I'm trying to access a shared property of a class passed as a parameter to a type-parametrised procedure. The reason why I'm doing this is so I can embed the various API call endpoints (among other class-specific things) as properties within the class itself. I've read some similar SO posts but nothing is close enough to be sure that it isn’t possible (which I think is likely).
Below is the essence of the structure - there's some pseudo code towards the end:
MustInherit Class BaseClass
Shared Property Endpoint As String
End Class
Class Person
Inherits BaseClass
Property Age As Integer
Property Name As String
Sub New()
_Endpoint = "/GetPerson"
End Sub
End Class
Class Event
Inherits BaseClass
Property When As Date
Property Type As String
Sub New()
_Endpoint = "/GetEvent"
End Sub
End Class
Function Retrieve(T As BaseClass)(Id As String) As T
Dim oResp As HttpResponse = MakeGetCall(T.Endpoint, Id) <- T.Endpoint throws a compile error
Return Deserialize(Of T)(oResp.Content)
End Function
Dim oPerson As Person = Retrieve(Of Person)("123")
Dim oEvent As Event = Retrieve(Of Event)("123")
To my tiny mind, I would have thought that, since T’s base class is BaseClass which contains the property Endpoint, I’d be ok. But seemingly not.
I've tried a fair few things from here on SO and other places to overcome this to no avail. Yes, I realize I could perform some kind of endpoint look-up based on the type of T but the above represents a very clean solution and I’d like to get it to work if possible.
Any ideas?
Assuming you want EndPoint to be different for each subclass, you should use MustOverride instead of Shared...
MustInherit Class BaseClass
Public MustOverride Property EndPoint As String
End Class
Then return a constant in each subclass
Class Person
Inherits BaseClass
Public Overrides Property EndPoint As String
Get
Return "/Person"
End Get
You might want to declare EndPoint as ReadOnly too.
The small limitation is that you'll need an instance of the class to access EndPoint (since it isn't Shared). If you have a parameterless constructor, you could use (New Person).EndPoint where needed.
Related
So to start this off; I'm a beginner in VisualBasic.Net and my classes require me to learn it. The current subject is object constructors and constructor methods. The current exercise (it's not graded or an exam) is requiring us to make a parent class with a constructor method, and a child class with a new() that calls said function. It looks a bit like this;
Protected MustInherit Class Vehicle()
Protected ReadOnly Property Serial_No As Integer
Protected Property Mileage As Integer
Protected Property Color As String
Protected Function CreateVehicle() As Object
End Function
End Class
Public Class Car
Inherits Vehicle
Public ReadOnly Property Car_Type As String
Public Sub New()
End Sub
End Class
The thing I'm having issues with is that I'm not sure how to go about it? Can't ReadOnly properties ONLY be edited in the constructor itself, and doesn't the object need to be initialized in the constructor? Is there something particular I need to add in the CreateVehicle function?
I did ask the teacher but his answer was 'just give up on it and go do something else', which is ultimately pretty unhelpful.
Edit: (added the inheritance to the child class)
So, after being asked for clarification on what I'm trying to do; the exercise itself is not entirely about doing this, but it is the thing in the exercise that I'm struggling with. The goal is to create a Car object utilizing the constructor (New()), but the constructor must call a secondary function located inside the parent class, Vehicle.
My issue is the following : I'm not sure how to go about implementing the function inside the constructor. I know how to call methods/subs/functions and how to get returns from them, but I'm not sure on how I would go about returning a ReadOnly property's values from a secondary function. Don't readonly properties become uneditable outside of the constructor?
I could always return each value separately instead of as an object, and then set the Car object's values to be equal to the return of the function, individually. But then what's the point of calling a separate function instead of just passing everything as a parameter and doing it directly in the constructor?
This is probably what your teacher is looking for:
Public MustInherit Class Vehicle
Protected ReadOnly Property Serial_No As Integer
Protected Sub New(serialNumber As Integer)
Me.Serial_No = serialNumber
End Sub
End Class
Public Class Car
Inherits Vehicle
Public ReadOnly Property Car_Type As String
Public Sub New(serialNumber As Integer, carType As String)
MyBase.New(serialNumber)
Me.Car_Type = carType
End Sub
End Class
Both constructors take in parameters so the ReadOnly properties can be set.
I have two classes to handle database operations, one for MySQL (DBMySQL), and another for SQLite (DBSQLite). The user chooses which database system to use.
The functions within the classes have the same names, but are obviously slightly different to handle the variations in databases.
I would like to refer to the chosen class by one name throughout the application. I have set a global variable DB.
In a procedure I can: Dim DB as New DBMySQL (or DBSQLite). This works within the procedure, but not globally, but I can see all the functions when coding.
If I instead use: DB = New DBMySQL, this works globally, but no class functions are displayed when coding.
Any alternatives?
Use the concept of inheritance, and create a MustInherit class with MustOverride methods and/or properties.
Public MustInherit Class AbstractDB
Public MustOverride Function MyQuery(input As Object) As Object
Public MustOverride Sub MyUpdateMethod(input As Object)
End Class
Public Class DBMySQL
Inherits AbstractDB
Public Overrides Function MyQuery(input As Object) As Object
End Function
Public Overrides Sub MyUpdateMethod(input As Object)
End Sub
End Class
Public Class DBSQLite
Inherits AbstractDB
Public Overrides Function MyQuery(input As Object) As Object
End Function
Public Overrides Sub MyUpdateMethod(input As Object)
End Sub
End Class
Then, when you want to use your classes, make your DB global variable of type AbstractDB. You could then create either DBMySql or DBSQLite and assign it to your DB variable. The method names will all be the same, because they all inherit the same base class. But each derived class must fill out the content of those methods on its own.
Dim DB as AbstractDB = New DBMySQL
You could also use an interface.
Public Interface IRepository
' common functions of MySQL and SQLLiteclasses
End Interface
Public Class MySQLRepository
Implements IRepository
End Class
Public Class SQLLiteRepository
Implements IRepository
End Class
Public Function GetDB(userChoice As String) As IRepository
If userChoice = "MySQL" Then
Return New MySQLRepository()
Else
Return New SQLLiteRepository
End if
End Function
Dim DB As IRepository = GetDB(userChoice)
This is a basic implementation of the Repository pattern. The example in the link is in C#, but, as you're probably aware, it's not easy finding examples in VB. Fortunately, there are lots of C# to VB converters.
The abstract example Sean Skelly gave should also work. You may want to research the difference between abstract classes and interfaces.
Folks;
Code looks like:
Public Class MasterA
Inherits Underling
End Class
Public Class MasterB
Inherits Underling
End Class
Public Mustinherit Class Underling
Sub DoSomething()
Me.GetType 'Using the instance, I can get the class.
end sub
Shared function() as ???? 'How can I define the return type based on the class that inherited me?
'Me.GetType 'Won't work as this is a shared function with no instance 'Me'
End Function
End class
OK. The question is: is there a way to get at the class type from within a shared function that was inherited by another class?
What I'm building is an XML serializer/desrializer as an inheritable class so that classes that inherit it can be serilized to an XML file, and back again. Rather than writing a serializer/deserializer for each type of class I want to do this with, I'd like to just inherit the functionality.
To do that, though, requires that I be able to ascertain the clas that inherited me in the shared function.
You could get the desired behavior with a generic base class, my VB is a little rusty so you might find stray parens or brackets. This would really be the only way to get a type reference to an inheriting class in a shared base class function.
Public Mustinherit Class Underling(Of T)
Sub DoSomething()
Me.GetType 'Using the instance, I can get the class.
end sub
Shared function() As T
' GetType(T) should get the type at this point
End Function
End class
Public Class MasterA
Inherits Underling(Of MasterA)
End Class
Public Class MasterB
Inherits Underling(Of MasterB)
End Class
As a side note it does seem like a rather weird solution to handle XmlSerialization rather than through your own serializer implementation or XmlSerializer
Disclaimer: I am fairly new to working with generics so I am not entirely sure if what I am trying to do even makes sense or is possible.
I have a bunch of user controls in a project. All of these user controls share a similar property so I want to move it into a base class. The only difference is the return type of the property.
I have three classes interacting in this scenario. The first class is a base type, which inherits from CompositeControl and will be inherited by other classes in my project:
Friend Class MyBaseClass(Of T As {New})
Inherits CompositeControl
Private _someProperty As T = Nothing
Protected ReadOnly Property SomeProperty As T
Get
// dumbed down for the sake of example
If _someProperty Is Nothing Then
_someProperty = New T()
End If
Return _someProperty
End Get
End Property
End Class
Then I have this control class, which inherits from MyBaseClass:
Public Class MyControlClass
Inherits MyBaseClass(Of MyReturnTypeClass)
// snip...
End Class
And finally MyReturnTypeClass which is what the base's SomeProperty should return:
Friend Class MyReturnTypeClass
Public Property AutoProperty1 As Boolean = False
Public Property AutoProperty2 As String = String.Empty
// etc
End Class
When I attempt to build the project, I get this error from MyControlClass:
Inconsistent accessibility: type argument 'MyReturnTypeClass' is less accessible than Class 'MyControlClass'.
I need MyControlClass to be Public so it can be consumed by other projects, and I also want the MyBaseClass and MyReturnTypeClass to be Friend so they cannot be seen/used by consumers. Am I just missing some special keyword somewhere or is this not possible?
You cannot inherit from a base class that is less accessible than the derived class. So for instance, this won't work:
Friend Class MyBase
End Class
Public Class MyDerived
Inherits MyBase ' Won't compile because MyBase is less accessible
End Class
Therefore, since in your example, MyBaseClass(T) has is a friend type, but you are trying to inherit from it into a public MyControlClass type. Therefore, even if you took generics and MyReturnTypeClass out of the "equation", it still wouldn't work.
However, with generics, even if no member of the public interface of the class actually uses the generic type, the type must still be at least as accessible as the derived type. For instance:
Public Class MyBase(Of T)
' T not actually used at all
End Class
Friend Class MyOtherType
End Class
Public Class MyDerived
Inherits MyBase(MyOtherType) ' Won't compile because MyOtherType is less accessible
End Class
The base class must be at least as accessible as the derived class. This is a language restriction (see here).
If you intend to avoid MyBaseClass being instantiated by consumers, consider marking it Public MustInherit instead of Friend. Hope this helps.
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?
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
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.
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