Resuming interrupted transfers 

When a connection is lost or aborted, you may end up with incompletely transferred file. Instead of transferring in all over again, it's possible to resume the operation and only transfer the missing part.

Upload and Download methods can do this - just specify ActionOnExistingFiles.ResumeIfPossible when calling. This will:

• transfer files which were not transferred yet (don't exist at the destination)
• transfer remaining parts of files which are smaller at the destination than on source
• skip all other files

// auto-resume failed directory upload
ftp.Upload(@"C:\MyData\*", "/MyData",
    0, 0, ActionOnExistingFiles.ResumeIfPossible);

// auto-resume failed single file download
ftp.Download("/MyData/file.txt", @"C:\MyData",
    0, 0, ActionOnExistingFiles.ResumeIfPossible);

' auto-resume failed directory upload
ftp.Upload("C:\MyData\*", "/MyData",
    0, 0, ActionOnExistingFiles.ResumeIfPossible)

' auto-resume failed single file download
ftp.Download("/MyData/file.txt", "C:\MyData",
    0, 0, ActionOnExistingFiles.ResumeIfPossible)

Overwriting existing files 

When transferring multiple files using Upload or Download method, use ActionOnExistingFiles argument to specify what to do when a target file already exists.

To overwrite existing files, use one of these options:

• OverwriteAll - Overwrites all files.
• OverwriteOlder - Overwrites files which are older that source files.
• OverwriteDifferentSize - Overwrites files which have different size.
• OverwriteDifferentChecksum - Overwrites files which have different checksum.
  Note: Only some FTP servers support checksums.

Alternatively, in order not to overwrite existing files, use one of these instead:

• SkipAll - Skip files.
• ResumeIfPossible - Resume the transfer if possible.
• Rename - Rename files using "filename[number].extension" pattern.
• ThrowException - Fail with an error.

// set desired overwrite mode according to a .config file
ActionOnExistingFiles action = ActionOnExistingFiles.ThrowException;
switch (ConfigurationManager.AppSettings["OverwriteMode"])
{
    case "skip": action = ActionOnExistingFiles.SkipAll; break;
    case "all": action = ActionOnExistingFiles.OverwriteAll; break;
    case "older": action = ActionOnExistingFiles.OverwriteOlder; break;
    case "size": action = ActionOnExistingFiles.OverwriteDifferentSize; break;
    case "checksum": action = ActionOnExistingFiles.OverwriteDifferentChecksum; break;
}

// upload directory with specific overwrite mode
ftp.Upload(@"C:\MyData\*", "/MyData", 0, 0, action);

' set desired overwrite mode according to a .config file
Dim action = ActionOnExistingFiles.ThrowException
Select Case ConfigurationManager.AppSettings("OverwriteMode")
    Case "skip" : action = ActionOnExistingFiles.SkipAll
    Case "all" : action = ActionOnExistingFiles.OverwriteAll
    Case "older" : action = ActionOnExistingFiles.OverwriteOlder
    Case "size" : action = ActionOnExistingFiles.OverwriteDifferentSize
    Case "checksum" : action = ActionOnExistingFiles.OverwriteDifferentChecksum
End Select

' upload directory with specific overwrite mode
ftp.Upload("C:\MyData\*", "/MyData", 0, 0, action)

Auto-renaming existing files 

When transferring multiple files using Upload or Download method, specify the ActionOnExistingFiles.Rename argument to automatically rename transferred file if the target file already exists. This will add a number between the filename and extension ("filename[number].extension" pattern).

// upload directory,
// auto-rename transferred files
// if target file already exists
ftp.Upload(@"C:\MyData\*", "/MyData",
    0, 0, ActionOnExistingFiles.Rename);

// download file,
// rename it if "C:\MyData\file.txt" already exists
// (e.g., "C:\MyData\file[1].txt" might be created)
ftp.Download("/MyData/file.txt", @"C:\MyData",
    0, 0, ActionOnExistingFiles.Rename);

' upload directory,
' auto-rename transferred files
' if target file already exists
ftp.Upload("C:\MyData\*", "/MyData",
    0, 0, ActionOnExistingFiles.Rename)

' download file,
' rename it if "C:\MyData\file.txt" already exists
' (e.g., "C:\MyData\file[1].txt" might be created)
ftp.Download("/MyData/file.txt", "C:\MyData",
    0, 0, ActionOnExistingFiles.Rename)

Progress reporting 

Displaying progress of both single-file and multi-file transfers is simple with TransferProgressChanged event. It keeps you updated with:

• ProgressPercentage - percentage of the whole operation that has been completed (useful when transferring multiple files)
• CurrentFileProgressPercentage - percentage of the current file that has been transferred
• BytesPerSecond - approximate current transfer speed
• Action - current action (upload or download)
• SourceItem - information about the currently processed item
• TransferState - current operation state (processing file or directory, data block, whole file transferred, ...)
• And much more (TargetPath, FilesProcessed, FilesTotal, ...)

Sample transfer progress event handler:

System.Windows.Forms.ProgressBar fileProgressBar;
System.Windows.Forms.ProgressBar totalProgressBar;
System.Windows.Forms.Label transferSpeedLabel;

void client_TransferProgressChanged(object sender, FtpTransferProgressChangedEventArgs e)
{
    // update ProgressBar of the current file
    fileProgressBar.Value = (int)e.CurrentFileProgressPercentage;

    // update ProgressBar of the whole transfer
    totalProgressBar.Value = (int)e.ProgressPercentage;

    // update transfer speed
    transferSpeedLabel.Text = e.BytesPerSecond.ToString();
}

Dim fileProgressBar As System.Windows.Forms.ProgressBar
Dim totalProgressBar As System.Windows.Forms.ProgressBar
Dim transferSpeedLabel As System.Windows.Forms.Label

Sub client_TransferProgressChanged(ByVal sender As Object, ByVal e As FtpTransferProgressChangedEventArgs)
    ' update ProgressBar of the current file
    fileProgressBar.Value = e.CurrentFileProgressPercentage

    ' update ProgressBar of the whole transfer
    totalProgressBar.Value = e.ProgressPercentage

    ' update transfer speed
    transferSpeedLabel.Text = e.BytesPerSecond.ToString()
End Sub

Registering the event:

// register TransferProgressChanged event handler which is raised
// when a significant action occurs
ftp.TransferProgressChanged += client_TransferProgressChanged;

// transfer files
// ...

' register TransferProgressChanged event handler which is raised
' when a significant action occurs
AddHandler ftp.TransferProgressChanged, AddressOf client_TransferProgressChanged

' transfer files
' ...

Transfer problem handling 

When a problem occurs in Upload or Download method, your code can get notified about it using the ProblemDetected event and react accordingly. See TransferProblemType enum for a list of possible problems you can handle.

The FtpProblemDetectedEventArgs argument of the event makes it possible to determine what is going on and choose an appropriate reaction. These are some of the properties available:

• Exception - an exception (if any) which will be raised if the problem is not handled
• ProblemType - the kind of problem that has occured
• Action - current action (upload, download, ...)
• Reaction - selected reaction (fail, skip, overwrite, ...)

Sample ProblemDetected event handler

The following code shows a ProblemDetected event handler that provides a custom 'FileExists' problem handling:

• Already-existing ".txt" files are renamed
• Other already-existing files are overwritten, but only if they are older than 7 days
• All other problems will be handled using the default behavior (see below)

void client_ProblemDetected(object sender, FtpProblemDetectedEventArgs e)
{
    // handle only existing files problem
    if (e.ProblemType != TransferProblemType.FileExists)
        return;

    // rename ".txt" files
    if (Path.GetExtension(e.LocalPath) == ".txt")
    {
        e.Rename();
        return;
    }

    // initialize source and target dates (use whole date granularity)
    DateTime source = e.LocalItem.LastWriteTime.Value.Date;
    DateTime target = e.RemoteItem.LastWriteTime.Value.Date;

    // overwrite files which are older than 7 days
    if (source > target && target < DateTime.Now.AddDays(-7))
    {
        e.Overwrite();
        return;
    }
}

Sub client_ProblemDetected(ByVal sender As Object, ByVal e As FtpProblemDetectedEventArgs)
    ' handle only existing files problem
    If e.ProblemType <> TransferProblemType.FileExists Then Return

    ' rename ".txt" files
    If Path.GetExtension(e.LocalPath) = ".txt" Then
        e.Rename()
        Return
    End If

    ' initialize source and target dates (use whole date granularity)
    Dim source = e.LocalItem.LastWriteTime.Value.Date
    Dim target = e.RemoteItem.LastWriteTime.Value.Date

    ' overwrite files which are older than 7 days
    If source > target AndAlso target < DateTime.Now.AddDays(-7) Then
        e.Overwrite()
        Return
    End If
End Sub

Registering the event and setting the default behavior:

// register ProblemDetected event handler which is raised
// when target file already exists (besides other things)
ftp.ProblemDetected += client_ProblemDetected;

// upload directory (skip existing files by default)
ftp.Upload(@"C:\MyData\*", "/MyData", 0, 0, ActionOnExistingFiles.SkipAll);

' register ProblemDetected event handler which is raised
' when target file already exists (besides other things)
AddHandler ftp.ProblemDetected, AddressOf client_ProblemDetected

' upload directory (skip existing files by default)
ftp.Upload("C:\MyData\*", "/MyData", 0, 0, ActionOnExistingFiles.SkipAll)

Restoring date&time after transfer 

By default, file times (creation, last access or last write time) are not synchronized after a successful file transfer. Use Settings.RestoreDateTime property to enable file time synchronization.

// set to restore last modified time
ftp.Settings.RestoreDateTime = ItemDateTimes.LastWriteTime;

// transfer files
// ...

' set to restore last modified time
ftp.Settings.RestoreDateTime = ItemDateTimes.LastWriteTime

' transfer files
' ...

FTP protocol supports LastWriteTime only (CreationTime and LastAccessTime are not supported).

Please note that the time granularity of FTP server and local file system can differ. For example, many FTP servers ignore the millisecond part of file date/time.

Different time zones on client and server can cause discrepancies as well with some FTP servers. Check out a blog post about handling file modification date/time in FTP for more information.

ASCII and binary transfer modes 

The default transfer mode is binary, which means data is transferred as is without any processing. However, when transferring text data between platforms which use different end-of-line sequences (Windows use <CR><LF> Unix uses <LF>), it's useful to have the sequences automatically converted by setting the transfer mode to ASCII.

Use TransferType property to specify the desired transfer type.

// set transfer type to ASCII
ftp.TransferType = FtpTransferType.Ascii;

// transfer files
// ...

' set transfer type to ASCII
ftp.TransferType = FtpTransferType.Ascii

' transfer files
' ...

Active and passive transfer modes 

Passive transfer mode is used by default because it operates nicely with most firewalls.
To use Active transfer mode, set Passive property to false.

// set to active transfers
ftp.Passive = false;

// transfer files
// ...

' set to active transfers
ftp.Passive = False

' transfer files
' ...

ZLIB transfer compression (MODE Z) 

FTP protocol supports ZLIB compression to speed up data transfers. This is particularly useful when transferring highly compressible data such as text files or sparse files. However, it can slow down the transfer speed slightly for already-compressed, multimedia or less compressible files.

// set transfer compression
ftp.TransferMode = FtpTransferMode.Zlib;

// transfer files
// ...

' set transfer compression
ftp.TransferMode = FtpTransferMode.Zlib

' transfer files
' ...

Block mode support (MODE B) - reusable data channel 

In stream mode (default), each data transfer requires opening and closing the data connection.
In block mode blocks of data are sent with a header that makes it possible to reuse the data connection for subsequent transfers.

However, block mode is only supported by several FTP servers (IIS 7.0+ or WFTPD Pro).

// set block mode
ftp.TransferMode = FtpTransferMode.Block;

// transfer files
// ...

' set block mode
ftp.TransferMode = FtpTransferMode.Block

' transfer files
' ...

Large file support 

Rebex FTP/SSL supports very large files - the theoretical maximum file size supported by the FTP protocol is 2^63 bytes, which is 9,223,372,036,854,775,807, or 9,223,372 TB.

However, in practice, the maximum file size might limited by the underlying filesystem. For example, FAT16's maximum file size is 2GB, FAT32's limit is 4GB. Check out this table for other filesystems.

But is it possible to transfer files larger than 4GB even when the local filesystem can't store them?

Actually, it is - you can use the stream-based API, which makes it possible to develop a custom stream to work around the filesystem's limitation (by splitting data into multiple smaller files, for example).