The Situation-
Have EF database entities with collections of other database entities as navigation properties all of which are not exposed outside of the project. Each of these entities are nested in base classes that allow public read and protected set access to the Db entity's property values and are the classes the business layer will work with. So these base classes also need collections of their Db entity's navigation properties.
What I have now-
I am still working on how I want the data layer to interact with with the other layers that reference it. So I have been experimenting with creating collection classes that can be set equal to a collection of Db entities and create a collection of the base class that the Db entity is nested in. The collection is then exposed outside the class though an interface as well as the base class. The interface gives access to some custom methods of collection class like accepting DTOs and prebuilt quires but would hide any Add methods.
The Problem-
I would like to create a generic base class for the collections but cant seem to find a way. I have gotten close to something workable but it was ugly and confusing. Below is an outline of what I'm working with. The narrowing operators in the entity class work well. I would rather have widening operators and narrowing operators in the base class so I could keep the New method private for the base classes but I don't want to expose the actual database entities outside the Data Layer.
Interops Namespace referenced by all projects in solution-
Public Interface IGenericEntity
End Interface
Public Interface INavigationPropertyEntity : Inherits IGenericEntity
End Interface
Public Interface IDbEntityToExposedEntityCollection(Of TEntity As IGenericEntity)
'Used to hide Add method
End Interface
Public Interface IPublicEntity
ReadOnly Property NavProps As IDbEntityToExposedEntityCollection(Of INavigationPropertyEntity)
End Interface
Data Layer project-
-Database Entities
Friend Class DbEntity1
Public Shared Narrowing Operator CType(ByVal value As DbEntity1) As NavigationPropertyEntity
Return New NavigationPropertyEntity(value)
End Operator
End Class
Friend Class DbEntity2
Public Sub New()
NavigationPropertyDbEntities = New HashSet(Of DbEntity1)
End Sub
Public Property NavigationPropertyDbEntities As ICollection(Of DbEntity1)
Public Shared Narrowing Operator CType(ByVal value As DbEntity2) As PublicEntity
Return New PublicEntity(value)
End Operator
End Class
-Exposed Base Classes
Public Class NavigationPropertyEntity : Implements INavigationPropertyEntity
Private _value As DbEntity1
Friend Sub New(value As DbEntity1)
_value = value
End Sub
End Class
Public Class PublicEntity : Implements IPublicEntity
Dim _value As DbEntity2
Friend _NavProps As DbEntityToPublicEntityCollection(Of INavigationPropertyEntity, DbEntity1, NavigationPropertyEntity)
Public ReadOnly Property NavProps As IDbEntityToExposedEntityCollection(Of INavigationPropertyEntity) Implements IPublicEntity.NavProps
Get
Return _NavProps
End Get
End Property
Friend Sub New(value As DbEntity2)
_value = value
_NavProps = new DbEntityToPublicEntityCollection(Of INavigationPropertyEntity, DbEntity1, NavigationPropertyEntity)(_value.NavigationPropertyDbEntities)
End Sub
End Class
-Entity Specific Collection
Friend Class DbEntityToPublicEntityCollection(Of IEnt As INavigationPropertyEntity, DbEnt As DbEntity1, ExpEnt As NavigationPropertyEntity)
Inherits HashSet(Of IEnt)
Implements IDbEntityToExposedEntityCollection(Of IEnt)
Public Sub New(value As HashSet(Of DbEnt))
For Each ent In value
Dim exp As NavigationPropertyEntity = ent
Dim i As INavigationPropertyEntity = exp
MyBase.Add(i)
Next
End Sub
End Class
Additional Info Edit
The main issue is being able to have a generic collection declared something like,
Friend Class DbEntityToPublicEntityCollection(Of IEnt, DbEnt as class, ExpEnt As class)
because in the New sub ExpEnt cannot be converted to DbEnt and a TryCast will just result in a runtime error.
Like I said, I tried setting up classes that are inherited by the exposed and entity classes so a new method with a parameter for the exposed class could be set up but with trying to have the interfaces in there it got unmanageable pretty quick. Then again I have not worked much with making my own generic types before so I'm not sure if this is even a proper strategy.
Related
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.
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.
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.
I'm refactoring, and have run into a roadblock.
Background:
I have a base class and several inherited derived classes. The derived classes don't always need to have the same properties. If any properties are shared among the derived classes, those properties would live at the base class level ('Contents', for example).
Similarly, GoodDocument below has 'GoodThings' but would not want/need to have 'BadThings'.
I want to treat instances of both 'GoodDocument' and 'BadDocument' as type 'Document'
public mustinherit class Document
public property Contents as string
public sub new()...
end class
public class GoodDocument
inherits Document
public property GoodThings as string
public sub new()...
end class
public class BadDocument
inherits Document
public property BadThings as string
public sub new()...
end class
The 'DocumentWriter' class will also have several derived classes: ('GoodDocumentWriter' and 'BadDocumentWriter').
I need to pass around the DocumentWriter.Doc as a 'Document' to a number of other places in the code. Doc.GoodThings would only be called from within an instance of either 'GoodDocument' or 'GoodDocumentWriter'.
public mustinherit class DocumentWriter
public property Doc as Document
public sub new()...
end class
public class GoodDocumentWriter
inherits DocumentWriter
public sub new
mybase.Doc = new GoodDocument
end sub
end class
public class BadDocumentWriter
inherits DocumentWriter
public sub new
mybase.Doc = new BadDocument
end sub
end class
Question:
Is there a design pattern that allows for derived classes to have members that don't exist at the base class level?
Do all properties have to live at the base class level?
Revised
I was trying to be brief with my initial question and I made the mistake of over simplifying the situation. In short, I did realize that it should be possible to have different properties on each of the derived classes. (I typed that in a tongue-in-cheek manor and didn't mean to keep it in the final post).
I realize now that the problem that I was experiencing was really symptomatic of a larger issue which needed addressing.
It appears that I was encountering compiler complaints that could be corrected by further refactoring and looser coupling. While others answered the basic question that I posed, Ryan Gross' example really helped kick start some new ideas.
Thanks!
What you should do in this case is define the operations that can be performed on instances of Document in an interface. In your case maybe there is a WriteThings operation, so you would have:
public interface Writeable {
public sub WriteThings();
}
Then in your derived classes you would implement the method to utilize the internal data of the class. For example:
public mustinherit class Document implements Writeable
public property Contents as string
public sub new()...
public sub WriteThings();
end class
public class GoodDocument
inherits Document
public property GoodThings as string
public sub new()...
public sub WriteThings()
//Do something with GoodThings
end sub
end class
public class BadDocument
inherits Document
public property BadThings as string
public sub WriteThings()
//Do something with BadThings
end sub
public sub new()...
end class
Finally, client code that needs to call WriteThings accesses it through an interface:
public mustinherit class DocumentWriter
public property Doc as Writable
public sub new()...
public sub PerformWrite()
Doc.WriteThings();
end sub
end class
It is generally not a good idea to build several parallel class hierarchies. In this case, one DocumentWriter class should be able to write any class that implements Writeable by invoking its WriteThings method.
If all the properties live at the base class level, then I'm not sure what the point of a derived class would be. :) You'd be able to do everything with the base class.
So, yes. If something is applicable only to GoodDocument and not to Document, then it should be in GoodDocument.
To answer your question specifically:
Yes, you just create multiple layers in the inheritance hierarchy: You have a base class, and then many two “branches” (good and bad, to use your terminology). Any properties that are only relevant to either branch, you declare in the class that inherits from the base class. Those properties will only be visible to that class and any classes inheriting from it.
No, properties can be declared anywhere within your inheritance hierarchy.
I have a function which I need to call for three different types, with the underlying logic remaining the same for all the different types, so I figured it would be best to write this function using generics.
Here is the basic outline of the classes and functions involved:
'PO Base class'
Public MustInherit Class ProductionOrder
Public MustInherit Class Collection(Of T)
Inherits System.Collections.Generic.Dictionary(Of Long, T)
End Class
'....'
End Class
Public Class ProfileProductionOrder
Inherits ProductionOrder
Public Class Collection
Inherits ProductionOrder.Collection(Of ProfileProductionOrder)
End Class
'....'
End Class
Public Class UnitProductionOrder
Inherits ProductionOrder
Public Class Collection
Inherits ProductionOrder.Collection(Of UnitProductionOrder)
End Class
'....'
End Class
Public Class CrateProductionOrder
Inherits ProductionOrder
Public Class Collection
Inherits ProductionOrder.Collection(Of CrateProductionOrder)
End Class
'....'
End Class
'Generic function, intended to work on profile, unit, and crate production orders.'
'This method resides in the base class of the GUI.'
Protected Sub FillPOCells(Of T As ProductionOrder.Collection(Of ProductionOrder)) _
(ByVal dgv As DataGridView, ByVal ProductionOrders As T)
'...do some stuff'
End Sub
'This function resides in the Profile child GUI class.'
Protected Sub LoadDataGridViewPOs()
Dim dgv As DataGridView
Dim ProductionOrders As ProfileProductionOrder.Collection
'....'
'Fill PO Cells'
FillPOCells(Of ProfileProductionOrder.Collection)(dgv, ProductionOrders)
'....'
End Sub
The ProductionOrder base and child classes compile, as does the FillPOCells function. But when I call FillPOCells inside LoadDataGridViewPOs the compiler complains that "Type argument 'ProfileProductionOrder.Collection' does not inherit from or implement the constraint type 'ProductionOrder.Collection(Of ProductionOrder)'.
Also, here is some explanation about why things are set up this way. My predecessor set up the convention of putting the collection of an object as a subclass within it, so it's easy to refer to it as Obj.Collection. Next, the reason we need three different types of production orders is because they are treated differently and stored in different tables and such on the back end. Lastly, I realize I could implement this fairly easily without getting this particular generic function to work, but I'm looking at this as a learning experience to improve my understanding of generics and OO design.
So the question is, why am I getting that compiler error and how should I change my class and generics design to accomplish what I have in mind?
If you need any further explanation about what I'm trying to do or how I have things set up let me know. The main idea is to have a function that can take a collection who's elements belong to one of the ProductionOrder child classes, and run operations on these elements that only use the functionality held in their ProductionOrder base class (hence why either of the child types is okay to operate on in the function).
'PO Base class'
Public MustInherit Class ProductionOrder(Of T)
Public MustInherit Class Collection
Inherits System.Collections.Generic.Dictionary(Of Long, T)
End Class
'....'
End Class
Public Class ProfileProductionOrder
Inherits ProductionOrder(Of ProfileProductionOrder)
'....'
End Class
Public Class UnitProductionOrder
Inherits ProductionOrder(Of UnitProductionOrder)
'....'
End Class
Public Class CrateProductionOrder
Inherits ProductionOrder(Of CrateProductionOrder)
'....'
End Class
That is a lot simpler according to me.
But I higly doubt you need the CollectionClass.