Interfaces and Casting

M

mandelbrot

Centurion
Joined
Jul 1, 2005
Messages
194
Location
UK North East
Dear All,


Because of the way my current app works I've used an interface to supply basic details between objects. Obviously the data is common to all and key to the execution of the system.

If an object implements the interface then is it possible to cast an interface variable to an implementing object type?

For instance:

Visual Basic: 
Interface IPerson

    Property Name() As System.String
    Property Age() As Int32
    Property Born() As System.DateTime

End Interface

Class Employee
    Implements IPerson
...
    Public Property Id() As Int32
        Get
            Return _Id
        End Get
        Set(ByVal Value As Int32)
            _Id = Value
        End Set
    End Property
    Public Property Name() As System.String Implements IPerson.Name
        Get
            Return _Age
        End Get
    End Property
...
End Class
...
    Dim manager As IPerson = New Employee()
...
    [color=red]MessageBox.Show("Employee " & DirectCast(manager, Employee).Id & " has been updated.", "Employee Updated")[/color]
I've tried this (or something very similar) and cannot get it to work...

Any advice would be appreciated.


Regards,
Paul.
 
Edit: Ignore this - I didn't read the quesion properly :(

Not really an answer this - why are you needing to cast the interface back into a particular class?
Couldn't you just do
Visual Basic: 
MessageBox.Show("Employee " & manager.Id & " has been updated.", "Employee Updated")
 
Last edited:
Well strictly, no, I suppose, as I've casted manager as IPerson, technically IPerson doesn't have an Id...

Basically, I've got several different types of class all reliant on the same data, but I do need to know other data from the specific class types besides that available in the interface itself.

For instance, I could define a customer class which implements IPerson and give them a customerId which is a string. Obviously, from this, if I (for instance) run through an array of IPersons I may wish to obtain their employee or customer id, both of which are different underlying types, etc etc...

(Hope I'm making myself clear...).


Paul.
 
DirectCast... yes thats fine!

I'm interested as to why the employee's Name property returns their age, but thats academic. ;)

DirectCast should work fine. I just tried your code and it works as expected. Note that with Option Strict On, you will need to call the Id.ToString() method when appending Id to a string. What error(s) are you getting?

Good luck :cool:
 
LOL! Typo!!!

Hmmm... Well that's rather odd!

I've got a tree like data class that that implements a supporting interface which passes data to it. Other objects and controls require access to similar information, and obviously, by identifying the parent of your tree as being an interface, you've got the ability to create a root for your tree. So, I've created the interface to manage the fact that my tree has to grow from something.

The interface itself passes the required data...
Visual Basic: 
Public Interface IOccurrenceSupport

    ReadOnly Property User() As NTLogin
    ReadOnly Property AllCodes() As sysCode
    ReadOnly Property AllOccurrences() As sysCode
    ReadOnly Property AllEvents() As sysCode
    ReadOnly Property AllActions() As sysCode
    ReadOnly Property AllCausalities() As sysCode
    ReadOnly Property StartOfDay() As sysCode
    ReadOnly Property Connection() As String

End Interface
The tree class that I'm working with implements the IOccurrenceSupport class, as does the ultimate parent (OccurrenceContainer).

However, when I try to save the data an Invalid Cast exception raises it's head at this point...
Visual Basic: 
localParams.Add("@pParentId", DirectCast(_Parent, Occurrence).Id)
_Parent is defined as an IOccurrenceSupport interface, and the Occurrence class actually implements IOccurrenceSupport.

I've tried breaking it down to individual statements; replace DirectCast with CType; and finally actually implemented the Id as part of the interface (current solution!). But like you say - I shouldn't have to do this...
 
D'Oh! Excuse me for being a numpty!

I've realised where the problem lies! It's in the fact that the root level occurrence itself will attempt to reference the Id of it's parent, but it won't have one, as it's parent is a different class, containing different properties!

Gentlemen - many thanks for your patience! You can both beat me up later!


Paul.
 
Back
Top