如何：按扩展名对文件进行分组 (LINQ) (Visual Basic)

  • 项目

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。

另请参阅