Interfaces and Casting

[mandelbrot]

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:

 

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.

[PlausiblyDamp]

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

MessageBox.Show("Employee " & manager.Id & " has been updated.", "Employee Updated") 

Edited August 4, 2006 by PlausiblyDamp

[mandelbrot]

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.

[MrPaul]

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:

[mandelbrot]

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...

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...

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...

[mandelbrot]

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.