Question

我一直在尝试使用LINQ对我在下面定义的自定义类Aggregate_Table的IQueryable上执行求和。我不知道如何实现这一点，因为通常的sum函数在应用于我的自定义类的IQueryable（Of T）时会给出语法错误。

我开始使用以下查询：

Dim initial_SIs = From si In database._58k_SIs
                      Group By si.From_Firm, si.From_Account_Number, si.To_Account_Number, si.To_Firm, si.Security_Code, si.Settlement_Ccy
                   Into Quantity = Sum(si.Quantity), Settlement_Amount = Sum(si.Settlement_Amount)

这是来自datacontext对象数据库中的表_58k_SIs的简单查询。由于此查询结果的类型是匿名数据类型，并且我需要创建一个接受特定表类型的函数，因此我继续创建一个类名称Aggregate_SI，其属性与上面的查询结果相同。（每个这个类的属性有自己的相应属性，为简洁起见，我省略了它。

 Public Class Aggregated_SI
   Private _From_Firm As String
   Private _From_Account_Number As String
   Private _To_Account_Number As String
   Private _To_Firm As String
   Private _Security_Code As String
   Private _Quantity As Integer
   Private _Settlement_Amount As Decimal
   Private _Settlement_Ccy As String

   Public Property Quantity
    Get
        Return _Quantity
    End Get
    Set(value)
        _Quantity = value
    End Set
    End Property
 End Class

我创建了一个Aggregated_SI对象列表，并将所有查询结果传输到该列表，因为list（Of T）对象可用于LINQ查询。

Dim test_List = New List(Of Aggregated_SI)
    For Each si In initial_SIs
        test_List.Add(New Aggregated_SI(si.From_Firm, si.From_Account_Number, si.To_Account_Number, si.To_Firm, si.Security_Code, si.Quantity, si.Settlement_Amount, si.Settlement_Ccy))
    Next

聚合：

Dim outflow = From si In test_List
                  Group By si.From_Account_Number, si.Security_Code
                  Into Total_outflow = Sum(si.Quantity)

给我一条错误消息

 Error  BC36594 Definition of method 'Sum' is not accessible in this context.   

        Error   BC30519 Overload resolution failed because no accessible 'Sum' can be called without a narrowing conversion:
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Integer)) As Integer' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Integer'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Integer?)) As Integer?' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Integer?'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Long)) As Long' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Long'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Long?)) As Long?' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Long?'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Single)) As Single' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Single'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Single?)) As Single?' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Single?'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Double)) As Double' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Double'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Double?)) As Double?' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Double?'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Decimal)) As Decimal' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Decimal'.
Extension method 'Public Function Sum(selector As Func(Of Aggregated_SI, Decimal?)) As Decimal?' defined in 'Enumerable': Return type of nested function matching parameter 'selector' narrows from 'Object' to 'Decimal?'. Settlement_Optimisation C:\Users\chris\AppData\Local\Temporary Projects\WindowsApplication1\Form1.vb    22  Active

我一直试图解决这个问题几个小时而无济于事。任何人都有任何建议我如何解决这个问题？

Answer 1

您已将数量属性声明为Public Property Quantity而非Public Property Quantity As Integer。这会导致Quantity将结果强制转换为对象，并且没有可以汇总对象的Sum（）扩展名。使用正确的返回类型，它应该工作：

Public Class Aggregated_SI
   Private _From_Firm As String
   Private _From_Account_Number As String
   Private _To_Account_Number As String
   Private _To_Firm As String
   Private _Security_Code As String
   Private _Quantity As Integer
   Private _Settlement_Amount As Decimal
   Private _Settlement_Ccy As String

   Public Property Quantity As Integer
    Get
        Return _Quantity
    End Get
    Set(value As Integer)
        _Quantity = value
    End Set
    End Property
 End Class

PS：自从我编写VB.Net以来已经很久了，我不确定在setter的值参数中包含类型，但如果编译器抱怨

在自定义类IQueryable（Of T）上执行总和

1 个答案: