Encrypting .txt File Erroring - Process Cannot Access File

Question

I'm trying to encrypt a .txt file which is created in my application.

To create this file, I'm using the following code:

Dim fileExists As Boolean = File.Exists(directorypath & "dbpw.txt")
If File.Exists(directorypath & "dbpw.txt") = False Then
   Using sw As New StreamWriter(File.Open(directorypath & "dbpw.txt", FileMode.Create))
         IIf(fileExists, "", "")
         sw.Close()
   End Using
End If

Then, to write and encrypt the file, I'm using the following code, calling subroutines that I tweaked from examples on the internet.

bytKey = CreateKey(txtCode.Text)
bytIV = CreateIV(txtCode.Text)

EncryptOrDecryptFile(directorypath & "dbpw.txt", directorypath & "dbpw.txt", bytKey, bytIV, CryptoAction.ActionEncrypt)

When the code gets to the final line, calling the EncryptOrDecrypt subroutine, an error is thrown saying

The process cannot access the file 'myDirectoryPath\dbpw.txt' because it is being used by another process

What do I need to do to release the file?

I also tried just using File.Create along with File.Encrypt but the same error was thrown either way.

Code for EncryptOrDecryptFile()

Public Sub EncryptOrDecryptFile(ByVal strInputFile As String, ByVal strOutputFile As String, ByVal bytKey() As Byte, ByVal bytIV() As Byte, ByVal Direction As CryptoAction)

    Try
        fsInput = New System.IO.FileStream(strInputFile, FileMode.Open, FileAccess.Read)
        fsOutput = New System.IO.FileStream(strOutputFile, FileMode.OpenOrCreate, FileAccess.Write)
        fsOutput.SetLength(0)
        ' Currently fails, file not being released for read/write once it's created.

        Dim bytBuffer(4096) As Byte
        Dim lngBytesProcessed As Long = 0
        Dim lngFileLength As Long = fsInput.Length
        Dim intBytesInCurrentBlock As Integer
        Dim csCryptoStream As CryptoStream

        Dim cspRijndael As New System.Security.Cryptography.RijndaelManaged

        Select Case Direction
            Case CryptoAction.ActionEncrypt
                csCryptoStream = New CryptoStream(fsOutput, _
                cspRijndael.CreateEncryptor(bytKey, bytIV), _
                CryptoStreamMode.Write)

            Case CryptoAction.ActionDecrypt
                csCryptoStream = New CryptoStream(fsOutput, _
                cspRijndael.CreateDecryptor(bytKey, bytIV), _
                CryptoStreamMode.Write)
        End Select

        While lngBytesProcessed < lngFileLength
            intBytesInCurrentBlock = fsInput.Read(bytBuffer, 0, 4096)

            csCryptoStream.Write(bytBuffer, 0, intBytesInCurrentBlock)

            lngBytesProcessed = lngBytesProcessed + _
                                    CLng(intBytesInCurrentBlock)
        End While

        csCryptoStream.Close()
        fsInput.Close()
        fsOutput.Close()

    Catch ex As Exception
        errorLog(ex)

    End Try
End Sub

Answer 1

Together with the parameters you pass it, in your EncryptOrDecryptFile() method you are trying to open two streams (fsInput and fsOutput) to the same file. Once you've opened fsInput it has exclusive access to the file, making fsOutput unable to open it for writing.

Your only options are:

• Rename either the input file or the output file so that the two streams open two different files, or:

• Keep a "global" encryption stream open to the file, which you use every time you want to write data to it.

NOTE: Someone in the future might suggest that you open fsInput with FileShare.Write in order to allow other processes to open the file for writing, thus also allowing the fsOutput stream to write to it as well. - DON'T DO THAT!

Allowing other processes to write to the file while you're reading from it can cause problems, even if it's your fsOutput stream. This is because you'll never be able to know how much data fsOutput is going to write. What if the encrypted data is longer than the original? That would result in it overwriting what fsInput is going to read next, thus corrupting the file.