VB.NET Using Hashes To Check If A File Changed

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Public Class frmCheckFile Private fileToCheck As String = "" Private fileHash As Byte() ' Select file button opens a file dialog to select a file to check. Private Sub btnSelectFile_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSelectFile.Click If dlgOpenFile.ShowDialog() = DialogResult.OK Then If Not String.IsNullOrEmpty(dlgOpenFile.FileName) Then ' Save the file name so we can check later. ' Then compute its hash and save that. fileToCheck = dlgOpenFile.FileName fileHash = ComputeFileHash(fileToCheck) Else MessageBox.Show("Please select a valid file to compute the hash for.") End If End If End Sub ' Check File button recomputes hash for the saved file name and compares it to the old saved hash. Private Sub btnCheckFileChanges_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCheckFileChanges.Click If Not String.IsNullOrEmpty(fileToCheck) Then Dim newFileHash As Byte() = ComputeFileHash(fileToCheck) If CompareByteHashes(newFileHash, fileHash) Then MessageBox.Show("They are the same, no changes were made.") Else MessageBox.Show("The file " + fileToCheck + " was changed.") End If Else MessageBox.Show("Please select a file to hash before you attempt to check it for changes.") End If End Sub ' Simply opens up the file name we have stored and appends a byte onto the end of it to change it slightly. Private Sub btnChangeSlightly_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnChangeSlightly.Click If Not String.IsNullOrEmpty(fileToCheck) Then Try Dim fileToChange As FileStream = New FileStream(fileToCheck, FileMode.Open, FileAccess.Write) ' Write a byte representing "A" onto the end of the file. fileToChange.Seek(fileToHash.Length, SeekOrigin.Begin) fileToChange.WriteByte(65) fileToChange.Close() MessageBox.Show("File successfully modified.") Catch ex As IOException MessageBox.Show("Error opening or writing to file: " & ex.Message) End Try Else MessageBox.Show("Please select a file to check/change first before trying to change it.") End If End Sub ' Calculates a file's hash value and returns it as a byte array. Private Function ComputeFileHash(ByVal fileName As String) As Byte() Dim ourHash(0) As Byte ' If file exists, create a HashAlgorithm instance based off of MD5 encryption ' You could use a variant of SHA or RIPEMD160 if you like with larger hash bit sizes. If File.Exists(fileName) Then Try Dim ourHashAlg As HashAlgorithm = HashAlgorithm.Create("MD5") Dim fileToHash As FileStream = New FileStream(fileName, FileMode.Open, FileAccess.Read) 'Compute the hash to return using the Stream we created. ourHash = ourHashAlg.ComputeHash(fileToHash) fileToHash.Close() Catch ex As IOException MessageBox.Show("There was an error opening the file: " & ex.Message) End Try End If Return ourHash End Function ' Return true/false if the two hashes are the same. Private Function CompareByteHashes(ByVal newHash As Byte(), ByVal oldHash As Byte()) As Boolean ' If any of these conditions are true, the hashes are definitely not the same. If newHash Is Nothing Or oldHash Is Nothing Or newHash.Length <> oldHash.Length Then Return False End If ' Compare each byte of the two hashes. Any time they are not the same, we know there was a change. For i As Integer = 0 To newHash.Length - 1 If newHash(i) <> oldHash(i) Then Return False End If Next i Return True End Function End Class