I recently added an interface to some custom user controls I have implemented. The interface is pretty basic. It has one method that supports chaining:
Public Interface IMyInterface(Of T As WebControl)
Function DoSomething() As T
End Interface
The implementations are also pretty basic:
Public Class MyCustomControl
Inherits CompositeControl
Implements IMyInterface(Of MyCustomControl)
Public Function DoSomething() As MyCustomControl _
Implements IMyInterface(Of MyCustomControl).DoSomething
' do stuff
Return Me
End Class
Everything works fine up to this point. The issues arise when I attempt to loop over a collection of controls that all implement the IMyInterface interface, like so:
Dim myList = New List(Of IMyInterface(Of WebControl))
myList.Add(someCustomControl)
myList.ForEach(Sub(i) i.DoSomething())
someCustomControl is a MyCustomControl which implements IMyInterface(Of MyCustomControl) instead of IMyInterface(Of WebControl).
I am getting this error on the second line (where I try to add someCustomControl):
Option Strict On disallows implicit conversions from 'MyCustomControl' to 'IMyInterface(Of WebControl)'.
Is there any way to get around this error? I am close to having it working but I do not know enough about generics to get beyond this point.
Covariance is a language feature that was introduced in VS 2010, and solves your problem. You need to define your generic such that the type T has the Out keyword in front of it:
Public Interface IMyInterface(Of Out T As WebControl)
Function DoSomething() As T
End Interface
When you use the Out keyword, you are using covariance. It allows generics of a more derived type to be used in place of a generic with the base type. So in your case it will allow a IMyInterface(Of MyCustomControl)) object in places where the code would normally expect IMyInterface(Of WebControl)), such as your for loop.
Note that covariance has a restriction. The covariant type T can only be used as a function return value, and not as a parameter into a function (or sub). For example, if the DoSomething signature in IMyInterface looked like this the compiler would complain:
' Here the type T is used as an input param - compiler error
Sub DoSomething(ByVal sampleArg As T)
Given your chaining scenario, I don't think the above restriction is a problem.
More Info at MSDN:
Covariance and Contravariance
Creating Variant Generic Interfaces
I don't know what your function DoSomething does, but I try assigning the instance's CssClass in there for testing purpose.
Declare the interface as follows:
Public Interface IMyInterface(Of Out T As WebControl)
Function DoSomething() As T
End Interface
Notice the Out T parameter.
Create 2 controls that implement the interface:
Public Class MyCustomControl1
Inherits CompositeControl
Implements IMyInterface(Of MyCustomControl1)
Public Function DoSomething() As MyCustomControl1 Implements IMyInterface(Of MyCustomControl1).DoSomething
' do stuff
Me.CssClass = "XXX"
Return Me
End Function
End Class
Public Class MyCustomControl2
Inherits CompositeControl
Implements IMyInterface(Of MyCustomControl2)
Public Function DoSomething() As MyCustomControl2 Implements IMyInterface(Of MyCustomControl2).DoSomething
' do stuff
Me.CssClass = "YYY"
Return Me
End Function
End Class
On a test page's PageLoad event:
Dim someCustomControl As New MyCustomControl1
Dim someCustomControl2 As New MyCustomControl2
Dim myList = New List(Of IMyInterface(Of WebControl))
myList.Add(someCustomControl)
myList.Add(someCustomControl2)
myList.ForEach(Sub(i) Literal1.Text &= i.DoSomething.CssClass & "<br />")
The result is, the CssClass property of both someCustomControl & someCustomControl2 are set to the respective values.
This shows that the interface function DoSomething was successfully called and the instance changed.
You will need to cast the object before adding it:
myList.Add(CType(someCustomControl, IMyInterface(Of WebControl)))
You may also want to concider making the interface not generic and your "DoWork" method return type as the interface itself.
Public Interface IMyInterface
Function DoSomething() As IMyInterface
End Interface
When you have to specify the type in the interface definition it kind of takes away from the power of interfaces (not having to know about the implementation).
Related
what I'm trying to archive with the code below is to have the GetInstance generic function take in an interface type that SystemVars implements (say IAuthentication) then create an instance of SystemVars and return it as interface type T.
The problem I an having is that no matter what casting method I try I can't find a way to return the new instance of SystemVars as T. The line in the GetInstance method Return <CastingFunction>(New SystemVars,T) always fails to compile with the error message saying Value of type SystemVars cannot be converted to 'T'.
How do I return the instance of the class as the interface type that was passed into T?
Imports System.Drawing
Public Class SystemVars
Implements IAuthentication,
IAuthorization,
IApplicationStarting
Private Sub New()
End Sub
Public Shared Function GetInstance(Of T)() As T
Return DirectCast(New SystemVars, T)
End Function
Public ReadOnly Property Username As String _
Implements IAuthentication.Username,
IAuthorization.Username
Get
Return _userName
End Get
End Property
Public ReadOnly Property Rolls As List(Of String) _
Implements IAuthorization.Rolls
Get
Return _rolls
End Get
End Property
Public ReadOnly Property InstallationId As Guid _
Implements IAuthentication.InstallationId,
IApplicationStarting.InstallationId
Get
Return _installationId
End Get
End Property
Public ReadOnly Property MainWindowStartUpPlacement As Rectangle _
Implements IApplicationStarting.MainWindowStartUpPlacement
Get
Return _mainWindowStartUpPlacement
End Get
End Property
'........
Private Shared _userName As String
Private Shared _rolls As List(Of String)
Private Shared _installationId As Guid
Private Shared _mainWindowStartUpPlacement As Rectangle
End Class
You can make an otherwise illegal cast work by passing through Object.
Public Shared Function GetInstance(Of T)() As T
Return DirectCast(CObj(New SystemVars), T)
End Function
You will get a runtime error if the cast isn't possible; as noted in the comments, this strategy is chucking type safety out the window and basically telling the compiler, "Don't bother me, I know what I'm doing." The runtime will throw an InvalidCastException on failure if you don't test and throw yourself. You can test using Type.IsAssignableFrom if you want to create a more developer-friendly error message; there isn't much context available in the debugger at the point of failure, though it may be pretty obvious if you look up the call stack.
For just three interfaces, it might be better to do three separate specific functions rather than a generic version, especially considering that the functions are necessarily Shared (and thus can't themselves be part of an interface).
You might also consider a design that includes a Dependency Injection container. In this kind of design, there would be a configuration step that would associate the interfaces with SystemVars as the implementation, then the client would ask the container for an instance of the interface and receive a SystemVars object.
The rough way that the three options (the third being to cast the SystemVars object to the requested interface) would look in code is:
'Casting a received object to a requested interface
Dim asInterface = DirectCast(SystemVars.GetInstance(), IAuthorization)
'Using a custom casting function on SystemVars
Dim asInterface = SystemVars.GetInstance(Of IAuthorization)
'Using a DI container
'Behavior if the interface isn't supported depends on the container
Dim asInterface = container.GetInstance(Of IAuthorization)
Note that TryCast could be used instead of DirectCast, in which case the result would be Nothing if the interface isn't supported.
Is using the Named Constructor Idiom possible in VB.NET? I've found many examples in C#/C++ but can't quite wrap my head around how to use it in vb.net. Seems like a better method of keeping my code readable when involving a lot of constructors with similar argument types.
I've never heard this term before, but after a quick search it sounds vaguely like the Static Factory Pattern. The idea is you make the constructor private and use a shared (static in c#) public function to create the new object.
Public Class Foo
Private Sub New()
End Sub
Public Shared Function CreateNew(param as Object) as Foo
Dim obj as New Foo()
obj.Prop = param
return obj
End Function
End Class
You sure can make Named Constructors in VB. The pattern uses a static (Shared in VB) factory method on the class itself, so that the method can be named. (Other Factory patterns involve using a separate Factory class to provide the static method.)
System.Drawing.Color is a simple example. The pattern is implemented underneath as a static (Shared) property. Since no arguments are necessary, the Get method of a Property works just fine:
Public Shared ReadOnly Property Chartreuse As Color
Usage:
Dim favoriteColor as Color = Color.Chartreuse
Or you can make static factory methods to do the same thing.
Public Class TheClass
Public Sub New()
End Sub
Public Sub New(input As String)
'do something with input
End Sub
Public Shared Function MyNamedConstructor() As TheClass
Return New TheClass
End Function
Public Shared Function AnotherNamedConstructor() As TheClass
Return New TheClass("Another Name")
End Function
End Class
As for whether this pattern is "better" than overloading constructors, that's really an opinion. Personally, I would just overload the constructors. As you can see in the example above, the constructors need to be there anyway.
I suggest using the Named Constructor pattern when you have only a few possible ways to construct your class/struct, but consumers of your class/struct will be using those few constructors often, and with different input values to those constructors (as in the System.Drawing.Color example).
The Name in 'Named Constructor' doesn't represent a name for the constructor itself, but for the object resulting from the constructor. If your named constructor can be used to create two objects that don't feel right to give the same name to, then don't give the constructor that name.
I am trying to implement a derived interface in a class. My interfaces and class are similar to the following. The Namespaces relate to different projects that hold these interfaces and the class:
Namespace ns1
Public Interface IParent
Function myFunction() As Double
End Interface
End ns1
Namespace ns2
Public Interface IDerived
Inherits ns1.IParent
Overloads / Shadows Function myFunction(ByRef myObject as Object) As Double
End Interface
End ns2
Namespace ns3
Public Class myClass
Implements ns2.IDerived
Public Function myFunction(ByRef obj as Object) As Double Implements ns2.IDerived.myFunction
End Function
End ns3
In the derived interface, I am trying to overload the function in a way that when I implement the derived interface, I only have to implement the function as defined therein - as is done with the code above on "myClass". However, I am getting an error saying I have to also implement the function from the parent interface (with the empty argument list). The error exists regardless of my using Overloads or Shadows on the function in the derived interface - both cause the error.
Is there anyway to accomplish what I am trying to do - implement only the derived interface's function in my class - using interfaces? If there is not a way using interfaces, can anyone suggest an alternate way? We really need to use interfaces and are trying to avoid using classes. That said, abstract classes my allow us to do all we need to do with these.
I have read a lot of info on all the topics covered by this question as every concept is pretty basic and well covered in online help. But, I have not found anything that I recognize as a direct solution to my specific issue.
Thanks in advance for any help.
I don't know if this is a typo but you have two distinct methods: one that takes no parameter, and another that takes an object, so the compiler requirement is legitimate.
If this is a typo and that you have only one method, say "myFunction()", then I fear VB.Net does not act like C# by simply hiding the base interface and allowing to only implement the derived one.
But you could easily fix this by forwarding:
Namespace ns1
Public Interface IParent
Function myFunction() As Double
End Interface
End Namespace
Namespace ns2
Public Interface IDerived
Inherits ns1.IParent
Function myFunction() As Double
End Interface
End Namespace
Namespace ns3
Public Class Class1
Implements ns2.IDerived
Public Function myFunction() As Double Implements ns2.IDerived.myFunction
Return 42
End Function
Private Function myFunction1() As Double Implements ns1.IParent.myFunction
Return myFunction()
End Function
End Class
End Namespace
Module Module1
Sub Main()
Dim cp As ns1.IParent = New ns3.Class1
cp.myFunction()
Dim cd As ns2.IDerived = New ns3.Class1
cd.myFunction()
End Sub
End Module
EDIT:
So was not a typo, here is the standard (good/best practice?) fix:
Public Class Class1
Implements ns2.IDerived
Public Function myFunction(ByRef obj As Object) As Double Implements ns2.IDerived.myFunction
End Function
Public Function myFunction() As Double Implements ns1.IParent.myFunction
Throw New NotImplementedException("'ns1.IParent.myFunction' has not been implemented because unicorns can't fly!")
End Function
End Class
I don't believe that what you want to accomplish is possible the way you are trying... As I recall when you inherit an Interface any class that implements your derived Interface is actually being told that it must implement both Interfaces rather allowing the options you have in a full class.
So effectively what you have in myClass is:
Namespace ns3
Public Class myClass
Implements ns2.IDerived
Implements ns1.IParent
Public Function myFunction(ByRef obj as Object) As Double Implements ns2.IDerived.myFunction
End Function
End ns3
So inheriting an interface is really just a way to enforce that a class implementing the derived interface must also implement the base interface.
I have a parent class that is also a factory. For example:
Public Class Factory
Public Function clone() as Factory
' Some logic here
' return something
End Function
Public Function all() as List (Of Factory)
' Some logic here
' return something
End Function
End Class
And then an inherited one
Public Class BookFactory
inherits Factory
End Class
I can use inflection in the Factory class to generate the proper extended objects when called by the inherited one. myBookFactory.clone() will then return a BookFactory instance and not only a Factory instance.
The problem: this BookFactory instance will be cast as Factory, since the type of the function is Factory and not BookFactory.
I'd like to do something like
Public Class Factory
Public Function clone() as Me.GetType()
' Some logic here
' return something
End Function
Public Function all() as List (Of Me.GetType())
' Some logic here
' return something
End Function
End Class
So the returned value would be correctly cast and avoid having to do this each time:
Dim myBookFactory2 = DirectCast(myBookFactory1.clone(), myBookFactory1.getType())
How can I do this?
This seems to be a variation on asking for covariant return types. As you have noticed, this is not supported by VB.NET (or C# for that matter). Typically this is asked in the context of overriding virtual methods, where it is still not allowed. There are several alternatives, each with their own pros and cons.
Use a generic template argument to specify the derived class
This is similar to the way IComparable<T> is most commonly implemented.
Public Class Factory(Of T As Factory)
Public Function Clone() As T
'use GetType(T) to determine derived type
End Function
End Class
Public Class BookFactory
Inherits Factory(Of BookFactory)
End Class
Additionally, if you can add a New constraint to the Factory (eg: Factory(Of T {New, Factory(Of T)})) base class, you may be able to avoid using reflection.
However, this does not prevent the accidental (or potentially malicious) mistake of declaring a class like this:
Public Class EvilFactory
Inherits Factory(Of BookFactory)
'hmmm, now clone will be making the wrong type
End Class
Also, this approach makes it impossible to create a list of factories of different types without resorting to another base class below Factory(Of T) or declaring the list as being of object.
Make new methods on the derived classes that return the specific type you want.
Public Class Factory
Public Function Clone() As Factory
'create derived class, but return as base
End Function
End Class
Public Class BookFactory
Inherits Factory
Public Function CloneBooks() As BookFactory
Return CType(Me.Clone(), BookFactory)
End Function
End Class
This allows you to hide the cast for those times when you know you have a BookFactory and want to get another BookFactory. It also lets you treat all factory types polymorphically in the normal inheritance sense. However, if you have an object typed as Factory, you will still get back an object type as Factory.
Reconsider the inheritance relationship
Depending on how these classes are used, it may not make sense to use the inheritance relationship here. If you are only concerned with not retyping code, you may want to look into code generation instead.
You could potentially use generics to make the problem easier, but it won't remove the requirement to cast at some point. For example:
Public Class Factory(Of T)
Public Function clone() As Factory(Of T)
' Some logic here
' return something
End Function
Public Function all() As Collections.Generic.List(Of T)
' Some logic here
' return something
End Function
End Class
Public Class BookFactory
Inherits Factory(Of Book)
End Class
I have a class which I have marked as MustInherit (called BasePage), with a generic method that is marked as MustOverride:
Protected MustOverride Function SaveData(Of T As {BaseClass})(ByVal item As T) As T
What I want to do is force the users of this method to only supply a type of BaseClass, or anything derived from it. Also, when a class derives from BasePage, it should work on only one derived class from BaseClass:
Protected Overrides Function SaveData(Of T As BaseClass)(ByVal item As T) As T
Dim grad As DerivedClass = CType(item, DerivedClass)
Return grad
End Function
However, when I try to do the cast, it flags up the following error:
Value of type 'T' cannot be converted to 'DerivedClass'.
All the documentation I have read suggests that this should work. However, it's not a big problem if it doesn't work, as I can work around by making a non-generic method that only accepts BaseClass.
Any ideas?
All the documentation I have read suggests that this should work.
On the contrary: it can’t work. The type T derives from BaseClass – but nothing in your code tells the compiler that it is convertible to DerivedClass. For example, it could be of type IndependentlyDerivedClass which is a sibling of DerivedClass.
However, the following cast works:
Dim grad As DerivedClass = DirectCast(DirectCast(item, BaseClass), DerivedClass))
Notice that I’m using DirectCast in place of CType. This is a best-practice when casting in class hierarchies since DirectCast only allows such casts (these, and boxing/unboxing conversions) so you minimize the risk of accidentally calling a conversion operator (which can happen when you’re using CType on non-related types).
You typically do something like this
Public MustInherit Class BasePage(Of T As BaseClass)
Public MustOverride Function Savedata(ByVal Item As T) As T
End Class
Public Class derivedPage
Inherits BasePage(Of DerivedClass)
Public Overrides Function Savedata(ByVal Item As DerivedClass) As DerivedClass
Dim grad As DerivedClass = Item
Return grad
End Function
End Class
Public MustInherit Class BaseClass
End Class
Public Class DerivedClass
Inherits BaseClass
End Class