to do

  Object Get Object.Property to hResult

whereas it would be soooo easy to have

If object.property = ... then

maybe something for the next release.

[Petr Schreiber jr]

Steven,

did you have a look at ISINTERFACE?
It allows to test whether object has specified interface.

So you can do:

IF ISINTERFACE(obj, ICheckBox) THEN
ELSEIF ISINTERFACE(obj, ILabel) THEN
...
END IF

Petr

[Steven Pringels 3]

Hi Peter,

Sure, I checked it out but that doesn't do much good. I need to have the name of the interface. However, the use of a property solves the issue quite nicely.

Thanks for looking into this !

Steven