GetLastWriteTime

I have a line of code in an onChanged event for a FileSystemWatcher, and have not gotten the result that I want. Here's the code:

Code:

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                str = "SELECT Base, Target FROM Backup WHERE Base = " & "'" & getDir & "'" & "OR Target = " & "'" & getDir & "'"
                cmd.Connection = myConn
                cmd.CommandText = str
                myConn.Open()

                Dim lstReader As OleDbDataReader = cmd.ExecuteReader()

                lstReader.Read()
                baseDir = lstReader.Item(0).ToString()
                targetDir = lstReader.Item(1).ToString()
                baseTime = IO.File.GetLastWriteTime(lstReader.Item(0).ToString() + "\" + e.Name)
                targetTime = IO.File.GetLastWriteTime(lstReader.Item(1).ToString() + "\" + e.Name)
                MessageBox.Show(baseTime.ToString() + "--" + targetTime.ToString())

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I'm finding that the GetLastWriteTime that is returned to me is not the lastest write time; for instance, I open a graphics file, change it, and then save it, but the last write time that is returned to me from the onChanged event is not the write time from my last save, but earlier. Does anyone know why this may be? I didn't post the whole sub, but if anyone needs me to I will.

The code you posted is db code, not fsw code. In your fsw code, be sure to subscribe to the last change event.

Quote:

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Originally Posted by Arjay

The code you posted is db code, not fsw code. In your fsw code, be sure to subscribe to the last change event.

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I'm not sure what you mean by subscribe to the last change event, Arjay.

Look at the msdn docs for fsw. You can enable different events like onCreated, onUpdated, onDeleted, etc. I believe you can do the same for LastChanged.

Actually, you set up a NotifyFilter.LastWrite. Take a look at the msdn docs - it has an example.

Quote:

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Originally Posted by Arjay

Actually, you set up a NotifyFilter.LastWrite. Take a look at the msdn docs - it has an example.

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I read through the fsw material again, but that's pretty much what I have coded. I'm still wondering how to take into account that multiple events are delivered when one file is changed; for instance, I change a graphics file, save it, and get four different notifications for the event. I've been trying to figure out how I can weed out the last notification which is the most recent one.

Quote:

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Originally Posted by KGCole

I read through the fsw material again, but that's pretty much what I have coded. I'm still wondering how to take into account that multiple events are delivered when one file is changed; for instance, I change a graphics file, save it, and get four different notifications for the event. I've been trying to figure out how I can weed out the last notification which is the most recent one.

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I've been using the fsw for quite a while. Previously, I was subscribing several different NotifyFilters and the Changedand Created events. To determine if it was the *last event, I would attempt to open the file with an exclusive lock for read access. I didn't want to do anything at that point with the file, I just wanted to get the lock. If I could get the lock, then I could be as sure as possible that the external operation was finished writing to the file (I don't have this code handy, but I can post this approach later).

Recently, on another project, I needed a fsw and started to copy my old code. The previous code handled large files, and the new code handled (relatively) tiny files. I ran into an article that simply does the following in a test project:

Code:

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static void Main(string[] args)
{
  var watcher = new FileSystemWatcher();
           
  watcher.Path = @"path goes here";
  watcher.NotifyFilter = NotifyFilters.FileName | NotifyFilters.Size;

  // Only watch text files.
  watcher.Filter = "*.txt";

  // Add event handlers.
  watcher.Changed += OnChanged;

  // Begin watching.
  watcher.EnableRaisingEvents = true;
           
  Console.WriteLine("FileSystemWatcher test. Press any key quit the sample.");
  Console.Read();

}

private static void OnChanged(object source, FileSystemEventArgs e)
{
  Console.WriteLine("File: " + e.FullPath + " " + e.ChangeType);
}

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The above code worked fine and seemed to give me a single event after the file gets written. However, in my testing today (before posting this code), it doesn't trigger when I paste a file into the folder.

So let me dig up the old code that does the check with the file lock. Sorry about the C# code, but I don't code in VB.Net.

Here is the code for the locked file check..

Code:

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private void InitializeFileSystemWatcher( )
{
  _fileSystemWatcher = new FileSystemWatcher( _destinationFolder );

  _fileSystemWatcher.NotifyFilter = NotifyFilters.LastWrite;
  _fileSystemWatcher.EnableRaisingEvents = true;
  _fileSystemWatcher.Changed += new FileSystemEventHandler( OnChanged );
}

protected virtual void OnChanged( object sender, FileSystemEventArgs args )
{
  if( VerifyFileTransferComplete( args) )
  {
    // Do work with file
  }
}

private bool VerifyFileTransferComplete( FileSystemEventArgs args )
{
  bool transferCompleted = false;

  try
  {
    using( FileStream fs = new FileStream( args.FullPath, FileMode.Open, FileAccess.Read, FileShare.None ) )
    {
      // Debug.WriteLine( args.FullPath );
      transferCompleted = true;
    }
  }
  catch( IOException )
  {
    // Ignore exceptions where we can't open the file
    // This is because the file is still being copied
  }
  return transferCompleted;
}

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Quote:

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Originally Posted by Arjay

Here is the code for the locked file check..

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Thanks Arjay, I'll look this over and see how I can implement the idea to solve my notification issues. I'll probably have questions later on.

Quote:

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Originally Posted by Arjay

Here is the code for the locked file check..

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I integrated your idea into my code, Arjay...I'm having a problem with file access, though. I've got a file open in a graphics editor, and when I save the file the right notifications come up, but I can't copy the file because it's still open in that process. Do you know any way around that?

Quote:

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Originally Posted by KGCole

I integrated your idea into my code, Arjay...I'm having a problem with file access, though. I've got a file open in a graphics editor, and when I save the file the right notifications come up, but I can't copy the file because it's still open in that process. Do you know any way around that?

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If a program has a file locked because it's using the file, your file watcher program won't be able to do anything until the first program releases the file. This is true for any program that has an exclusive lock on a file, i.e., other programs won't be able to access the file until the lock is released. Some programs aren't real good about immediately cleaning up file handles after the user closes the file in the program (but keeps the program running). In this case, the lock stays until the program is closed. I would avoid the scenario where you are monitoring files in folders where the user is saving them with programs that retain locks.

Quote:

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Originally Posted by Arjay

If a program has a file locked because it's using the file, your file watcher program won't be able to do anything until the first program releases the file. This is true for any program that has an exclusive lock on a file, i.e., other programs won't be able to access the file until the lock is released. Some programs aren't real good about immediately cleaning up file handles after the user closes the file in the program (but keeps the program running). In this case, the lock stays until the program is closed. I would avoid the scenario where you are monitoring files in folders where the user is saving them with programs that retain locks.

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I dunno, Arjay...I know what you mean about a file being locked. I've run into that scenario on a general basis with open files. But I downloaded a popular syncing program on a trial basis to see what it did with open files, and it was able to sync the open file with no problem. So I guess there's a way to do this. I don't know what it is, but it can be done. I'll have to think about this, and do some more research. I'll post my results if I find anything regarding it.

When you say the trial program synced the files, what does that mean? Did it copy an open file to a different location it was changed (but locked) or did it cooy after the file was unlocked. Copying files that are in use but changed is an easier problem to solve vs. processing a file after its last change. Withe the former, you don't care if the file is complete because it eventually will be, but that is a very different thing from waiting until the last change to process. My requirement was was for the 2nd scenario. You'll need to alter or change the code for your requirement.

Quote:

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Originally Posted by Arjay

When you say the trial program synced the files, what does that mean? Did it copy an open file to a different location it was changed (but locked) or did it cooy after the file was unlocked. Copying files that are in use but changed is an easier problem to solve vs. processing a file after its last change. Withe the former, you don't care if the file is complete because it eventually will be, but that is a very different thing from waiting until the last change to process. My requirement was was for the 2nd scenario. You'll need to alter or change the code for your requirement.

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It copied the open file to the target location while it was open and immediately after it's last saved change. I've done some reading on this over the net and I think how this is being accomplished is by way of VSS shadow file system. I think the shadow image of the last change is in the VSS file system by way of Windows that crawls the file system for changes and updates the volume image, and that file image is what is accessed and copied before a locked file is closed and released.

I've also run into some C# code that a poster showed as being a cmd line copy using the Process class of System namespace. I'll post that here for you to look at. I haven't had a chance to try it in a C# codepage yet, but I know that's your language forte', Arjay. Here's the code snippet...well, couldn't copy and paste it so here's the link to the page with the info:

https://stackoverflow.com/questions/...nother-process

Can you explain (again) what happens to the file after the last change is detected and why it is important to work with a locked file?

Quote:

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Originally Posted by Arjay

Can you explain (again) what happens to the file after the last change is detected and why it is important to work with a locked file?

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It's conjecture that the file is copied, but the open, locked file is synced to another location immediately after a change is saved. Whether it was by VSS or not, I can't say. I don't have access to the program code of the program. I didn't notice any difference between the open file and the saved, sync file, if there was any integrity issue. I don't know if an integrity issue would evolve after multiple syncs, either.

I suppose it would work to sync a file after it has been released, it's just interesting that someone has done the sync work on the fly as changes are made. Whether that's advisable or not, I don't know. I just noticed that's it's being done. But wouldn't system notifications have to be ignored or otherwise dealt with as they pop up during the save process on locked files? That I don't know anything about. I also don't know anything about asynchronous processes, which is something else I'll have to deal with later, I think.

My question is toward what you are trying to accomplish. You may have mentioned this in some other thread already.

Quote:

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Originally Posted by Arjay

My question is toward what you are trying to accomplish. You may have mentioned this in some other thread already.

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I think my objective is to create a backup utility that syncs between two folders on a pc in a timely manner. I suppose I'm comparing what I can create with other applications, but I'm learning what I want to produce as I find out what's possible. Overall, though, the primary objective is to provide that mens to backup information.

Sounds like vss is the way to go.

Quote:

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Originally Posted by Arjay

Sounds like vss is the way to go.

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I have doubts about that after what you said about partial file info. I don't want to jeopardize file integrity by using a means like VSS if it leads to inferior process. Quality is preferable to expediency. I'd like to know anything you've got to tell me about this.

Quote:

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Originally Posted by KGCole

I have doubts about that after what you said about partial file info. I don't want to jeopardize file integrity by using a means like VSS if it leads to inferior process. Quality is preferable to expediency. I'd like to know anything you've got to tell me about this.

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I'm not sure I have anything left to add. Time to start coding and try out some different approaches.

Quote:

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Originally Posted by Arjay

I'm not sure I have anything left to add. Time to start coding and try out some different approaches.

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Sorry, had to be away from things for a while. On this subject, I put a method together to test whether the changed file is released, but I haven't been able to get the exception for the error ignored if it's still locked. The exception still interrupts the process and says the file is in an open process. So basically I don't know how to have the exception ignored. Here's the sub, don't know how I can change it to get the desired result:

Code:

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        Private Function VerifyLastWrite(e As FileSystemEventArgs) As Boolean
            Dim fileDone As Boolean

            Try
                Dim fs As FileStream = New FileStream(e.FullPath, FileMode.Open, FileAccess.Read, FileShare.None)
                fileDone = True

            Catch ex As Exception
                fileDone = False

            End Try
            Return fileDone

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Quote:

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Originally Posted by KGCole

Sorry, had to be away from things for a while. On this subject, I put a method together to test whether the changed file is released, but I haven't been able to get the exception for the error ignored if it's still locked. The exception still interrupts the process and says the file is in an open process. So basically I don't know how to have the exception ignored. Here's the sub, don't know how I can change it to get the desired result:

Code:

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        Private Function VerifyLastWrite(e As FileSystemEventArgs) As Boolean
            Dim fileDone As Boolean

            Try
                Dim fs As FileStream = New FileStream(e.FullPath, FileMode.Open, FileAccess.Read, FileShare.None)
                fileDone = True

            Catch ex As Exception
                fileDone = False

            End Try
            Return fileDone

---

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You need to put the FileStream check inside a using block like it is in my example code. The using block will cause the FileStream to release the file object when it goes out of scope (which is immediately at the end of the using block). If you don't have a using block, the FileStream object you are using to check the file will itself keep a lock on the file.

From my code snippet:

Code:

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try
{
  using( FileStream fs = new FileStream( args.FullPath, FileMode.Open, FileAccess.Read, FileShare.None ) )
  {
    // Debug.WriteLine( args.FullPath );
    transferCompleted = true;
  } // IMPORTANT: the using block will release the FileStream object (and the lock for this object) here
} catch( IOException )
{
  // Ignore exceptions where we can't open the file
  // This is because the file is still being copied
}

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Quote:

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Originally Posted by Arjay

You need to put the FileStream check inside a using block like it is in my example code. The using block will cause the FileStream to release the file object when it goes out of scope (which is immediately at the end of the using block). If you don't have a using block, the FileStream object you are using to check the file will itself keep a lock on the file.

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I'm not sure I constructed this using statement correctly. My code still gets interrupted with the locked process error. Could you tell me what is wrong with this snippet?

Code:

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        Private Function VerifyLastWrite(e As FileSystemEventArgs) As Boolean
            Dim fileDone As Boolean

            Try
                Using fs As FileStream = New FileStream(e.FullPath, FileMode.Open, FileAccess.Read, FileShare.None)
                    fileDone = True

                End Using
            Catch ex As Exception
                fileDone = False

            End Try
            Return fileDone

        End Function

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What happens if you debug and step through the code?

Quote:

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Originally Posted by Arjay

What happens if you debug and step through the code?

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If I put a breakpoint on the using declaration and let it run to that point, then run the code through after the break, I don't get the error. Don't know quite what to make of that. If I don't have a breakpoint, I always get the error...