Incompatible interface - vb.net

Question

Please see the code below:
Public Class clsCar
Implements IVehicle
Public Function getWheels() As Integer Implements IVehicle.getWheels
Return 4
End Function
End Class
Public Interface IVehicle
Function getWheels() As Integer
End Interface
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
Dim list As List(Of IVehicle) = New List(Of IVehicle)
Dim v1 As IVehicle = New clsCar
Dim v2 As IVehicle = New clsBus
list.Add(v1)
list.Add(v2)
Dim IVehicle As IVehicle = New clsCar
Dim IVehicle2 As IVehicle = New clsBus
For Each IVehicle In list
MsgBox(IVehicle.getWheels())
Next
End Sub
End Class
I want to add a new function to the clsBus class:
Public Function getWheels(ByVal strCarType As String) As Integer
If strCarType = "Robin Ryliant" Then
Return 3
Else
Return 4
End If
End Function
How do I call this from the FOR EACH statement? At the moment it will call getWheels with no arguments.

Answer 1

You will have to add the method overload to the interface in order to be able to call it from a IVehicle variable.
Public Interface IVehicle
Function getWheels() As Integer
Function getWheels(ByVal strCarType As String) As Integer
End Interface
But probably it is better to have different, more specialized car types
Public Class clsCar
Implements IVehicle
Public Overridable Function getWheels() As Integer Implements IVehicle.getWheels
Return 4
End Function
End Class
Public Class clsRobinRyliantCar
Inherits clsCar
Public Overrides Function getWheels() As Integer
Return 3
End Function
End Class
This does not break the inheritance hierarchy and is purely polymorphic.

Answer 2

I think I would go for something more like this, with the number of wheels being an instance property (code in LINQPad format):
Sub Main
Dim list As List(Of IVehicle) = New List(Of IVehicle)()
list.Add(New clsCar("Ford Focus", 4))
list.Add(New clsCar("Robin Ryliant", 3))
list.Add(New clsBus())
For Each v In list
Console.WriteLine(String.Format("{0}:{1}", v.GetVehicleType(), v.GetWheels()))
Next
End Sub
' Define other methods and classes here
Public Interface IVehicle
Function GetVehicleType() As String
Function GetWheels() As Integer
End Interface
Public MustInherit Class clsVehicle
Implements IVehicle
Protected Property VehicleType As String
Protected Property Wheels As Integer
Protected Sub New(vehicleType As String, wheels As Integer)
Me.VehicleType = vehicleType
Me.Wheels = wheels
End Sub
Public Function GetVehicleType() As String Implements IVehicle.GetVehicleType
Return Me.VehicleType
End Function
Public Function GetWheels() As Integer Implements IVehicle.GetWheels
Return Me.Wheels
End Function
End Class
Public Class clsCar
Inherits clsVehicle
Public Sub New(vehicleType As String, wheels As Integer)
MyBase.New(vehicleType, wheels)
End Sub
End Class
Public Class clsBus
Inherits clsVehicle
Public Sub New()
MyBase.New("Bus", 4)
End Sub
End Class

Answer 3

If your clsBus and clsCar are meant to refer to a specific car then the type should be a member of that class already, not something you pass in when you want to get information. To this end I'd suggest that you have "type" as something you can pass in the constructor and then the method on the bus would have no parameters and would just refer to its internal type to determine how many wheels it has.
I'm not too fluent with VB.NET so would probably make mistakes in example code but hopefully you get what I mean. If not I'll knock up some code. :)