Question

如果我使用

    Dim kun = From cus In customers
    Group By cus.Country
    Into Group

Northwind上的

（我得到IEnumerable.Collections.Generic.Ienumerable(of <anonymous Type>)）我可以执行以下循环：

    For Each kdGroup In kun
        ListBox1.Items.Add(kdGroup.Country)
        For Each kd In kdGroup.Group
            ListBox1.Items.Add("      " & kd.CompanyName)
        Next
    Next

我可以访问该成员（.Country和.CompanyName）。

但是如果我使用Extension Method（或Dot Notation）语法：

Dim kun = customers.GroupBy(Function(k) k.Country)

（我得到一个IEnumerable（Of System.Linq.IGrouping（String，... CustomerRow）））我无法执行上面显示的循环，因为我现在无法访问成员。这是一本书的例子，所以我认为它应该有效。任何人都可以向我解释一下，为什么DotNotation在这里不起作用以及这个片段有什么问题？

Answer 1

以下是展示如何访问Country和CountryName使用GroupBy

的示例

 Sub Main
  Dim empList As New List(Of Employee)()
    empList.Add(New Employee() With _
    {.ID = 1, .FName = "John", .Age = 21, .Sex = "M"c}) 
    empList.Add(New Employee() With _
    {.ID = 2, .FName = "Mary", .Age = 25, .Sex = "F"c})
    empList.Add(New Employee() With _
    {.ID = 3, .FName = "Amber", .Age = 23, .Sex = "M"c}) 
    empList.Add(New Employee() With _
    {.ID = 4, .FName = "Kathy", .Age = 25, .Sex = "F"c})
    empList.Add(New Employee() With _
    {.ID = 5, .FName = "Lena", .Age = 27, .Sex = "F"c}) 
    empList.Add(New Employee() With _
    {.ID = 6, .FName = "Bill", .Age = 28, .Sex = "M"c})
    empList.Add(New Employee() With _
    {.ID = 7, .FName = "Celina", .Age = 27, .Sex = "F"c}) 
    empList.Add(New Employee() With _
    {.ID = 8, .FName = "John", .Age = 28, .Sex = "M"c})

Dim query = empList.GroupBy(Function(x)  New with{.Age= x.Age}) 


For Each employee In query 
         Console.WriteLine(employee.Key.Age)
         For Each kd In employee
             Console.WriteLine("      " & kd.FName)
        Next
Next employee 
End Sub


Public Class Employee 
    Private privateID As Integer
    Public Property ID() As Integer 

        Get 
            Return privateID
        End Get 

        Set(ByVal value As Integer) 
            privateID = value
        End Set 

    End Property 

    Private privateFName As String 
    Public Property FName() As String
        Get 
            Return privateFName
        End Get 

        Set(ByVal value As String) 
            privateFName = value
        End Set 
    End Property  

    Private privateAge As Integer 
    Public Property Age() As Integer
        Get 
            Return privateAge
        End Get 

        Set(ByVal value As Integer) 
            privateAge = value
        End Set 
    End Property  

    Private privateSex As Char 
    Public Property Sex() As Char
        Get 
            Return privateSex
        End Get 

        Set(ByVal value As Char) 
            privateSex = value
        End Set 
    End Property
End Class

结果：

21
      John
25
      Mary
23
      Amber
25
      Kathy
27
      Lena
28
      Bill
27
      Celina
28
      John

Answer 2

你仍然可以让你的循环工作：

For Each kdGroup In kun
    ListBox1.Items.Add(kdGroup.Key)
    For Each kd In kdGroup
        ListBox1.Items.Add("      " & kd.CompanyName)
    Next
Next

查询与扩展方法 - 使用GroupBy

2 个答案: