如何：从多个源填充对象集合 (LINQ) (Visual Basic)
本示例演示如何将来自不同源的数据合并到一系列新的类型。
注意
请勿尝试将内存中数据或文件系统中的数据与仍在数据库中的数据进行联接。 这种跨域联接可能产生未定义的结果，因为可能为数据库查询和其他类型的源定义了联接操作的不同方式。 此外，如果数据库中的数据量足够大，这样的操作还存在可能导致内存不足的异常的风险。 若要将数据库中的数据联接到内存数据，首先对数据库查询调用 ToList 或 ToArray，然后对返回的集合执行联接。
创建数据文件
- 按照如何：联接不同文件中的内容 (LINQ) (Visual Basic) 中的说明，将 names.csv 和 scores.csv 文件复制到项目文件夹中。
示例
下面的示例演示如何使用命名类型 Student 存储来自两个内存字符串集合（模拟 .csv 格式的电子表格数据）的合并数据。 第一个字符串集合代表学生姓名和 ID，第二个集合代表学生 ID（在第一列）和四次考试分数。 此 ID 用作外键。
Imports System.Collections.Generic
Imports System.Linq
Class Student
Public FirstName As String
Public LastName As String
Public ID As Integer
Public ExamScores As List(Of Integer)
End Class
Class PopulateCollection
Shared Sub Main()
' Merge content from spreadsheets into a list of Student objects.
' These data files are defined in How to: Join Content from
' Dissimilar Files (LINQ).
' Each line of names.csv consists of a last name, a first name, and an
' ID number, separated by commas. For example, Omelchenko,Svetlana,111
Dim names As String() = System.IO.File.ReadAllLines("../../../names.csv")
' Each line of scores.csv consists of an ID number and four test
' scores, separated by commas. For example, 111, 97, 92, 81, 60
Dim scores As String() = System.IO.File.ReadAllLines("../../../scores.csv")
' The following query merges the content of two dissimilar spreadsheets
' based on common ID values.
' Multiple From clauses are used instead of a Join clause
' in order to store the results of scoreLine.Split.
' Note the dynamic creation of a list of integers for the
' ExamScores member. The first item is skipped in the split string
' because it is the student ID, not an exam score.
Dim queryNamesScores = From nameLine In names
Let splitName = nameLine.Split(New Char() {","})
From scoreLine In scores
Let splitScoreLine = scoreLine.Split(New Char() {","})
Where Convert.ToInt32(splitName(2)) = Convert.ToInt32(splitScoreLine(0))
Select New Student() With {
.FirstName = splitName(1), .LastName = splitName(0), .ID = splitName(2),
.ExamScores = (From scoreAsText In splitScoreLine Skip 1
Select Convert.ToInt32(scoreAsText)).ToList()}
' Optional. Store the query results for faster access in future
' queries. This could be useful with very large data files.
Dim students As List(Of Student) = queryNamesScores.ToList()
' Display each student's name and exam score average.
For Each s In students
Console.WriteLine("The average score of " & s.FirstName & " " &
s.LastName & " is " & s.ExamScores.Average())
Next
' Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.")
Console.ReadKey()
End Sub
End Class
' Output:
' The average score of Svetlana Omelchenko is 82.5
' The average score of Claire O'Donnell is 72.25
' The average score of Sven Mortensen is 84.5
' The average score of Cesar Garcia is 88.25
' The average score of Debra Garcia is 67
' The average score of Fadi Fakhouri is 92.25
' The average score of Hanying Feng is 88
' The average score of Hugo Garcia is 85.75
' The average score of Lance Tucker is 81.75
' The average score of Terry Adams is 85.25
' The average score of Eugene Zabokritski is 83
' The average score of Michael Tucker is 92
在 Select 子句中，对象初始值设定项使用来自两个源的数据来实例化每个新的 Student 对象。
如果不需要存储查询的结果，那么和命名类型相比，匿名类型使用起来更方便。 如果在执行查询的方法外部传递查询结果，则需要使用命名类型。 下面的示例执行与前面的示例相同的任务，但使用的是匿名类型，而不是命名类型：
' Merge the data by using an anonymous type.
' Note the dynamic creation of a list of integers for the
' ExamScores member. We skip 1 because the first string
' in the array is the student ID, not an exam score.
Dim queryNamesScores2 =
From nameLine In names
Let splitName = nameLine.Split(New Char() {","})
From scoreLine In scores
Let splitScoreLine = scoreLine.Split(New Char() {","})
Where Convert.ToInt32(splitName(2)) = Convert.ToInt32(splitScoreLine(0))
Select New With
{.Last = splitName(0),
.First = splitName(1),
.ExamScores = (From scoreAsText In splitScoreLine Skip 1
Select Convert.ToInt32(scoreAsText)).ToList()}
' Display each student's name and exam score average.
For Each s In queryNamesScores2
Console.WriteLine("The average score of " & s.First & " " &
s.Last & " is " & s.ExamScores.Average())
Next
