Casting interfaces and MEF - vb.net

Question

I have the following problem with MEF:
Interface definition to be used by host:
Public Interface IExecuteDoSomething
Inherits IAddinSettings
Event DataReceived As EventHandler(Of DataReceivedEventArgs)
Function DoSomething() As Boolean
End Interface
Public Class DataReceivedEventArgs
Inherits EventArgs
Public Sub New(ByVal message As String)
Me.Message = message
End Sub
Public Message As String
End Class
extra interface needed by some other code inside the host:
Public Interface IAddinSettings
ReadOnly Property Setting() As AddinSettings
End Interface
Public Class AddinSettings
Private _Name As String
Public Property Name() As String
Get
Return _Name
End Get
Set(ByVal value As String)
_Name = value
End Set
End Property
Public Sub New(ByVal name As String)
Me.Name = name
End Sub
End Class
The class that provides the export:
<Export(GetType(SharedLibrary.IExecuteDoSomething))> Public Class Class1
Implements SharedLibrary.IExecuteDoSomething
Implements SharedLibrary.IAddinSettings
Private _Addinsettings As New SharedLibrary.Addinsettings("Test")
Public Function DoSomething() As Boolean Implements SharedLibrary.IExecuteDoSomething.DoSomething
MsgBox("i did something")
Return True
End Function
Public Event DataReceived(ByVal sender As Object, ByVal e As SharedLibrary.DataReceivedEventArgs) Implements SharedLibrary.IExecuteDoSomething.DataReceived
Public ReadOnly Property Setting() As SharedLibrary.AddinSettings Implements SharedLibrary.IAddinSettings.Setting
Get
Return _Addinsettings
End Get
End Property
End Class
The host:
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim catalog As New Hosting.AggregateCatalog
Dim d As New Hosting.DirectoryCatalog("..path to dlll..")
catalog.Catalogs.Add(d)
Dim container = New Hosting.CompositionContainer(catalog)
Dim batch As New Hosting.CompositionBatch
batch.AddPart(Me)
container.Compose(batch)
For Each dd In dos
AddHandler dd.DataReceived, AddressOf testevent
Next
End Sub
<Import()> Public dos As IEnumerable(Of SharedLibrary.IExecuteDoSomething)
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
For Each d In dos
d.DoSomething()
Next
End Sub
Private Sub testevent(ByVal sender As Object, ByVal e As SharedLibrary.DataReceivedEventArgs)
MsgBox("Event received: " & e.Message)
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
Dosomethingelse(DirectCast(dos, System.Collections.Generic.List(Of SharedLibrary.IAddinSettings)))
End Sub
Private Sub Dosomethingelse(byval settings as IEnumerable(Of SharedLibrary.IAddinSettings))
End Sub
End Class
Everything seems to work fine until the Button2_Click routine is executed, then an InvalidCastException is thrown with the info:
Unable to cast object of type 'System.Collections.Generic.List1[SharedLibrary.IExecuteDoSomething]' to type 'System.Collections.Generic.List1[SharedLibrary.IAddinSettings]'.
How can i solve this problem, because the imported object implements both of the interfaces?

Answer 1

I suspect you're actually running into a covariance issue - that's the typical cause of problems like this. A List<IFoo> is not a List<IBar> even if IBar extends IFoo.
If you're using .NET 3.5, the easiest way to get round this in your case is to remove the DirectCast and instead use Enumerable.Cast:
Dosomethingelse(dos.Cast(Of SharedLibrary.IAddinSettings))