vb.net - LINQ - 选择某些列

Question

我是 LINQ 新手。请看下面的代码:

Public Class Person
    Public Name As String
    Public Address As String
    Public Age As String
End Class

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
        Try
            Dim p1 As New Person
            Dim p2 As New Person
            Dim p3 As New Person

            p1.Name = "Ian"
            p1.Age = 32
            p1.Address = "8 street"

            p2.Name = "Fred"
            p2.Age = 62
            p2.Address = "5 road"

            p3.Name = "Bernie"
            p3.Age = 59
            p3.Address = "5 road"

            Dim list As New List(Of Person)
            list.Add(p1)
            list.Add(p2)
            list.Add(p3)

            Dim test As IEnumerable(Of Person) = From p In list
                       Where p.Name = "Ian"
                       Select p.Name, p.Age
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try

    End Sub
End Class

我将如何循环执行测试？

我知道我可以做这样的事情:

Dim test As IEnumerable(Of Person) = From p In list
                       Where p.Name = "Ian"
                       Select p
            Dim List2 As List(Of Person) = test.ToList

但是，SELECT 子句的意义何在，因为基于此您只能选择完整对象。

Answer 1

如果您的结果不再包含地址，则它不再是人。

以下确实有效:

Dim test = From p In list
           Where p.Name = "Ian"
           Select New With { .Name = p.Name, .Age = p.Age }

这将创建一个新的匿名类型，其中仅包含您感兴趣的信息。test 的类型为 IEnumerable(Of SomeAnonymousTypeThatDoesNotHaveAName)。

<小时/>

一般来说，Select 子句的目的是进行投影。例如，您可以将人员投影到字符串:

Dim namesWithAges As IEnumerable(Of String) = 
    From p In list Select p.Name & " (" & p.Age & ")"

或者一些全新的对象:

Dim employees = 
    From p In list
    Select New With { .Name = p.Name, .Supervisor = DetermineSupervisor(p) }