610 Chapter 13 WORKING WITH FOLDERS AND FILES

Listing 13.20: Reading Records from a Binary File

Private Sub Button2_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button2.Click

Dim BR As BinaryReader Dim FS As FileStream

FS = New System.IO.FileStream(“Binarydata2.bin”, FileMode.Open, _ FileAccess.Read)

BR = New System.IO.BinaryReader(FS) BR.BaseStream.Seek(0, SeekOrigin.Begin) Dim p As New Product() TextBox1.Clear()

Dim c As Integer

c = BR.PeekChar

While FS.Position < FS.Length Console.WriteLine(c)

p = Nothing

‘Read fields and populate structure p.ProdID = BR.ReadString p.prodDescription = BR.ReadString p.listPrice = BR.ReadSingle p.available = BR.ReadBoolean p.minStock = BR.ReadInt32

‘Display structure

ShowRecord(p)

c = BR.PeekChar

End While

BR.Close()

FS.Close()

End Sub

Notice that the product’s price is read with the ReadSingle method, because it was saved as a Single variable. The ShowRecord() subroutine appends the fields of the current structure to the TextBox control at the bottom of the form.

Using a custom Structure to store the fields simplifies the structure of the application at large, but it doesn’t help the file I/O operation much. It’s quicker to use the Serializer object to store an entire collection to the file at once, rather than each member of the collection individually. The Serializer object was discussed in Chapter 11, and it addresses many of the file I/O needs of your applications. There will be situations, however, when you must store widely different pieces of information to a text or binary file, and the information presented in this chapter should be adequate for these situations.

The FileSystemWatcher Component

The FileSystemWatcher is a special component that has no visible interface and allows your application to watch for changes in the file system. You can use the FileSystemWatcher component to monitor changes in the local computer’s file system, a network drive, and even a remote computer’s file

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THE FILESYSTEMWATCHER COMPONENT 611

system (as long as the remote machine is running Windows NT or Windows 2000). The component exposes a few properties that let you specify what type of changes you want to monitor and the folders/files that will be monitored. Once activated, it fires an event every time one of the specified items has been changed.

The items you can monitor are folders and files. You can specify the folders you want to monitor as well as the file types to be monitored. You can also specify the types of actions you want to monitor; each action fires its own event. The actions you can monitor are the creation, deletion, and renaming of a file or folder and the modification of a file. The corresponding events are appropriately named: Changed, Created, Deleted, and Renamed. There’s also a special event, the Error event, that is fired when too many changes occur and the FileSystemWatcher component can’t keep track of them all (the internal buffer overflows and this condition is signaled with the Error event).

Properties

To use a FileSystemWatcher component in your project, open the Components tab of the Toolbox and double-click the FileSystemWatcher component’s icon. An instance of the component will be placed in your project, and you can set the following properties in the Properties window.

NotifyFilter

This property determines the types of changes you want to monitor; its value can have one of the values shown in the Table 13.8, the members of the IO.NotifyFilters enumeration.

Table 13.8: The NotifyFilters Enumeration

Value	Description
Attributes	The attributes of the file or folder
CreationTime	The date of file’s or folder’s creation
DirectoryName	The directory name
FileName	The filename
LastAccess	The date of file’s or folder’s last access
LastWrite	The date of file’s or folder’s last edit
Security	The security settings of the file or folder
Size	The size of the file or folder

You can combine multiple types of changes with the Or operator, but only in your code. The following statement prepares the FileSystemWatcher1 component to monitor for changes in the date and time of a file’s last write and last access:

FileSystemWatcher1.NotifyFilter = IO.NotifyFilters.LastWrite Or _

IO.NotifyFilters.LastAccess

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612 Chapter 13 WORKING WITH FOLDERS AND FILES

Path

Set this property to the path you want to monitor. The component will watch for changes in all the files in the specified path. If you want to include the path’s subfolders, set the IncludeSubdirectories property to True. The default value of this property is False.

Filter

This property filters the files you want to monitor through a string with wildcards. A Filter value of *.txt tells the component to monitor for changes in text files only. The default value of the Filter property is *.*, which includes all the files with an extension. To monitor all files, including the ones without extension, set the Filter property to an empty string. Notice that you can’t specify multiple extensions with the Filter property.

EnableRaisingEvents

To start monitoring for changes in the file system, set the EnableRaisingEvents property to True. While the EnableRaisingEvents property is True, the FileSystemWatcher component fires an event for the changes you have specified through its properties.

Events

To notify your application about the changes, the FileSystemWatcher component raises the following events, which you can handle from within your code: Changed, Created, Deleted, and Renamed. To code the handlers of these events, select the name of the FileSystemWatcher component in the Object drop-down list of the editor’s window and the name of the event you want to code in the Events drop-down list.

Like all events, they include two arguments: the sender and the e argument. The second argument of these events carries information about the type of the change through the ChangeType member. The

e.ChangeType member can be a member of the IO.WatcherChangeTypes enumeration: All, Changed,

Created, Deleted, and Renamed. The e.FullPath and e.Name properties are the path and filename of the file that was changed, created, or deleted. In the case of a folder, use the FullPath property to retrieve the name of the changed folder. Finally, the Renamed event’s argument exposes the OldFullPath and OldName members, which let you retrieve the old path and name of the renamed file.

You can write a common event handler for the Changed, Created, and Deleted events, because they share the same arguments. The Rename event must have its own handler, because the e argument is of a different type.

All the changes detected by the FileSystemWatcher component are stored in an internal buffer, which may overflow if too many changes take place in a short period of time. To avoid overflowing the buffer, you should limit the number of files you monitor by setting the Filter and Path properties appropriately. You should always limit the type of changes (you’ll rarely have to monitor for all types of changes in a folder). If the buffer overflows, the Error event will be raised. In this event’s handler you can increase the size of the buffer, by setting the InternalBufferSize property. You can double the buffer’s size from within the Error event handler to prevent the loss of additional events with the following statement:

FileSystemWatcher1.InternalBufferSize = 2 * FileSystemWatcher1.InternalBufferSize

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THE FILESYSTEMWATCHER COMPONENT 613

VB.NET at Work: The FileSystemWatcher Project

The FileSystemWatcher project, shown in Figure 13.3, demonstrates how to set up a FileSystemWatcher component and how to process the events raised by the component. The FileSystemWatcher component is initialized when the button is clicked. This button’s Click event handler prepares the FileSystemWatcher component to monitor changes in text files on the root of the C: drive. The Path property was set through the Properties window, but you may have to change it. I’ve chosen the root folder because it’s easy to locate and it has very few files on most systems. You can create, edit, rename, and then delete a few text files in the root folder to test the application.

Figure 13.3

The FileSystem-

Watcher project

After setting the properties at design time, the code sets the component’s EnableRaisingEvents property to True to start watching for changes. These changes will be signaled though the component’s events, which are programmed to print in the Output window the type of change detected and the name of the corresponding file. The type of change is reported to the event handler through the ChangeType member of the e argument. When a file is renamed, the program prints both the old and the new name.

The Start Monitoring button is a toggle. When clicked for the first time, its caption changes to Stop Monitoring and if you click it again, it will stop monitoring the file system. The properties of the FileSystemWatcher component are set in the form’s Load event, which is shown in Listing 13.21.

Listing 13.21: Programming the FileSystemWatcher Component

Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) _ Handles MyBase.Load

FileSystemWatcher1.Path = “c:\” FileSystemWatcher1.IncludeSubdirectories = False FileSystemWatcher1.Filter = “*.txt”

FileSystemWatcher1.NotifyFilter = IO.NotifyFilters.CreationTime Or _ IO.NotifyFilters.LastWrite Or IO.NotifyFilters.LastAccess Or _ IO.NotifyFilters.FileName

FileSystemWatcher1.EnableRaisingEvents = False End Sub

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614 Chapter 13 WORKING WITH FOLDERS AND FILES

The code behind the button’s Click event handler (Listing 13.22) toggles the EnableRaisingEvents property and the button’s caption. When this property is set to True, the FileSystemWatcher component starts monitoring the changes in the file system.

Listing 13.22: The Code of the Start Monitoring Button

Private Sub Button1_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles Button1.Click

If Button1.Text = “Start Monitoring” Then

FileSystemWatcher1.EnableRaisingEvents = True

Button1.Text = “Stop Monitoring”

Else

FileSystemWatcher1.EnableRaisingEvents = False

Button1.Text = “Start Monitoring”

End If

End Sub

Now you must program the handlers of the FileSystemWatcher component. You need not program all the events, only the ones you want to monitor. Since the Changed, Created, and Deleted event handlers have the same arguments, you can write a common handler for all three and a separate one for the Renamed event. Listing 13.23 details the event handlers of the sample applications.

Listing 13.23: The Event Handlers of the FileSystemWatcher Component

Private Sub WatcherHandler(ByVal sender As Object, _ ByVal e As System.IO.FileSystemEventArgs) _

Handles FileSystemWatcher1.Changed, FileSystemWatcher1.Created, _ FileSystemWatcher1.Deleted

ListBox1.Items.Add(e.ChangeType & vbTab & e.FullPath) End Sub

Private Sub FileSystemWatcher1_Renamed(ByVal sender As Object, _ ByVal e As System.IO.RenamedEventArgs) _ Handles FileSystemWatcher1.Renamed

ListBox1.Items.Add(e.ChangeType & vbTab & e.OldFullPath & “ TO “ & e.FullPath) End Sub

If you want to handle the Error event, you must stop monitoring the file system momentarily, double the value of the InternalBufferSize property, and then enable the monitoring again, as in Listing 13.24.

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