Question

下面的代码从Access 2010数据库中提取了一堆记录;因此滚动我自己的连接器位。我已经成功地完成了observablecollection并使它与我自己的对象中的拖放数据源完美结合。然而，就像一个愚蠢的人，我想异步这样做。然而，我有一个小的投射怪物问题，我不知道该喂它！任何人都可以告诉我 - 我已经尝试了很多阅读，但是周五下午的概念只是有点太多了，我正在努力取得任何真正的进展。

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WPF列表框，但只能通过注释掉ObservableCollectionEx类的一部分。这会导致同步问题，并在输入几百条记录后崩溃。

构建实体的线程列表的类 - 在本例中是一个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 - 从另一个在线源代码翻译而来。

'我使用PostSharp来尝试捕捉东西。 ` 公共类ObservableCollectionEx（Of T）继承ObservableCollection（Of T） '覆盖事件，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

从我所看到的，你有两个问题。

您将局部变量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。
您正在使用CType将目标投射到DispatcherObject，如果目标不是DispatcherObject，则该目标无效。请改用TryCast：
```
Dim dispatcherObject As DispatcherObject = TryCast(handler.Target, DispatcherObject)
```

您还可以使用IsNot：

If dispatcherObject IsNot Nothing AndAlso Not dispatcherObject.CheckAccess Then

Answer 2

警告 - 以下代码与C＃版本的行为不同。关键的区别似乎是在VB中你不能覆盖一个事件（为什么不在地球上？）但是在C＃中你可以。

结果是Handler在VB中没有任何东西但在C＃中没有:(。

因此语法构建没有错误，但VB版本不会做任何事情。

使用VB中的更新答案重做。谢谢！

注意我还无法使用Entity Framework。但我认为这是我和EF的问题，而不是集合。

代码本身适用于任何有兴趣的人。我的清单现在填充完好。但是，我会用一小撮盐来接受我的这个答案，直到我更新说我如何进行广泛测试：）

然而，预兆是好的 - 这是最初的C＃作者的网站：Original Site

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

结束班

DispatcherObject在WPF中抛出了问题和Async / ObservableCollection问题

2 个答案: