如何：按扩展名对文件进行分组 (LINQ) (Visual Basic)
本示例演示如何使用 LINQ 来执行高级分组和对文件或文件夹列表执行排序操作。 它还演示如何使用 Skip 和 Take 方法在控制台窗口中对输出进行分页。
示例
下面的查询演示如何按文件扩展名对指定的目录树的内容进行分组。
Module GroupByExtension
Public Sub Main()
' Root folder to query, along with all subfolders.
Dim startFolder As String = "C:\program files\Microsoft Visual Studio 9.0\VB\"
' Used in WriteLine() to skip over startfolder in output lines.
Dim rootLength As Integer = startFolder.Length
'Take a snapshot of the folder contents
Dim dir As New System.IO.DirectoryInfo(startFolder)
Dim fileList = dir.GetFiles("*.*", System.IO.SearchOption.AllDirectories)
' Create the query.
Dim queryGroupByExt = From file In fileList _
Group By file.Extension.ToLower() Into fileGroup = Group _
Order By ToLower _
Select fileGroup
' Execute the query. By storing the result we can
' page the display with good performance.
Dim groupByExtList = queryGroupByExt.ToList()
' Display one group at a time. If the number of
' entries is greater than the number of lines
' in the console window, then page the output.
Dim trimLength = startFolder.Length
PageOutput(groupByExtList, trimLength)
End Sub
' Pages console display for large query results. No more than one group per page.
' This sub specifically works with group queries of FileInfo objects
' but can be modified for any type.
Sub PageOutput(ByVal groupQuery, ByVal charsToSkip)
' "3" = 1 line for extension key + 1 for "Press any key" + 1 for input cursor.
Dim numLines As Integer = Console.WindowHeight - 3
' Flag to indicate whether there are more results to display
Dim goAgain As Boolean = True
For Each fg As IEnumerable(Of System.IO.FileInfo) In groupQuery
' Start a new extension at the top of a page.
Dim currentLine As Integer = 0
Do While (currentLine < fg.Count())
Console.Clear()
Console.WriteLine(fg(0).Extension)
' Get the next page of results
' No more than one filename per page
Dim resultPage = From file In fg _
Skip currentLine Take numLines
' Execute the query. Trim the display output.
For Each line In resultPage
Console.WriteLine(vbTab & line.FullName.Substring(charsToSkip))
Next
' Advance the current position
currentLine = numLines + currentLine
' Give the user a chance to break out of the loop
Console.WriteLine("Press any key for next page or the 'End' key to exit.")
Dim key As ConsoleKey = Console.ReadKey().Key
If key = ConsoleKey.End Then
goAgain = False
Exit For
End If
Loop
Next
End Sub
End Module
此程序的输出可能很长，具体取决于本地文件系统的详细信息和 startFolder 的设置。 为了能够查看所有结果，此示例演示如何对结果进行分页。 相同的方法适用于 Windows 和 Web 应用程序。 请注意，由于代码对组中的项进行分页，因此需要使用 For Each 循环。 此外，还有一些其他逻辑用于计算列表中的当前位置，以及使用户能够停止分页并退出程序。 在此特定情况下，根据原始查询的缓存结果运行分页查询。 在其他上下文中，如 LINQ to SQL，则不需要此类缓存。
编译代码
创建 Visual Basic 控制台应用程序项目，其中包含用于 System.Linq 命名空间的 Imports 语句。
