create and print barcode c# Advanced Win32 Techniques in .NET

Generator QR in .NET Advanced Win32 Techniques

Advanced Win32 Techniques
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The FileSystemWatcher component raises one event for each file and for each action on the file. For example, if you delete 10 files, you receive 10 distinct Deleted events. If you move 10 files from one directory to another, you receive 10 Deleted events from the source directories and 10 Created events from the destination directory.
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The WaitForChanged Method
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If your application doesn t perform any operation other than waiting for changes in the specified path, you can write simpler and more efficient code by using the WaitForChanged method. This method is synchronous in the sense that it doesn t return until a file change is detected or the (optional) timeout expires. This method returns a WaitForChangedResult structure, whose fields let you determine whether the timeout elapsed, the type of the event that occurred, and the name of the involved file:
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Create a *new* FileSystemWatcher component with values from
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the txtPath and txtFilter controls.
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Dim tmpFsw As New FileSystemWatcher(txtPath.Text, txtFilter.Text)
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Wait max 10 seconds for any file event.
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Dim res As WaitForChangedResult
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res = tmpFsw.WaitForChanged(WatcherChangeTypes.All, 10000)
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Check whether the operation timed out.
If res.TimedOut Then
LogMessage( 10 seconds have elapsed without an event ) Else LogMessage( Event: & res.Name & ( & res.ChangeType.ToString & ) ) End If
The WaitForChanged method traps changes only in the directory pointed to by the Path property and ignores the IncludeSubdirectories property. For this reason, the WaitForChangedResult structure includes a Name field but not a FullPath field. The first argument you pass to the WaitForChanged method lets you further restrict the kind of file operation you want to intercept:
Pause the application until the c:\temp\temp.dat file is deleted. Dim tmpFsw2 As New FileSystemWatcher( c:\temp", temp.dat ) tmpFsw2.WaitForChanged(WatcherChangeTypes.Deleted)
Buffer Overflows
You should be aware of potential problems when too many events fire in a short time. The FileSystemWatcher component uses an internal buffer to keep track of file system actions so that events can be raised for each one of them even if the application can t serve them fast enough. By default, this internal buffer is 8 KB long and can store about 160 events. Each event takes 16 bytes, plus 2 bytes for each character in the filename. (Filenames are stored as Unicode characters.) If you anticipate a lot of file activity, you should increase the size of the buffer by setting the InternalBufferSize to a larger value. The size should be an integer multiple of the operating system s page size (4 KB under Windows 2000 and later versions). Alternatively, you can use the NotifyFilter property
Part IV:
Win32 Applications
to limit the number of change operations that fire the Changed event or set IncludeSub directories to False if you don t really need to monitor an entire directory tree. (Use mul tiple FileSystemWatcher components to monitor individual subdirectories if you aren t interested in monitoring all the subdirectories under a given path.) You can t use the Filter property to prevent the internal buffer s overflow because this property filters out files only after they ve been added to the buffer. When the internal buffer overflows, you get an Error event:
Private Sub fsw_Error(ByVal sender As Object, _ ByVal e As ErrorEventArgs) Handles fsw.Error LogMessage( FileSystemWatcher error ) End Sub
By default, the Created, Deleted, Renamed, and Changed events run in a thread taken from the system thread pool. (See 12 for more information about the thread pool.) Because Windows Forms controls aren t thread safe, you should avoid accessing any control or the form itself from inside the FileSystemWatcher component s event handlers. If you find this limitation unacceptable, you should assign a Windows Form control to the component s SynchronizingObject property, as in this code:
Use the Form object as the synchronizing object. fsw.SynchronizingObject = Me
The preceding code ensures that all event handlers run in the same thread that serves the form itself. When you create a FileSystemWatcher component using the Visual Stu dio .NET designer, this property is automatically assigned the hosting form object. Here are a few more tips about the FileSystemWatcher component and the problems you might find when using it:
The FileSystemWatcher component starts raising events when the Path property is nonempty and the EnableRaisingEvents property is True. You can also prevent the component from raising unwanted events during the initialization phase by brack eting your setup statements between a call to the BeginInit method and a call to the EndInit method. (This is the approach used by the Visual Studio designer.) As I mentioned before, this component works only on Windows Me, Windows NT, Windows 2000, Windows XP, and Windows 2003 Server. It raises an error when it points to a path on machines running earlier versions of the operating system. Remote machines must have one of these operating systems to work properly, but you can t monitor a remote Windows NT system from another Windows NT machine. You can use UNC-based directory names only on Windows 2000 or later systems. The FileSystemWatcher component doesn t work on CD-ROM and DVD drives because their contents can t change.
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