Implement interface in abstract class for derived classes - vb.net

Question

Let's say I got this interface:
Public Interface IGsm
Function Run(filePath As String) As Boolean
End Interface
Now I want all child classes to use IGsm interface, therefore I decided to implement it by abstract class (hope it's good approach):
Public Class MustInherit GsmBase
Implements IGsm
Public Function Run(filePath As String) As Boolean Implements IGsm.Run
Throw New NotImplementedException()
End Function
End Class
First question:
What if I put Overridable to my function does it mean that I can define some implementation here in base class same for all derived classes and also ovveride rest implementation in derived classes? Is that correct approach?
Public Overridable Function Run(filePath As String) As Boolean Implements IGsm.Run
'Some shared code for all child classes
'......
End Function
so derived classes could do:
Public Ovverides Function Run(filePath As String) As Boolean Implements IGsm.Run
MyBase.Run
'add additional code for this specific child class...
End Function
Is that all correct what I just wrote?
Second question: If I do not mark Run as Overridable in base class. When inheriting from base class to my derived classes does it mean that my derived classes will just have to use the current implementation of that Run method from base class?
Third question: Why I cannot mark Run in base class as MustOverides?

Answer 1

Functions, Subs and properties and also Interface implementations are inherited in deriving classes, no matter whether the members are Overridable / Overridden or not.
Overridable means that a deriving class can just do nothing, if it is happy with the base implementation. A call to a Function will automatically call the base function. But it can override it, if another implementation is desired. You can then decide to call the base implementation or not, depending whether you want to add functionality or want to replace it completely.
Apply MustInherit to the class. This enables you to apply MustOverride to members. (This is a bit more complicated than in C#, where you can apply the same keyword abstract twice.)
Public MustInherit Class GsmBase
Implements IGsm
Public MustOverride Function Run(filePath As String) As Boolean Implements IGsm.Run
End Class
Note that you can combine different approaches. You could have several abstract base classes implementing the interface and also classes implementing the interface directly
Interface I
MustInherit Class BaseA Implements I
MustInherit Class BaseB Implements I
Class A1 Inherits BaseA
Class A2 Inherits BaseA
Class A2_1 Inherits A2
Class B1 Inherits BaseB
Class B2 Inherits BaseB
Class C Implements I
Class D Inherits C
Class E Inherits D
All objects of type A1, A2, A2_1,B1, B2, C, D and E are assigment compatible to I.

Answer 2

You didn't create an abstract class in your sample, it would look like this.
Public Interface IGsm
Function Run(filePath As String) As Boolean
End Interface
Public MustInherit Class GsmBase
Implements IGsm
Public MustOverride Function Run(filePath As String) As Boolean Implements IGsm.Run
End Class
Public Class Gsm
Inherits GsmBase
Public Overrides Function Run(filePath As String) As Boolean
Return False
End Function
End Class
Then you can use is as
Dim test As IGsm = New Gsm
test.Run("test") ' This will call Gsm.Run
or
Dim test As GsmBase = New Gsm
test.Run("test") ' This will call Gsm.Run
MustInherit is for class, not method. For method you would use MustOverride.