I am not a programmer...

[crokett]

which is why I am asking yet another question.
Apologies to Alex who probably already answered this.

I have this code:

public Interface iLinkable 
end interface
 
public class fibreport 
    implements iLinkable
 
 Dim portname As string
end class
 
public class ethernet 
    implements iLinkable
Dim portname As string
end class 
 
public class fabric
 
Dim ISLs As iLinkable()
 
'tests the iLinkable interface
sub tester 
   Redim ISLs(2)
   ISLs(1) = new fibreport
   ISLs(2) = new ethernet
end sub

Now when I try to do something like:

ISLs(1).portname = "tester"

I get a compiler error to the effect of no such property 'portname' for iLinkable. I am expecting it to resolve ISLs(1) to the fibreport object stored there.

but this works:

Dim newport As fibreport = ISLs(1)
fibreport.portname = "tester"

So why does first not work but second does and how do I get first to work? This has implications for other stuff I am trying to do. Google's got nothin'. Well I'm sure it has something, but I can't find it.

[Alex Franke]

You have two options.

Option 1:
"ISLs(1)" is a reference to an interface, not a specific object, and the interface doesn't know that the object has a property called portname. So, add portName to the interface definition. If everything that is ILinkable must have a property called PortName, then PortName can be part of the contract. Like this:

Interface ILinkable
    Property PortName() As String 
End Interface

' Implement the property required by the interface in the class that implements the interface
public class ethernet 
    implements iLinkable

    Private _Portname As string

    Property PortName() As String
        Get
            Return _PortName
        End Get
        Set(ByVal Value As String)
            _PortName = value
        End Set
    End Property
end class

Now the compiler will demand that everything that you say implements ILinkable does in fact have the PortName property -- it's part of the contract -- part of what it means to be ILinkable. And now ILinkable knows that everything that implements it has this property, so a reference to an ILinkable object in an array of ILinkables will be able to access it.

If there's other stuff that an ILinkable object should be able to do, then you should also add it to the interface definition. For example, an IDrivable interface might have properties called Speed and Direction and functions called Steer(), Accelerate() and Brake(). Then if you have a Car object that implements IDrivable, it must implement all those properties and functions, so that if you have an array of IDrivable objects (car, boat, bicycle, etc), you can access any of these properties or call any of these functions on those objects while only knowing that they're IDrivable. (You don't need to know it's a car or a boat -- only that it's IDrivable.)

Option 2:
...is exactly what you did in your second example. You're "casting" the object held in the interface array back to a specific object, and the computer knows that that object has a property called PortName.

[Alex Franke]

Dim newport As fibreport = ISLs(1)
fibreport.portname = "tester"

I think you mean this:

Dim newport As fibreport = ISLs(1)
[B]newport[/B].portname = "tester"

[crokett]

It seems a cleaner solution is multiple interfaces with the methods/properties I need declared in them.

For the casting for every port/switch/etc I wanted to update I'd have to cast it, make the change then cast back into the array.

thanks Alex

<edit>
On second thought I think I need to stick with casting or not use interfaces. There a few properties unique to each type of port that can't be implemented in the interface.
</edit>

[Alex Franke]

It seems a cleaner solution is multiple interfaces with the methods/properties I need declared in them.

Yeah, I agree. Or simple inheritance if in fact you're talking about two different flavors of the same object. For example, you might use an IDrivable interface and implement it for things like a Car, Boat, or even a Conversation or GolfBall. But if you're really talking about objects that represent a Coupe, FamilySedan, and PickupTruck, then you'd probably just use a Car base class with the properties and functions you want and inherit those for your subclasses.

For the casting for every port/switch/etc I wanted to update I'd have to cast it, make the change then cast back into the array.

Yes and no -- you'd have to cast out of ILinkable (and perhaps more importantly you'd have to know what type of object you want to cast to), but you shouldn't have to cast back to ILinkable to get it back into an ILinkable array. In fact, you probably won't have even removed the item from the array in the first place...

No problem, crokett -- always happy to help!