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I've finally solved a problem that's been bugging me for years. One of our file shares ended up with several undelete-able files. Attempting to delete them results in "Error Deleting File or Folder - Cannot delete file: Cannot read from the source file or disk".

Note: Windows 7's version of this message is something like:

Could not find this item: This is no longer located in C:\Blah. Verify the item's location and try again.

Even going to the file's properties to check permissions presented a very blank properties dialog. And a CHKDSK didn't sort thing out either.

It turns out the problem was: the filename ended with a dot, e.g. it was something like "C:\Temp\Stuff\Sales Agreement.". As far as Windows is concerned this is an invalid file name: so although it gets reported in a directory listing, the standard Windows APIs for manipulating files subsequently deny its existence.

So how did this file get created in the first place? The answer: a Mac. The file was on a file share which had been accessed by a Mac user. Macs tend to write all sorts of metadata to extra "._DSStore" files and suchlike and had left this file behind.

So if Windows doesn't appear to allow these file names, how did they get to be created? Well, it turns out that NTFS allows all sort of file name/path weirdness that Windows, or specifically the Win32 API, doesn't allow. For example, NTFS actually allows file paths up to 32K but Windows restricts file paths to no more than 260 characters (MAX_PATH). I suppose this is all for DOS/Windows 9x backwards compatibility. As these files were being accessed over a file share I guess the usual Win32 checks are bypassed.

But thankfully you can get Win32 to ignore these checks by prefixing your file paths with \?\, (ie. C:\Temp\SomeFile.txt becomes \?\C:\Temp\SomeFile.txt) which I discovered after reading this blog post about long paths in .NET.

So at a command prompt (Start > All Programs > Accessories > Command Prompt) I was able to delete the file using:

del "\\?\C:\Temp\Stuff\Sales Agreement."
Note: On Windows 7 it seems you can just use wildcards without the \\?\ trick to delete the offending files: e.g. del c:\temp\somefil*

If it's a folder/directory you're trying to delete use the rd or rmdir command, e.g.:

rd /s "\\?\C:\Documents and Settings\User\Desktop\Annoying Folder."

Tip: as you're typing the file/directory name use the TAB key to auto-complete the name (press TAB repeatedly to cycle through possible names).

 


Of course the corollary of all of this is that you could really annoy somebody by doing this:

 

echo Hi > "\\?\%USERPROFILE%\Desktop\Annoying file you can't delete."

But you wouldn't do that would you?

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At home, our PC running Vista is rarely rebooted and is either in a low power state sleeping or being used by either me or my wife. One thing we've taken great advantage since the XP days is Fast User Switching which allows someone else to log in to their desktop whilst yours is kept running in the background. My wife and I had got quite used to doing a quick WinKey+L (as you do) before relinquishing control of the PC to one another. In Windows XP WinKey+L is does a "lock workstation" which in non-domain machines takes you back to the Windows logon screen. Unfortunately on Vista it takes you to a "workstation locked" screen, so you then have another mouse click on the Switch User button (followed by monitor re-syncing itself - why does it do this? All users run at the same screen resolution) to take you to the users screen. Of course there is a "Switch User" command tucked away in the little menu next to the lock button on the start menu - but a quick keystroke is what we're after here.

So - I go off searching for a shortcut key that does a "switch user" rather than "lock workstation". After a 20 minutes fruitless Googling for some special key combination, I sat back, thought about it logically and came up with this solution:

  1. Create a shortcut on your desktop to TSDISCON (* see below) and call it something like "Switch User"
  2. Go the shortcut Properties page and assign a shortcut key. Note that unfortunately you can't use the Windows Key in your shortcut - so I went for CTRL + ALT + SHIFT + L
  3. Right click Start button and choose "Open All Users" and move the shortcut into the Programs folder (confirming the UAC prompt as you go).
  4. Log out, and back in again. This is necessary because Explorer hasn't noticed there's now a shortcut with a new shortcut key it should be taking notice of.

And that did it. CTRL + ALT + SHIFT + L isn't quite a neat as WinKey+L but it's a hell of a lot better than poking around in the Start Menu.

  • What is TSDISCON you ask? It's the Terminal Services Disconnect command. Fast User Switching is all made possible by the core Terminal Services technologies which introduced the concept of multiple Window Stations or "sessions" running concurrently on the one machine. It was of course originally designed to support multiple users connecting concurrently to these sessions over the network using the Remote Desktop Protocol (RDP), but Windows XP took advantage of the multi-session architecture to enable Fast User Switching. (The RDP stuff is still there but hobbled to only allow one user to connect at a a time.)

UPDATE: If you don't have tsdiscon.exe on your system for some reason (maybe it's only available in Business/Ultimate or something) then you can use the following C# code (compiled into a Windows EXE using C:\Windows\Microsoft.NET\Framework\v2.0.50727\csc.exe if you don't have Visual Studio) to do the same thing. Tsdiscon.exe is just a wrapper around WTSDisconnectSession. I used Dependency Walker (aka depends.exe) to find out what was being used:

UPDATE 2: For your convenience: I've compiled the below and packaged it into a ZIP along with the source for download here.

using System;
using System.Runtime.InteropServices;
using System.ComponentModel;

class Program
{
  [DllImport("wtsapi32.dll", SetLastError = true)]
  static extern bool WTSDisconnectSession(IntPtr hServer, int sessionId, bool bWait);

  const int WTS_CURRENT_SESSION = -1;
  static readonly IntPtr WTS_CURRENT_SERVER_HANDLE = IntPtr.Zero;

  static void Main(string[] args)
  {
    if (!WTSDisconnectSession(WTS_CURRENT_SERVER_HANDLE,
         WTS_CURRENT_SESSION, false))
      throw new Win32Exception();
  }
}