VB.NET Implement Multiple Contravariant interface types
.net, architecture, contravariance, generics, vb.net
Solution
The program runs just fine, but I want to understanding the specifics of why this is happening
The warning is there because of (Generic) contravariance with respect to Interfaces and/or classes with an pre-existing inheritance hierarchy. It doesn't specifically apply in the case you have given in your example (may in your real code), but here is why it is warning:
suppose MaterialB Inherited MaterialA and was in turn Inherited by SpecialB then
Public Class Composite
Implements ICopiesFrom(Of MaterialA)
Implements ICopiesFrom(Of MaterialB)
combined with
Public Interface ICopiesFrom(Of In TModel)
says (due to the 'In'): Composite can be an `ICopiesFrom(Of <anything Inheriting from MaterialA>)` (with one implementation) AND Composite can be an `ICopiesFrom(Of <anything Inheriting from MaterialB>)` (with second implementation)
so if I say:
Dim broken As ICopiesFrom(Of SpecialB) = New Composite()
Which implementation should it choose, both are valid (it seems natural to choose B, but it is ambiguous)
the Situation if perhaps more clear with Interfaces:
Public Class Composite2
Implements ICopiesFrom(Of IMaterialA)
Implements ICopiesFrom(Of IMaterialB)
...
Public Class Broken
Implements IMaterialA
Implements IMaterialB
...
Dim broken As ICopiesFrom(Of Broken) = New Composite()
Which implementation should the compiler use now??
Also there is nothing in you example that Requires the In keyword (perhaps there may be in the real code). Unless you need to 'pass around' Composite AS a `ICopiesFrom(Of SpecialB)` for example you are gaining nothing, the `ICopiesFrom(Of MaterialB)` can cope with a SpecialB without (Generic) contravariance, via the normal (non-generic) mechanisms.
Problem
Basic Question: Given an interface: ICopiesFrom(Of In TModel) where there is no type constraint on the generic argument, can that interface be implemented more than once on the same concrete type using a different type argument without a compiler warning? Background Info: My handle on covariance and contravariance has been increasing in recent years thanks to Mr. Eric Lippert, Google, and many hours of testing / experimenting. In a project I am working on, I have a need to separate different layers of the architecture and not expose base model / entity types to a higher layer (presentation). To accomplish this, I have been creating composite classes (MVC Models) that contain aspects of potentially multiple different base layer model types. I have a separate layer that will build these composite types from the base types (service layer). One important requirement is that the base types not be passed up via a reference, so properties must be duplicated in order to create a deep-copy of the base model class. To remove some of the lengthy and ugly code from the service layer, I created an interface that defines a common contract for composite types that allows for the property values to be copied in the composite object. When I want to implement this interface multiple times however, the VB compiler generates a warning. The program runs just fine, but I want to understanding the specifics of why this is happening. Particularly, if this is a fragile or poor design decision, I want to know now before I get too deep. Environment Details: - Language: VB.Net - .NET: 4.0 - IDE: VS2010 SP1 - Usage: Website (MVC2) In attempting to figure this out, I have done some reasearch on SO and the internet but nothing really addresses my question specifically. Here are some of (but not all) the resources I have consulted: - Contravariance explained - http://msdn.microsoft.com/en-us/library/dd799517(v=vs.110).aspx - http://ericlippert.com/2013/07/29/a-contravariance-conundrum/#more-1344 Summary: Is there a better / cleaner / more flexible way to achieve what I'm trying to or do I have to live with the compiler warning? Here is a run-able example (not the actual code) that illustrates the issue: ``` Public Module Materials Sub Main() Dim materials As New List(Of Composite)() Dim materialData As New Dictionary(Of MaterialA, MaterialB)() 'Load data from a data source 'materialData = Me.DataService.Load(.....'Query parameters'.....) Dim specificMaterial As New SpecialB() With {.Weight = 24, .Height = 12} Dim specificMaterialDesc As New MaterialA() With {.Name = "Silly Putty", .Created = DateTime.UtcNow.AddDays(-1)} Dim basicMaterial As New MaterialB() With {.Weight = 34.2, .Height = 8} Dim basicMaterialDesc As New MaterialA() With {.Name = "Gak", .Created = DateTime.UtcNow.AddDays(-2)} materialData.Add(specificMaterialDesc, specificMaterial) materialData.Add(basicMaterialDesc, basicMaterial) For Each item In materialData Dim newMaterial As New Composite() newMaterial.CopyFrom(item.Key) newMaterial.CopyFrom(item.Value) materials.Add(newMaterial) Next Console.WriteLine("Total Weight: {0} lbs.", materials.Select(Function(x) x.Weight).Sum()) Console.ReadLine() End Sub End Module ''' <summary> ''' Class that represents a composite of two separate classes. ''' </summary> ''' <remarks></remarks> Public Class Composite Implements ICopiesFrom(Of MaterialA) Implements ICopiesFrom(Of MaterialB) #Region "--Constants--" Private Const COMPOSITE_PREFIX As String = "Comp_" #End Region #Region "--Instance Variables--" Private _created As DateTime Private _height As Double Private _name As String Private _weight As Double #End Region #Region "--Constructors--" ''' <summary> ''' Creates a new instance of the Composite class. ''' </summary> ''' <remarks></remarks> Public Sub New() _created = DateTime.MaxValue _height = 1D _name = String.Empty _weight = 1D End Sub #End Region #Region "--Methods--" Public Overridable Overloads Sub CopyFrom(ByVal model As MaterialA) Implements ICopiesFrom(Of MaterialA).CopyFrom If model IsNot Nothing Then Me.Name = model.Name Me.Created = model.Created End If End Sub Public Overridable Overloads Sub CopyFrom(ByVal model As MaterialB) Implements ICopiesFrom(Of MaterialB).CopyFrom If model IsNot Nothing Then Me.Height = model.Height Me.Weight = model.Weight End If End Sub #End Region #Region "--Functions--" Protected Overridable Function GetName() As String Dim returnValue As String = String.Empty If Not String.IsNullOrWhiteSpace(Me.Name) Then Return String.Concat(COMPOSITE_PREFIX, Me.Name) End If Return returnValue End Function #End Region #Region "--Properties--" Public Overridable Property Created As DateTime Get Return _created End Get Set(value As DateTime) _created = value End Set End Property Public Overridable Property Height As Double Get Return _height End Get Set(value As Double) If value > 0D Then _height = value End If End Set End Property Public Overridable Property Name As String Get Return Me.GetName() End Get Set(value As String) If Not String.IsNullOrWhiteSpace(value) Then _name = value End If End Set End Property Public Overridable Property Weight As Double Get Return _weight End Get Set(value As Double) If value > 0D Then _weight = value End If End Set End Property #End Region End Class ''' <summary> ''' Interface that exposes a contract / defines functionality of a type whose values are derived from another type. ''' </summary> ''' <typeparam name="TModel"></typeparam> ''' <remarks></remarks> Public Interface ICopiesFrom(Of In TModel) #Region "--Methods--" ''' <summary> ''' Copies a given model into the current instance. ''' </summary> ''' <param name="model"></param> ''' <remarks></remarks> Sub CopyFrom(ByVal model As TModel) #End Region End Interface Public Class MaterialA #Region "--Instance Variables--" Private _created As DateTime Private _name As String #End Region #Region "--Constructors--" ''' <summary> ''' Creates a new instance of the MaterialA class. ''' </summary> ''' <remarks></remarks> Public Sub New() _created = DateTime.MaxValue _name = String.Empty End Sub #End Region #Region "--Properties--" Public Overridable Property Created As DateTime Get Return _created End Get Set(value As DateTime) _created = value End Set End Property Public Overridable Property Name As String Get Return _name End Get Set(value As String) _name = value End Set End Property #End Region End Class Public Class MaterialB #Region "--Instance Variables--" Private _height As Double Private _weight As Double #End Region #Region "--Constructors--" ''' <summary> ''' Creates a new instance of the MaterialB class. ''' </summary> ''' <remarks></remarks> Public Sub New() _height = 0D _weight = 0D End Sub #End Region #Region "--Properties--" Public Overridable Property Height As Double Get Return _height End Get Set(value As Double) _height = value End Set End Property Public Overridable Property Weight As Double Get Return _weight End Get Set(value As Double) _weight = value End Set End Property #End Region End Class Public Class SpecialB Inherits MaterialB Public Overrides Property Weight As Double Get Return MyBase.Weight End Get Set(value As Double) MyBase.Weight = value * 2 End Set End Property End Class ```