Question

我对MEF有以下问题：

主机使用的接口定义：

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

主机内部其他代码所需的额外接口：

    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

提供导出的类：

    <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

主持人：

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

在执行Button2_Click例程之前，一切似乎都能正常工作，然后抛出InvalidCastException并显示以下信息：
无法转换类型为'System.Collections.Generic.List 1[SharedLibrary.IExecuteDoSomething]' to type 'System.Collections.Generic.List 1 [SharedLibrary.IAddinSettings]'的对象。

我如何解决这个问题，因为导入的对象实现了这两个接口？

Answer 1

我怀疑你实际上遇到了协方差问题 - 这就是这类问题的典型原因。即使List<IFoo>延长List<IBar>，IBar 也不 IFoo。

如果您使用的是.NET 3.5，在您的情况下解决此问题的最简单方法是删除DirectCast，而是使用Enumerable.Cast：

Dosomethingelse(dos.Cast(Of SharedLibrary.IAddinSettings))

铸造界面和MEF

1 个答案: