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Drag&drop Shell Extension

WinSCP drag&drop shell extension is extension of the operation system (or rather Windows Explorer). It allows you to drag files from WinSCP directly to any folder (Windows Explorer window). Without it WinSCP needs to download the files first to temporary directory and only after that Windows Explorer transfers the files to their destination. While this happens transparently, it has several drawbacks.

Note that with or without the extension you can always transfer files directly by dragging them between local and remote panels of Commander interface.

Installation and Set up

The extension is installed by WinSCP installer by default (the portable WinSCP executable does not include the extension and it cannot be downloaded separately).

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Newly installed extension is loaded only the next time you log in to Windows. So you may need to restart your machine or log off and log in again to load it.1)

To check that the extension is loaded, run WinSCP and display Drag&drop page of Preferences window. If the Use shell extension checkbox is enabled (not grayed), the extension is loaded.

To tell WinSCP to use the extension, select Use shell extension checkbox. Note that unselecting it (or rather selecting Use temporary folder checkbox) does not uninstall the extension. It will be still loaded into memory and will be loaded every time the operating system starts up.

Drawbacks

When the extension is enabled in WinSCP the only destination application you can drag files to is Windows Explorer (this includes Desktop). Of course you can still drag files within WinSCP.

In addition, as the files are actually transferred directly to the target folder, not “dropped” on the target window, the Window Explorer will not place them on the spot, where you have dropped them. Instead they will appear on automatically selected position.

Security Considerations

Note that the extension is registered in the operating system (or rather Windows Explorer) and thus it is loaded into memory even if you do not use WinSCP.

Technical Background

This chapter is for those who are curious why WinSCP needs shell extension to allow direct downloads using drag&drop.

Here is short explanation: Windows drag&drop mechanics does not allow source application of drag&drop operation to find out easily, where the files are dropped. It is up to target application (Windows Explorer usually) to transfer files to destination. It is rather reasonable, because source application can hardly transfer files to all possible destinations. Keep in mind that you can drop files not only to a directory, but even to ZIP file (or any other archive), remote directory (via FTP, SFTP, SCP, …), trash, …

Obviously even Windows Explorer (or any other target application, like WinZip) cannot download files from any possible source (particularly it does not know SFTP/SCP).

When the extension is not installed, WinSCP uses a trick to allow drag&drop downloads. It tells Windows Explorer that the files are in temporary folder (from where Windows Explorer knows how to get the files) and a moment before the Windows Explorer starts to copy files from there, WinSCP downloads the files there.

To allow direct drag&drop downloads, the shell extension was developed. It misuses Windows Explorer CopyHook’s. CopyHook is a COM object (DLL library) that is called by Windows Explorer whenever directory (not file) is transferred within file system. When you drag anything from WinSCP, it creates empty dummy folder in temporary directory and pretends that you as user drag that directory. Once your drop it to Windows Explorer, it calls the CopyHook’s (including the WinSCP shell extension), telling it what and where was dragged. This way WinSCP knows the actual destination. It cancels the drag&drop operation, so the dummy directory is not copied by Windows Explorer and transfers your actual selection to now-known destination.

This on the other hand explains why with the extension you cannot drag files to any other application except for the Windows Explorer. No other application would call the CopyHook.

1) Alternatively you can restart explorer process manually, by killing it from Task Manager and running it again.
 
  dragext.txt · Last modified: by prikryl
 

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