WPF中DispatcherObject的问题和Async / ObservableCollection问题

Question

下面的代码从Access 2010数据库中拉出一堆记录。 因此滚动我自己的连接器位。 我已经成功地进行了observablecollection，并通过我自己的对象中的漂亮的拖放数据源将其绑定在一起。 但是，我想像一个愚蠢的人那样异步地执行此操作。 但是，我遇到了一个小型怪物问题，而且我不知道该怎么喂！ 谁能告诉我-我已经尝试了很多阅读，但是每个星期五一次一次的概念太多了，我正在努力取得任何进展。

我遇到的问题是： Dim dispatcherObject As DispatcherObject = CType (handler.Target, DispatcherObject )

例外是： Unable to cast object of type '_Closure$__2[SomeRecord_Viewer.SomeRecord]' to type 'System.Windows.Threading.DispatcherObject'.

我设法通过下面的代码填充了WPF列表框，但是只能注释掉ObservableCollectionEx类的一部分。 输入数百条记录后，这将导致同步问题和崩溃。

用于构建实体的线程列表的类-在这种情况下，为ObservableCollectionEx（Of SomeRecord）：

Class SomeRecordSet
Inherits ObservableCollectionEx( Of  SomeRecord)

Private Shared Property _SomeRecordList As New ObservableCollectionEx(Of  SomeRecord )
Public Shared ReadOnly Property SomeRecordList As ObservableCollectionEx(Of  SomeRecord )
    Get
        If _SomeRecordList.Count = 0 Then BuildSomeRecordListAsync()
        Return _SomeRecordList
    End Get
End Property

Public Shared ReadOnly Property ReturnSingleSomeRecord(id As Integer) As SomeRecord
    Get
        Return ( From SomeRecord In _SomeRecordList Where SomeRecord.id = id Select        SomeRecord).First()
    End Get
End Property

Private Shared Async Sub BuildSomeRecordListAsync()
    Await Task.Run( Sub() BuildSomeRecordList())
    Return
End Sub

Private Shared Sub BuildSomeRecordList()
    Db.newcmd( "Select * from  RecordList ")
    While Db.read
        Dim SomeRecord As New SomeRecord
        With SomeRecord
            .id = Db.dbint( "ID")
            .type = Db.dbin( "type")
         End With
        _SomeRecordList.Add(SomeRecord)
    End While
End Sub`

SomeRecord类的部分代码：

Class SomeRecord
Implements INotifyPropertyChanged
Public Event PropertyChanged As PropertyChangedEventHandler Implements                   INotifyPropertyChanged.PropertyChanged
Private Sub NotifyPropertyChanged( ByVal info As String)
    RaiseEvent PropertyChanged(Me , New PropertyChangedEventArgs (info))
End Sub

...'lots of simple properties.
End Class

线程集合类代码-从另一个在线来源翻译而来。

``我使用PostSharp尝试捕捉东西。 `Public Class ObservableCollectionEx（Of T）继承ObservableCollection（Of T）'重写事件，以便此类可以访问它。Public Shadows Event CollectionChanged as System.Collections.Specialized.NotifyCollectionChangedEventHandler

Protected Overrides Sub OnCollectionChanged( ByVal e As System.Collections.Specialized.NotifyCollectionChangedEventArgs )

    Using BlockReentrancy()

        Dim eventHandler As System.Collections.Specialized.NotifyCollectionChangedEventHandler = Sub () RaiseEvent CollectionChanged(Me , e)
        If (eventHandler Is Nothing) Then Return

        Dim delegates() As [Delegate] = eventHandler.GetInvocationList
*******If I comment this out I can populate the Listbox via a CollectionView, however it dies with issues to do with the list not staying synchronised :).
        'Walk thru invocation list 
        For Each handler As System.Collections.Specialized.NotifyCollectionChangedEventHandler     In delegates
            Dim dispatcherObject As DispatcherObject = CType (handler.Target, DispatcherObject)
            ' If the subscriber is a DispatcherObject and different thread
            If (( Not (dispatcherObject) Is Nothing) AndAlso (dispatcherObject.CheckAccess = False )) Then
                ' Invoke handler in the target dispatcher's thread
                dispatcherObject.Dispatcher.Invoke(DispatcherPriority .DataBind, handler, Me, e)
            Else
                handler( Me, e)
            End If
        Next

*******End of stuff I comment out to get working partially***

    End Using
End Sub
End Class

Answer 1

据我所知，您有两个问题。

1. 您正在将局部变量eventHandler分配给匿名方法，而不是实际的事件处理程序。 它应该是：

    Dim eventHandler As NotifyCollectionChangedEventHandler = CollectionChangedEvent 

   注意：您需要在VB中使用CollectionChangedEvent ，而不是CollectionChanged 。

2. 您正在使用CType到目标转换为DispatcherObject ，如果目标不是将不起作用DispatcherObject 。 改用TryCast ：

    Dim dispatcherObject As DispatcherObject = TryCast(handler.Target, DispatcherObject) 

您还可以使用IsNot整理下一行的测试：

If dispatcherObject IsNot Nothing AndAlso Not dispatcherObject.CheckAccess Then

Answer 2

警告-以下代码的行为与C＃版本不同。 关键的区别似乎是，在VB中，您不能重写事件（为什么不是呢？），而在C＃中，您可以重写。

结果是Handler在VB中什么都没有，但在C＃中却没有:(。

因此，语法的构建没有错误，但是VB版本从来没有做任何事情。

在VB中重做更新的答案。 谢谢！

注意我还不能使用实体框架。 但是我认为这是我和EF的问题，而不是收藏。

该代码本身适用于任何有兴趣的人。 我的列表现在填充得很好。 但是，我会用一小撮盐来回答我的问题，直到我更新说我如何进行了广泛测试：)

但是，预兆很好-这是原始C＃作者的网站： 原始网站

Public Class ObservableCollectionEx(Of T)
Inherits ObservableCollection(Of T)



'Override the event so this class can access it
Public Shadows Event CollectionChanged As NotifyCollectionChangedEventHandler

Protected Overrides Sub OnCollectionChanged(ByVal e As NotifyCollectionChangedEventArgs)
    Using BlockReentrancy()
        Dim eventHandler As System.Collections.Specialized.NotifyCollectionChangedEventHandler = CollectionChangedEvent
        If eventHandler Is Nothing Then
            Return
        End If


        Dim delegates() As [Delegate] = CollectionChangedEvent.GetInvocationList
        'Walk thru invocation list
        For Each handler As NotifyCollectionChangedEventHandler In delegates
            Dim dispatcherObject As DispatcherObject = TryCast(handler.Target, DispatcherObject)
            ' If the subscriber is a DispatcherObject and different thread
            If dispatcherObject IsNot Nothing AndAlso Not dispatcherObject.CheckAccess Then
                ' Invoke handler in the target dispatcher's thread
                dispatcherObject.Dispatcher.Invoke(DispatcherPriority.DataBind, handler, Me, e)
            Else
                handler(Me, e)
            End If
        Next
    End Using
End Sub

末级