Question

I’m writing some code in VB.Net which I hope demonstrate to colleagues (not to say familiarise myself a little more) with various design patterns – and I’m having an issue with the FactoryMethod Pattern.

Here’s my code:

Namespace Patterns.Creational.FactoryMethod

    ''' <summary>
    ''' This is the Factory bit - the other classes are merely by way of an example...
    ''' </summary>
    Public Class CarFactory
        ''' <summary>
        ''' CreateCar could have been declared as Shared (in other words,a Class method) - it doesn't really matter.
        ''' Don't worry too much about the contents of the CreateCar method - the point is that it decides which type
        ''' of car should be created, and then returns a new instance of that specific subclass of Car.
        ''' </summary>
        Public Function CreateCar() As Car
            Dim blnMondeoCondition As Boolean = False
            Dim blnFocusCondition As Boolean = False
            Dim blnFiestaCondition As Boolean = False

            If blnMondeoCondition Then
                Return New Mondeo()
            ElseIf blnFocusCondition Then
                Return New Focus()
            ElseIf blnFiestaCondition Then
                Return New Fiesta()
            Else
                Throw New ApplicationException("Unable to create a car...")
            End If

        End Function
    End Class

    Public MustInherit Class Car
        Public MustOverride ReadOnly Property Price() As Decimal
    End Class

    Public Class Mondeo Inherits Car

        Public ReadOnly Overrides Property Price() As Decimal
            Get
                Return 17000
            End Get
        End Property
    End Class

    Public Class Focus Inherits Car
        Public ReadOnly Overrides Property Price() As Decimal
            Get
                Return 14000
            End Get
        End Property
    End Class

    Public Class Fiesta Inherits Car
        Public ReadOnly Overrides Property Price() As Decimal
            Get
                Return 12000
            End Get
        End Property
    End Class

End Namespace

When I try to compile this, I am getting errors (BC30311) in the CarFactory.CreateCar telling me that it can’t convert Fiesta, Mondeo, and Focus into Car. I don’t see what the issue is – they’re all subclasses of Car.

Doubtless I’m overlooking something simple. Can anyone spot it?

Cheers,

Martin.

Answer 1

Your Inherits keyword must be on a new line. This is documented by Microsoft on their help and Support. http://support.microsoft.com/kb/307222

Change the SavingsAccount class
definition as follows, so that
SavingsAccount inherits from Account
(note that the Inherits keyword must
appear on a new line):