Force multiple classes to have a specific shared member - vb.net

Question

I'm already searching for hours, but I cannot find any solution or even a possible way, that fits.
I need a way to enforce, that multiple classes have a shared member. This classes can not be inherits another class, because they are entity data model classes and i work on a partial class file beside the autogenerated files.
I tried to use an interface, but interfaces does not provide the option to declare a member as shared.
My try was like:
Public Interface IInterfaceA
ReadOnly Property PropA as String
End Interface
Public Partial Class ClassA
Implements IInterfaceA
Public Shared ReadOnly Property SPropA As String
Get
Return "FixedValueStringForClassA"
End Get
End Property
Public ReadOnly Property PropA As String Implements IInterfaceA.PropA
Get
Return SPropA
End Get
End Property
End Class
Public Partial Class ClassB
Implements IInterfaceA
Public Shared ReadOnly Property SPropA As String
Get
Return "FixedValueStringForClassB"
End Get
End Property
Public ReadOnly Property PropA As String Implements IInterfaceA.PropA
Get
Return SPropA
End Get
End Property
End Class
So I can call the PropA Member, when I get e.g. an array of types, that implements IInterfaceA.
But my first implementation does not enforce the shared property, only the normal property. To call the normal property, I would need an instance of that type, but this is not the way I searching for.
Maybe there is another solution to this problem.

Answer 1

Unfortunately you can't force a class to implement a shared member.
However, you CAN share extension methods from a module.
I had a similar problem recently with needing to share functions with multiple classes but still be able to run them as that class. Which I solved using extension methods.
Basically how it works is the module extends all objects that implement an interface, which can be conveniently included with the module, that interface will include anything the module will need access to from the calling class and the module will include anything that needs to be shared between all classes
Public Module SharedProp
Dim PropA As String
<Extension()>
Public Function GetPropA(Of T As IInterfaceA)(this As T) As String
Return PropA
End Function
<Extension()>
Public Sub SetPropA(Of T As IInterfaceA)(this As T, value As String)
PropA = value
End Function
Public Interface IInterfaceA
Property PropA As String
End Interface
End Module
so long as the module is included, any class which impliments your interface will have access to the module's extension methods, which in turn has access to the module's implicitly shared members
Imports SharedProp
Public Partial Class ClassA
Implements IInterfaceA
Public Property PropA As String Implements IInterfaceA.PropA
Get
Return Me.GetPropA()
End Get
Set(value As String)
Me.SetPropA(value)
End Set
End Property
End Class
Public Partial Class ClassB
Implements IInterfaceA
Public Property PropA As String Implements IInterfaceA.PropA
Get
Return Me.GetPropA()
End Get
Set(value As String)
Me.SetPropA(value)
End Set
End Property
End Class
Hope that helps solve your problem!
EDIT: Shared on Class basis instead of Interface
Public Module SharedProp
Dim PropA As Dictionary(Of Type, String)
<Extension()>
Public Function GetPropA(Of T As IInterfaceA)(this As T) As String
If PropA.ContainsKey(GetType(T)) Then
Return PropA(T)
Else
Return Nothing
End Function
<Extension()>
Public Sub SetPropA(Of T As IInterfaceA)(this As T, value As String) As String
If PropA.ContainsKey(GetType(T) Then
PropA(T) = value
Else
PropA.Add(GetType(T), value)
End If
End Function
Public Interface IInterfaceA
Property PropA As String
End Interface
End Module