Files: how to distinguish file lock and permission denied cases?

Question

How to distinguish cases when file operation failed due to file lock and lack of permissions?

Are the `Message` and `InnerException` properties of the exception object the same too? — Roger Rowland, May 20 '13 at 08:17
InnerException is null, Message is the same. Any more blind guesses, or better some real info? — user626528, May 20 '13 at 09:05
@user626528 Your comments here read as aggressive and dismissive. They are liable to result in potential answerers choosing not to help you. — David Heffernan, Jun 13 '13 at 08:53
@David Heffernan, I really don't believe blind guess is a type of answer that SO needs to encourage. I'm grateful to any real answers, though. — user626528, Jun 13 '13 at 09:02
That's fine, but in my view, it would be more politically sensible to keep those views to yourself. — David Heffernan, Jun 13 '13 at 09:13
@David Heffernan, I find it stupid to put votes due to "politics" rather than professional considerations. And I don't care about ratings much. — user626528, Jun 13 '13 at 09:21
If this was Win32 then you'd do it by distinguishing between `ERROR_SHARING_VIOLATION` and `ERROR_ACCESS_DENIED`. It seems odd that .net won't pass that information on to you. — David Heffernan, Jun 13 '13 at 09:25
@David Heffernan, probably it's encapsulated somewhere, but I can't find it anywhere. — user626528, Jun 13 '13 at 09:26
@DavidHeffernan: I'm not sure that Win32 always draws that distinction either. IIRC, you often get ERROR_ACCESS_DENIED when the real issue is a sharing violation. — Harry Johnston, Jun 17 '13 at 00:41
@Harry Johnston, that's not good at all. Any ideas how to distinguish it in another way? Check if current program has enough access rights to the file, maybe? — user626528, Jun 17 '13 at 08:43

score 4 · Accepted Answer · edited May 23 '17 at 12:25

As per my answer here: Is there a way to check if a file is in use?

const int ERROR_SHARING_VIOLATION = 32;
const int ERROR_LOCK_VIOLATION = 33;

private static bool IsFileLocked(Exception exception)
{
    int errorCode = Marshal.GetHRForException(exception) & ((1 << 16) - 1);
    return errorCode == ERROR_SHARING_VIOLATION || errorCode == ERROR_LOCK_VIOLATION;
}

...

FileStream stream = null;
try
{
stream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
}
catch (IOException ex)
{
   if (IsFileLocked(ex))
   {
    // you know the file is locked
   } 
}
catch (ArgumentNullException ex)
{
    HandleException(ex);
}
catch (SecurityException ex)
{
    HandleException(ex);
}
catch (ArgumentException ex)
{
    HandleException(ex);
}
catch (ObjectDisposedException ex)
{
    HandleException(ex);
}
catch (UnauthorizedAccessException ex)
{
    // you know you dont have permission to the file
    HandleException(ex);
}
catch (Exception ex) 
{
}

score 2 · Answer 2 · answered Jun 13 '13 at 09:07

2

Better you can catch the IOException instead of UnauthorizedAccessException. Then process it accordingly.

 try
 {
     File.Open(_fileName, FileMode.Open, fileAccess, Fileshare.None); 
 }
 catch (IOException e)
 {
     if (!IsFileLocked(e))
         throw;
 }

private static bool IsFileLocked(IOException exception)
{
    int errorNum = Marshal.GetHRForException(exception) & ((1 << 16) - 1);
    return errorNum == 32 || errorNum == 33;
}

This way you can check FileLocked exception. Same way try out for Access denied exception by checking the appropriate Error Number.

answered Jun 13 '13 at 09:07

Santosh Panda

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The code I call throws UnauthorizedAccessException rather than IOException. – user626528 Jun 13 '13 at 09:11
@user626528 Is it important for us to know what your code is? Perhaps different code would lead to `IOException` being thrown. – David Heffernan Jun 13 '13 at 09:30
As per MSDN link : http://msdn.microsoft.com/en-us/library/system.io.file.delete.aspx UnauthorizedAccessException occurs when the caller does not have the required permission. While IOException occurs when the specified file is in use. – Santosh Panda Jun 13 '13 at 09:46
However, I'm getting UnauthorizedAccessException when I'm trying to delete file which is in use (it's an exe file and application haven't stopped yet). – user626528 Jun 13 '13 at 09:48

score 1 · Answer 3 · answered Jun 17 '13 at 21:37

You can read the ACL from the file using GetFileSecurity, get a handle to the process token using OpenProcessToken and then check for the access you want using AccessCheck.

To the best of my knowledge, sharing rules never prevent you from reading the security, so if you get an access denied at that point it should be safe to assume that you don't have permission to access the file. (Strictly speaking, you might have permission to do other things but not to read the ACL, but that would be a rare edge case.)

If AccessCheck says that you have access but you get access denied when opening the file, it should be safe to assume it's a sharing violation. Again, there may be edge cases, such as if the permissions are being changed at the same time you're looking at them, but that should be rare.

Files: how to distinguish file lock and permission denied cases?

3 Answers3