Cara: Mengisi Kumpulan Objek dari Berbagai Sumber (LINQ) (Visual Basic)

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Contoh ini memperlihatkan cara menggabungkan data dari sumber yang berbeda ke dalam urutan jenis baru.

Catatan

Jangan mencoba menggabungkan data dalam memori atau data dalam sistem file dengan data yang masih berada dalam database. Gabungan lintas domain tersebut dapat menangguhkan hasil yang tidak terdefinisi karena berbagai cara di mana operasi gabungan mungkin ditentukan untuk kueri database dan jenis sumber lainnya. Selain itu, ada risiko bahwa operasi seperti itu dapat menyebabkan pengecualian di luar memori jika jumlah data dalam database cukup besar. Untuk menggabungkan data dari database ke data dalam memori, pertama-tama panggil ToList atau ToArray pada kueri database, lalu lakukan gabungan pada koleksi yang dikembalikan.

Untuk membuat file data

Contoh

Contoh berikut menunjukkan cara menggunakan jenis bernama Student untuk menyimpan data gabungan dari dua koleksi string dalam memori yang mensimulasikan data spreadsheet dalam format .csv. Kumpulan string pertama mewakili nama siswa dan ID, dan koleksi kedua mewakili ID siswa (di kolom pertama) dan empat skor ujian. ID digunakan sebagai kunci asing.

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

Dalam klausul Pilih Klausul, penginisialisasi objek digunakan untuk membuat instans setiap objek Student baru dengan menggunakan data dari dua sumber.

Jika Anda tidak perlu menyimpan hasil kueri, jenis anonim bisa lebih sesuai daripada jenis bernama. Jenis bernama diperlukan jika Anda meneruskan hasil kueri di luar metode di mana kueri dijalankan. Contoh berikut menjalankan tugas yang sama dengan contoh sebelumnya, tetapi menggunakan jenis anonim alih-alih jenis bernama:

' 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

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