在LINQ中使用匿名类型进行区分（在VB.NET中）

假设下面引用的列表包含2个元素：

Dim Countries = From c In List _
                Select New With { .Country = c.Country, .CountryID = c.CountryID }

上面的代码返回

.Country=Spain .CountryID = 1
.Country=Spain .CountryID = 1

如何获得不同的值？“国家/地区”查询应仅包含

.Country=Spain .CountryID = 1

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有一个名为Distinct的LINQ操作符，您可以这样调用它：

 Dim Countries = (From c In List _
             Select c.Country, c.CountryID).Distinct()

有关Distinct的更多信息

当我在调试器的最后一行停止时，这对我有效：

Imports System.Text

<TestClass()>
Public Class UnitTest1

    Class Test
        Public Country As String
        Public CountryID As Integer
    End Class

    <TestMethod()>
    Public Sub TestMethod1()

        Dim List(1) As Test
        List(0) = New Test With {.Country = "Spain", .CountryID = 1}
        List(1) = New Test With {.Country = "Spain", .CountryID = 1}

        Dim Countries = From c In List Select c.Country, c.CountryID

        Dim distinctCountries = Countries.Distinct()
    End Sub

End Class

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我只能假设你死定了要使用匿名类型，因为Alex Peck给出的答案是正确的。我已经投了更高的票

然而，这归结为VB.NET与C编译器的讨论

在VB.NET中，当遇到匿名类型时，只有声明为键属性的属性才能用于比较。因此，在没有键的VB.NET中，当您尝试进行不同的比较时，不会发生任何事情

首先，为了回答您的问题，这适用于匿名类型：

Dim Countries = From c In List Select New With {Key c.CountryId, c.Country} Distinct.ToList

这就是为什么freedompeace的答案不太管用

但是编译器有点不同

当遇到匿名类型并且需要进行比较操作时，c编译器将重写Equals和GetHashCode。它将迭代匿名类型的所有公共属性，以计算对象的哈希代码来测试是否相等

---

希望这能回答您的问题。

Distinct必须知道哪些对象是相同的。在这里选择匿名对象，它不知道哪些是相等的。我从来没有写过一行VB.Net，但我尝试了一些东西，它很有效：

Module Module1

    Sub Main()
        Dim countries As List(Of Country) = New List(Of Country)
        Dim spain1 As Country = New Country()
        Dim spain2 As Country = New Country()
        Dim spain3 As Country = New Country()
        Dim hungary As Country = New Country()
        spain1.ID = 1
        spain1.Name = "Spain"
        spain2.ID = 1
        spain2.Name = "Spain"
        spain3.ID = 2
        spain3.Name = "Spain"
        hungary.ID = 3
        hungary.Name = "Hungary"
        countries.Add(spain1)
        countries.Add(spain2)
        countries.Add(spain3)
        countries.Add(hungary)

        Dim ctr = From c In countries Select c.Name, c.ID
                  Distinct

        For Each c In ctr
            Console.WriteLine(c)
        Next
    End Sub

    Public Class Country

        Protected _name As String

        Protected _id As Long

        Public Property Name() As String
            Get
                Return _name
            End Get
            Set(ByVal value As String)
                _name = value
            End Set
        End Property

        Public Property ID() As Long
            Get
                Return _id
            End Get
            Set(ByVal value As Long)
                _id = value
            End Set
        End Property

    End Class
End Module

就你而言：

Dim Countries = From c In List 
                Select c.Country, c.CountryID Distinct

---

这原来是一个有趣的问题，乔科尔8。+1有趣的是，我更精通C语言，并且没有意识到这里的差异。@Khepri这太棒了！谢谢来自的重要说明：>由查询创建的匿名类型中的属性始终是键属性。另见。

Module Module1

    Sub Main()
        Dim countries As List(Of Country) = New List(Of Country)
        Dim spain1 As Country = New Country()
        Dim spain2 As Country = New Country()
        Dim spain3 As Country = New Country()
        Dim hungary As Country = New Country()
        spain1.ID = 1
        spain1.Name = "Spain"
        spain2.ID = 1
        spain2.Name = "Spain"
        spain3.ID = 2
        spain3.Name = "Spain"
        hungary.ID = 3
        hungary.Name = "Hungary"
        countries.Add(spain1)
        countries.Add(spain2)
        countries.Add(spain3)
        countries.Add(hungary)

        Dim ctr = From c In countries Select c.Name, c.ID
                  Distinct

        For Each c In ctr
            Console.WriteLine(c)
        Next
    End Sub

    Public Class Country

        Protected _name As String

        Protected _id As Long

        Public Property Name() As String
            Get
                Return _name
            End Get
            Set(ByVal value As String)
                _name = value
            End Set
        End Property

        Public Property ID() As Long
            Get
                Return _id
            End Get
            Set(ByVal value As Long)
                _id = value
            End Set
        End Property

    End Class
End Module

Dim Countries = From c In List 
                Select c.Country, c.CountryID Distinct