LINQ 中两个 Lambda 表达式的区别答案

【问题标题】：Difference between two Lamba Expressions in LINQLINQ 中两个 Lambda 表达式的区别
【发布时间】：2019-08-24 13:19:45
【问题描述】：

我正在尝试了解 LINQ Join 方法中这两个 Lambda 表达式之间的区别。

我看不懂的两行是Function(aaa) ...和Function(bbb) ...开头的那几行。

为什么我在第二个示例中为aaa 和bbb明确命名了字段Name，而在第一个示例中没有它也可以工作？

第一个示例（来自更改了变量名的 .Net Framework 文档）：

Structure Person
    Public Name As String
    Public SecondName As String
End Structure

Structure Pet
    Public Name As String
    Public Owner As Person
End Structure

Dim magnus As New Person With {.Name = "Hedlund, Magnus"}
Dim terry As New Person With {.Name = "Adams, Terry"}
Dim charlotte As New Person With {.Name = "Weiss, Charlotte"}

Dim barley As New Pet With {.Name = "Barley", .Owner = terry}
Dim boots As New Pet With {.Name = "Boots", .Owner = terry}
Dim whiskers As New Pet With {.Name = "Whiskers", .Owner = charlotte}
Dim daisy As New Pet With {.Name = "Daisy", .Owner = magnus}

Dim people As New List(Of Person)(New Person() {magnus, terry, charlotte})
Dim pets As New List(Of Pet)(New Pet() {barley, boots, whiskers, daisy})

Dim query = people.Join(pets,
    Function(aaa) aaa,
    Function(bbb) bbb.Owner,
    Function(ccc, ddd) _
        New With {.OwnerName1 = ccc.Name, .Pet1 = ddd.Name})

第二个例子（我的代码）

Structure MyObject
    Public Name As String
    Public Value As Integer
End Structure

Dim Test1 As New List(Of MyObject) From {
    New MyObject With {.Name = "a", .Value = 1},
    New MyObject With {.Name = "b", .Value = 2},
    New MyObject With {.Name = "c", .Value = 3}
    }

Dim Test2 As New List(Of MyObject) From {
    New MyObject With {.Name = "a", .Value = 11},
    New MyObject With {.Name = "b", .Value = 22},
    New MyObject With {.Name = "c", .Value = 33}
    }

Dim Joined = Test1.Join(Test2,
   Function(aaa) aaa.Name,
   Function(bbb) bbb.Name,
   Function(ccc, ddd) New With {
       .Name1 = ccc.Name,
       .Value1 = ccc.Value,
       .Value2 = ddd.Value})

【问题讨论】：

标签： vb.net linq lambda

【解决方案1】：

如果您查看documentation，您会发现第一个函数是外部序列的选择器，第二个函数是内部序列的选择器。

内部和外部选择器的值必须具有相同的类型 (TKey)，并且将使用该类型的默认比较器进行比较（有一个 overload 允许您传递比较器）。

第三个函数是结果选择器，它接收一个TOuter 和一个TInner 实例，根据所选键匹配并返回一个TResult 值。

所以，在第一个示例中，TKey 是 Person。 outerSelector 和 innerSelector 必须返回 Person，因此 aaa 和 bbb。

在第二个示例中，您选择比较 Name 字段而不是整个 MyObject 结构。

【讨论】：