How to: Group Files by Extension (LINQ) (Visual Basic)
This example shows how LINQ can be used to perform advanced grouping and sorting operations on lists of files or folders. It also shows how to page output in the console window by using the Skip and Take methods.
Example
The following query shows how to group the contents of a specified directory tree by the file name extension.
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
The output from this program can be long, depending on the details of the local file system and what the startFolder is set to. To enable viewing of all results, this example shows how to page through results. The same techniques can be applied to Windows and Web applications. Notice that because the code pages the items in a group, a nested For Each loop is required. There is also some additional logic to compute the current position in the list, and to enable the user to stop paging and exit the program. In this particular case, the paging query is run against the cached results from the original query. In other contexts, such as LINQ to SQL, such caching is not required.
Compile the code
Create a Visual Basic console application project, with an Imports statement for the System.Linq namespace.
