Can I override an interface property? - vb.net

Question

Shell example is below. Basically, I want a client and employee to implement the SSN property from IPerson. However, I want client to have get and set (which isn't an issue), but I want employee to have get only.
Public Interface IPerson
Property SSN As String
End Interface
Public Class Client
Implements IPerson
Public Property SSN As String Implements AELName.IPerson.SSN
Get
Return _SSN
End Get
Set(value As String)
_SSN = value
End Set
End Property
End Class
Public Class Employee
Implements IPerson
Public Readonly Property SSN As String Implements AELName.IPerson.SSN
Get
Return _SSN
End Get
End Property
End Class
Employee generates an error of "'SSN' cannot implement 'SSN' because there is not matching property on interface 'IPerson'". Is there a somewhat simple way to override the SSN implementation for Employee?

Answer 1

You can implement an empty Set - one that doesn't update anything.
Public Class Employee
Implements IPerson
Public Readonly Property SSN As String Implements AELName.IPerson.SSN
Get
Return _SSN
End Get
Set
' Make read only for Employee
End Set
End Property
End Class

Answer 2

I would suggest splitting the interface into IReadablePerson and IReadWritePerson, with the latter inheriting the former. Note that the former interface is not IImmutablePerson, since the latter name would imply to consumers of the class that they should not expect any of its properties ever to change; an object which implements IReadWritePerson would not abide such expectation, but would abide the expectation that the person should be readable.
One slight annoyance with splitting the interface is that it will be necessary for the IReadWritePerson to include the modifier Shadows in the declarations of its read/write properties, and implementers of IReadWritePerson will have to provide both a read-only implementation of IReadablePerson and a read-write implementation of IReadWritePerson. In C#, a public implementation of a read-write property can automatically generate implementations for any like-named read-only, write-only, or read-write properties which are part of any interfaces the class implements, but when explicitly declaring which interface is being implemented, the style of the interface (read-only, write-only, read-write) must precisely match that of the implementation. Annoying.
The annoyance is made worse by the fact that one cannot simply declare IReadableFoo with a read-only property, IWriteOnlyFoo with a write-only property, and have IReadWriteFoo simply inherit both. If an interface implements a read-only property and a write-only property with the same name, neither property will be usable because the compiler will announce that in statements like somevariable = object.someProperty or someObject.someProperty = someVariable, it's "ambiguous" which implementation to use. Not that I can see any ambiguity--I can't see how the first could use anything but a getter, or the latter anything but a setter, but the compiler can't resolve it.

Answer 3

To answer your title question "Can I override an interface property" ... Absolutely. Here's an example of how to do so. You simply add the Overridable keyword to your base concrete implementation. I know that doesn't solve changing the property to ReadOnly, but I figured I'd point out that overriding a base classes concrete implementation of an interface is possible.
Module Module1
Sub Main()
Dim iEntity As IEntity = New MyEntity
iEntity.SetMessage(iEntity)
Console.WriteLine(iEntity.Message)
Console.ReadKey()
End Sub
End Module
Public Interface IEntity
Property Message As String
Sub SetMessage(entity As IEntity)
End Interface
Public Class MyEntity
Inherits BaseEntity
Public Overrides Property Message As String
Get
Return String.Format("{0}.. and overroad.", MyBase.Message)
End Get
Set(value As String)
MyBase.Message = value
End Set
End Property
Public Overrides Sub SetMessage(entity As IEntity)
Me.Message = "I was set from MyEntity."
End Sub
End Class
Public Class BaseEntity
Implements IEntity
Public Overridable Property Message As String Implements IEntity.Message
Public Overridable Sub SetMessage(entity As IEntity) Implements IEntity.SetMessage
Me.Message = "I was set from BaseEntity."
End Sub
End Class