如何：查詢目錄樹狀中的最大檔案 (LINQ) (Visual Basic)

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此範例顯示五個與檔案位元組大小相關的查詢：

  • 如何擷取最大檔案的位元組大小。

  • 如何擷取最小檔案的位元組大小。

  • 如何從所指定根資料夾下的一或多個資料夾中，擷取最大或最小檔案的 FileInfo 物件。

  • 如何擷取序列，例如 10 個最大檔案。

  • 如何根據檔案位元組大小將檔案分組排序，並略過小於指定大小的檔案。

範例

下列範例包含五個不同的查詢，以示範如何根據檔案位元組大小來查詢和分組檔案。 您可以輕鬆修改這些範例，使查詢根據 FileInfo 物件的其他某個屬性。

Module QueryBySize  
    Sub Main()  
  
        ' Change the drive\path if necessary  
        Dim root As String = "C:\Program Files\Microsoft Visual Studio 9.0"  
  
        'Take a snapshot of the folder contents  
        Dim dir As New System.IO.DirectoryInfo(root)  
        Dim fileList = dir.GetFiles("*.*", System.IO.SearchOption.AllDirectories)  
  
        ' Return the size of the largest file  
        Dim maxSize = Aggregate aFile In fileList Into Max(GetFileLength(aFile))  
  
        'Dim maxSize = fileLengths.Max  
        Console.WriteLine("The length of the largest file under {0} is {1}", _  
                          root, maxSize)  
  
        ' Return the FileInfo object of the largest file  
        ' by sorting and selecting from the beginning of the list  
        Dim filesByLengDesc = From file In fileList _  
                              Let filelength = GetFileLength(file) _  
                              Where filelength > 0 _  
                              Order By filelength Descending _  
                              Select file  
  
        Dim longestFile = filesByLengDesc.First  
  
        Console.WriteLine("The largest file under {0} is {1} with a length of {2} bytes", _  
                          root, longestFile.FullName, longestFile.Length)  
  
        Dim smallestFile = filesByLengDesc.Last  
  
        Console.WriteLine("The smallest file under {0} is {1} with a length of {2} bytes", _  
                                root, smallestFile.FullName, smallestFile.Length)  
  
        ' Return the FileInfos for the 10 largest files  
        ' Based on a previous query, but nothing is executed  
        ' until the For Each statement below.  
        Dim tenLargest = From file In filesByLengDesc Take 10  
  
        Console.WriteLine("The 10 largest files under {0} are:", root)  
  
        For Each fi As System.IO.FileInfo In tenLargest  
            Console.WriteLine("{0}: {1} bytes", fi.FullName, fi.Length)  
        Next  
  
        ' Group files according to their size,  
        ' leaving out the ones under 200K  
        Dim sizeGroups = From file As System.IO.FileInfo In fileList _  
                         Where file.Length > 0 _  
                         Let groupLength = file.Length / 100000 _  
                         Group file By groupLength Into fileGroup = Group _  
                         Where groupLength >= 2 _  
                         Order By groupLength Descending  
  
        For Each group In sizeGroups  
            Console.WriteLine(group.groupLength + "00000")  
  
            For Each item As System.IO.FileInfo In group.fileGroup  
                Console.WriteLine("   {0}: {1}", item.Name, item.Length)  
            Next  
        Next  
  
        ' Keep the console window open in debug mode  
        Console.WriteLine("Press any key to exit.")  
        Console.ReadKey()  
  
    End Sub  
  
    ' This method is used to catch the possible exception  
    ' that can be raised when accessing the FileInfo.Length property.  
    ' In this particular case, it is safe to ignore the exception.  
    Function GetFileLength(ByVal fi As System.IO.FileInfo) As Long  
        Dim retval As Long  
        Try  
            retval = fi.Length  
        Catch ex As FileNotFoundException  
            ' If a file is no longer present,  
            ' just return zero bytes.
            retval = 0  
        End Try  
  
        Return retval  
    End Function  
End Module

若要傳回一或多個完整的 FileInfo 物件，查詢必須先檢查資料來源中的每個物件，再根據其 Length 屬性值進行排序。 然後，它會傳回具有最大長度的單一物件或物件序列。 使用 First 可傳回清單中的第一個項目。 使用 Take 則傳回前 n 個項目。 指定遞減排序次序，則會將最小的項目放在清單的開頭。

查詢會呼叫外面另一個取得檔案位元組大小的方法，以解決可能會因下列狀況引發的例外狀況：自呼叫 GetFiles 而建立 FileInfo 物件以來的這段期間，有另一個執行緒刪除了檔案。 即使已建立 FileInfo 物件，還是可能會發生這個例外狀況，原因是 FileInfo 物件會在它的 Length 屬性第一次受到存取時，嘗試使用目前最新的位元組大小來重新整理這個屬性。 讓這個作業進入查詢外部的 try-catch 區塊，就會遵循規則，以避免查詢中會造成副作業的作業。 一般而言，處理例外狀況時需要十分小心，以確定應用程式不是處於未知狀態。

編譯程式碼

建立 Visual Basic 主控台應用程式專案，其中包含 System.Linq 命名空間的 Imports 陳述式。

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