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How to: Compare the Contents of Two Folders (LINQ) (Visual Basic)

  • Article

This example demonstrates three ways to compare two file listings:

  • By querying for a Boolean value that specifies whether the two file lists are identical.

  • By querying for the intersection to retrieve the files that are in both folders.

  • By querying for the set difference to retrieve the files that are in one folder but not the other.

    Note

    The techniques shown here can be adapted to compare sequences of objects of any type.

The FileComparer class shown here demonstrates how to use a custom comparer class together with the Standard Query Operators. The class is not intended for use in real-world scenarios. It just uses the name and length in bytes of each file to determine whether the contents of each folder are identical or not. In a real-world scenario, you should modify this comparer to perform a more rigorous equality check.

Example

Module CompareDirs
    Public Sub Main()

        ' Create two identical or different temporary folders
        ' on a local drive and add files to them.
        ' Then set these file paths accordingly.
        Dim pathA As String = "C:\TestDir"
        Dim pathB As String = "C:\TestDir2"

        ' Take a snapshot of the file system.
        Dim dir1 As New System.IO.DirectoryInfo(pathA)
        Dim dir2 As New System.IO.DirectoryInfo(pathB)

        Dim list1 = dir1.GetFiles("*.*", System.IO.SearchOption.AllDirectories)
        Dim list2 = dir2.GetFiles("*.*", System.IO.SearchOption.AllDirectories)

        ' Create the FileCompare object we'll use in each query
        Dim myFileCompare As New FileCompare

        ' This query determines whether the two folders contain
        ' identical file lists, based on the custom file comparer
        ' that is defined in the FileCompare class.
        ' The query executes immediately because it returns a bool.
        Dim areIdentical As Boolean = list1.SequenceEqual(list2, myFileCompare)
        If areIdentical = True Then
            Console.WriteLine("The two folders are the same.")
        Else
            Console.WriteLine("The two folders are not the same.")
        End If

        ' Find common files in both folders. It produces a sequence and doesn't execute
        ' until the foreach statement.
        Dim queryCommonFiles = list1.Intersect(list2, myFileCompare)

        If queryCommonFiles.Count() > 0 Then

            Console.WriteLine("The following files are in both folders:")
            For Each fi As System.IO.FileInfo In queryCommonFiles
                Console.WriteLine(fi.FullName)
            Next
        Else
            Console.WriteLine("There are no common files in the two folders.")
        End If

        ' Find the set difference between the two folders.
        ' For this example we only check one way.
        Dim queryDirAOnly = list1.Except(list2, myFileCompare)
        Console.WriteLine("The following files are in dirA but not dirB:")
        For Each fi As System.IO.FileInfo In queryDirAOnly
            Console.WriteLine(fi.FullName)
        Next

        ' Keep the console window open in debug mode
        Console.WriteLine("Press any key to exit.")
        Console.ReadKey()
    End Sub

    ' This implementation defines a very simple comparison
    ' between two FileInfo objects. It only compares the name
    ' of the files being compared and their length in bytes.
    Public Class FileCompare
        Implements System.Collections.Generic.IEqualityComparer(Of System.IO.FileInfo)

        Public Function Equals1(ByVal x As System.IO.FileInfo, ByVal y As System.IO.FileInfo) _
            As Boolean Implements System.Collections.Generic.IEqualityComparer(Of System.IO.FileInfo).Equals

            If (x.Name = y.Name) And (x.Length = y.Length) Then
                Return True
            Else
                Return False
            End If
        End Function

        ' Return a hash that reflects the comparison criteria. According to the
        ' rules for IEqualityComparer(Of T), if Equals is true, then the hash codes must
        ' also be equal. Because equality as defined here is a simple value equality, not
        ' reference identity, it is possible that two or more objects will produce the same
        ' hash code.
        Public Function GetHashCode1(ByVal fi As System.IO.FileInfo) _
            As Integer Implements System.Collections.Generic.IEqualityComparer(Of System.IO.FileInfo).GetHashCode
            Dim s As String = fi.Name & fi.Length
            Return s.GetHashCode()
        End Function
    End Class
End Module

Compile the code

Create a Visual Basic console application project, with an Imports statement for the System.Linq namespace.

See also