VB.Net Linq組InvalidCastException由

Question

我有一個Linq組通過查詢的工作。這裏的查詢：

 Dim query = From fb As Feedback In lst Where fb.Seller.login_name.ToLower = UserName.ToLower 
        Order By fb.transaction_id Descending, fb.creation Descending _ 
        Group fb By fb.transaction_id _ 
        Into Group 

我不喜歡用匿名類型工作，所以我想delare結果和我打的InvalidCastException的。

這裏的類型信息：

Debug.Print(query.GetType.ToString) 

返回：

System.Linq.GroupedEnumerable`4[Feedback,System.Nullable`1[System.Int32],Feedback,VB$AnonymousType_0`2[System.Nullable`1[System.Int32],System.Collections.Generic.IEnumerable`1[Feedback]]] 

和內型：

Debug.Print(item.GetType.ToString) 

返回：

VB$AnonymousType_0`2[System.Nullable`1[System.Int32],System.Collections.Generic.IEnumerable`1[Feedback]] 

所以，有了這些信息，這裏的二手聲明：

Dim query As IEnumerable(Of IGrouping(Of Int32?, IEnumerable(Of Feedback))) 

這裏的錯誤返回：

Unable to cast object of type 'System.Linq.GroupedEnumerable`4[Feedback,System.Nullable`1[System.Int32],Feedback,VB$AnonymousType_0`2[System.Nullable`1[System.Int32],System.Collections.Generic.IEnumerable`1[Feedback]]]' to type 'System.Collections.Generic.IEnumerable`1[System.Linq.IGrouping`2[System.Nullable`1[System.Int32],System.Collections.Generic.IEnumerable`1[Feedback]]]'. 

醜陋的解決方案是定義一個對象，並通過結果步驟如下：

Class FeedbackDataItem 

    Sub New(ByVal transaction_id As Integer) 
     _transaction_id = transaction_id 
     _feedbacks = New Feedbacks(Of Feedback) 
    End Sub 

    Private _transaction_id As Integer 
    Public Property transaction_id() As Integer 
     Get 
      Return _transaction_id 
     End Get 
     Set(ByVal value As Integer) 
      _transaction_id = value 
     End Set 
    End Property 

    Private _feedbacks As Feedbacks(Of Feedback) 
    Public Property Feedbacks() As Feedbacks(Of Feedback) 
     Get 
      Return _feedbacks 
     End Get 
     Set(ByVal value As Feedbacks(Of Feedback)) 
      _feedbacks = value 
     End Set 
    End Property 

End Class 

並加載了集合：

Dim FeedbackData As New List(Of FeedbackDataItem) 

    For Each item In query 
     Dim fbi As New FeedbackDataItem(item.transaction_id) 
     fbi.Feedbacks.AddRange(item.Group) 
     FeedbackData.Add(fbi) 
    Next 

我很難理解爲什麼我得到這個錯誤。只是定義結果並反思它們而不是人爲處理數據會更好。我錯過了什麼嗎？

Answer 1

查詢的結果是IEnumerable（匿名類型）。如果你不希望它是匿名的，你需要強烈地輸入它。

一種方式是通過使用擴展方法語法組：

Dim feedbacks As IEnumerable(Of Feedback) = 
     From fb As Feedback In lst Where fb.Seller.login_name.ToLower = username.ToLower 
     Order By fb.transaction_id Descending, fb.creation Descending 

    Dim grouped As IEnumerable(Of IGrouping(Of Integer?, Feedback)) = 
     feedbacks.GroupBy(Function(fb) fb.transaction_id) 

Answer 2

在語句末尾加上一個選擇，將其轉換爲您想要的任何類型。

Dim query = From fb As Feedback In lst _ 
      Where fb.Seller.login_name.ToLower = UserName.ToLower 
      Order By fb.transaction_id Descending, fb.creation Descending _ 
      Group gr By fb.transaction_id 
      Into Group 
      Select new FeedbackDataItem with { 
       .transaction_id = gr.transaction_id, _ 
       .FeedBacks = ... 
      }