Stop FTP file downloading

Question

Hi all, This is my sample code run under thread to download file from ftp server. In that, if the user want to stop file download, i tried to abort the thread. If the control in the while loop, its hang up.

     How to close the binaryreader and Stream, when reader in the middle of stream

     Using response As FtpWebResponse = CType(ftp.GetResponse, FtpWebResponse)
               Using input As Stream = response.GetResponseStream()
                        Using reader As New BinaryReader(input)
                            Try
                                Using writer As New BinaryWriter(File.Open(targetFI.FullName, FileMode.Create)) 'output)
                                    Dim buffer(2048) As Byte '= New Byte(2048)
                                    Dim count As Integer = reader.Read(buffer, 0, buffer.Length)
                                    While count <> 0
                                        writer.Write(buffer, 0, count)
                                        count = reader.Read(buffer, 0, buffer.Length)
                                   End While
                                    writer.Close()
                                End Using

                            Catch ex As Exception
                                'catch error and delete file only partially downloaded
                                targetFI.Delete()
                                'Throw
                                ret = False
                            End Try
                            reader.Close()
                        End Using

                        input.Close()
                    End Using

            response.Close()
            End Using

Answer 1

You will need to add "polling" within your While loop to check if a certain condition (in your case, the user wishes to abort the download) is true. If the condition is true, you can exit from the while loop.

For example, you will presumably have a function that is called when the user wishes to stop the download (this is perhaps in response to a Button click on a user interface or some such mechanism).

Using a class level boolean variable (or property), you can simply set this variable to true in response to the user wishing to abort the download, then within your while loop that reads portions of the file from the FTP response stream, you check the value of this variable and if it's true, you simply exit from the while loop:

For example:

Somewhere at the class level, you declare:

Dim blnAbort as Boolean = False

When the user (for example) clicks a button to abort the download:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        blnAbort = True
    End Sub

And in your main While Loop from the code in your question, you add:

While count <> 0
  ' Check the value of the blnAbort variable.  If it is true, the user wants to abort, so we exit out of our while loop'
  If blnAbort = True Then
    Exit While
  End If
  writer.Write(buffer, 0, count)
  count = reader.Read(buffer, 0, buffer.Length)
End While

This is the basic mechanism (polling) by which you should abort a long running process. Of course, you should always ensure that the relevant clean-up code is performed in the event of aborting (in your case, closing the reader and writer, which you are already doing). You may also need to make a call to Application.DoEvents in your while loop if this is being done in the context of a Windows Forms based application, and the user aborting is controlled by some king of GUI interaction. This will ensure that Windows messages generated by (for example) a button click are processed in a timely fashion.