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How to: Use a Background Thread to Search for Files

The BackgroundWorker component replaces and adds functionality to the System.Threading namespace; however, the System.Threading namespace is retained for both backward compatibility and future use, if you choose. For more information, see BackgroundWorker Component Overview.

Windows Forms uses the single-threaded apartment (STA) model because Windows Forms is based on native Win32 windows that are inherently apartment-threaded. The STA model implies that a window can be created on any thread, but it cannot switch threads once created, and all function calls to it must occur on its creation thread. Outside Windows Forms, classes in the .NET Framework use the free threading model. For information about threading in the .NET Framework, see Managed Threading.

The STA model requires that any methods on a control that need to be called from outside the control's creation thread must be marshaled to (executed on) the control's creation thread. The base class Control provides several methods (Invoke, BeginInvoke, and EndInvoke) for this purpose. Invoke makes synchronous method calls; BeginInvoke makes asynchronous method calls.

If you use multithreading in your control for resource-intensive tasks, the user interface can remain responsive while a resource-intensive computation executes on a background thread.

The following sample (DirectorySearcher) shows a multithreaded Windows Forms control that uses a background thread to recursively search a directory for files matching a specified search string and then populates a list box with the search result. The key concepts illustrated by the sample are as follows:

  • DirectorySearcher starts a new thread to perform the search. The thread executes the ThreadProcedure method that in turn calls the helper RecurseDirectory method to do the actual search and to populate the list box. However, populating the list box requires a cross-thread call, as explained in the next two bulleted items.

  • DirectorySearcher defines the AddFiles method to add files to a list box; however, RecurseDirectory cannot directly invoke AddFiles because AddFiles can execute only in the STA thread that created DirectorySearcher.

  • The only way RecurseDirectory can call AddFiles is through a cross-thread call — that is, by calling Invoke or BeginInvoke to marshal AddFiles to the creation thread of DirectorySearcher. RecurseDirectory uses BeginInvoke so that the call can be made asynchronously.

  • Marshaling a method requires the equivalent of a function pointer or callback. This is accomplished using delegates in the .NET Framework. BeginInvoke takes a delegate as an argument. DirectorySearcher therefore defines a delegate (FileListDelegate), binds AddFiles to an instance of FileListDelegate in its constructor, and passes this delegate instance to BeginInvoke. DirectorySearcher also defines an event delegate that is marshaled when the search is completed.

Option Strict
Option Explicit

Imports System
Imports System.IO
Imports System.Threading
Imports System.Windows.Forms

Namespace Microsoft.Samples.DirectorySearcher
   ' <summary>
   '      This class is a Windows Forms control that implements a simple directory searcher.
   '      You provide, through code, a search string and it will search directories on
   '      a background thread, populating its list box with matches.
   ' </summary>
   Public Class DirectorySearcher
      Inherits Control
      ' Define a special delegate that handles marshaling
      ' lists of file names from the background directory search
      ' thread to the thread that contains the list box.
      Delegate Sub FileListDelegate(files() As String, startIndex As Integer, count As Integer)
      Private _listBox As ListBox
      Private _searchCriteria As String
      Private _searching As Boolean
      Private _deferSearch As Boolean
      Private _searchThread As Thread
      Private _fileListDelegate As FileListDelegate
      Private _onSearchComplete As EventHandler
      Public Sub New()
         _listBox = New ListBox()
         _listBox.Dock = DockStyle.Fill
         _fileListDelegate = New FileListDelegate(AddressOf AddFiles)
         _onSearchComplete = New EventHandler(AddressOf OnSearchComplete)
      End Sub
      Public Property SearchCriteria() As String
            Return _searchCriteria
         End Get
            ' If currently searching, abort
            ' the search and restart it after
            ' setting the new criteria.
            Dim wasSearching As Boolean = Searching
            If wasSearching Then
            End If
            _searchCriteria = value
            If wasSearching Then
            End If
         End Set
      End Property
      Public ReadOnly Property Searching() As Boolean
            Return _searching
         End Get
      End Property
      Public Event SearchComplete As EventHandler
      ' <summary>
      ' This method is called from the background thread.  It is called through
      ' a BeginInvoke call so that it is always marshaled to the thread that
      ' owns the list box control.
      ' </summary>
      ' <param name="files"></param>
      ' <param name="startIndex"></param>
      ' <param name="count"></param>
      Private Sub AddFiles(files() As String, startIndex As Integer, count As Integer)
         While count > 0
            count -= 1
            _listBox.Items.Add(files((startIndex + count)))
         End While
      End Sub
      Public Sub BeginSearch()
         ' Create the search thread, which 
         ' will begin the search.
         ' If already searching, do nothing.
         If Searching Then
         End If
         ' Start the search if the handle has
         ' been created. Otherwise, defer it until the
         ' handle has been created.
         If IsHandleCreated Then
            _searchThread = New Thread(New ThreadStart(AddressOf ThreadProcedure))
            _searching = True
            _deferSearch = True
         End If
      End Sub
      Protected Overrides Sub OnHandleDestroyed(e As EventArgs)
         ' If the handle is being destroyed and you are not
         ' recreating it, then abort the search.
         If Not RecreatingHandle Then
         End If
      End Sub
      Protected Overrides Sub OnHandleCreated(e As EventArgs)
         If _deferSearch Then
            _deferSearch = False
         End If
      End Sub
      ' <summary>
      ' This method is called by the background thread when it has
      ' finished the search.
      ' </summary>
      ' <param name="sender"></param>
      ' <param name="e"></param>
      Private Sub OnSearchComplete(sender As Object, e As EventArgs)
         RaiseEvent SearchComplete(sender, e)
      End Sub
      Public Sub StopSearch()
         If Not _searching Then
         End If
         If _searchThread.IsAlive Then
         End If
         _searchThread = Nothing
         _searching = False
      End Sub
      ' <summary>
      ' Recurses the given path, adding all files on that path to 
      ' the list box. After it finishes with the files, it
      ' calls itself once for each directory on the path.
      ' </summary>
      ' <param name="searchPath"></param>
      Private Sub RecurseDirectory(searchPath As String)
         ' Split searchPath into a directory and a wildcard specification.
         Dim directoryPath As String = Path.GetDirectoryName(searchPath)
         Dim search As String = Path.GetFileName(searchPath)
         ' If a directory or search criteria are not specified, then return.
         If directoryPath Is Nothing Or search Is Nothing Then
         End If
         Dim files() As String
         ' File systems like NTFS that have
         ' access permissions might result in exceptions
         ' when looking into directories without permission.
         ' Catch those exceptions and return.
            files = Directory.GetFiles(directoryPath, search)
         Catch e As UnauthorizedAccessException
         Catch e As DirectoryNotFoundException
         End Try
         ' Perform a BeginInvoke call to the list box
         ' in order to marshal to the correct thread. It is not
         ' very efficient to perform this marshal once for every
         ' file, so batch up multiple file calls into one
         ' marshal invocation.
         Dim startingIndex As Integer = 0
         While startingIndex < files.Length
            ' Batch up 20 files at once, unless at the
            ' end.
            Dim count As Integer = 20
            If count + startingIndex >= files.Length Then
               count = files.Length - startingIndex
            End If
            ' Begin the cross-thread call. Because you are passing
            ' immutable objects into this invoke method, you do not have to
            ' wait for it to finish. If these were complex objects, you would
            ' have to either create new instances of them or 
            ' wait for the thread to process this invoke before modifying
            ' the objects.
            Dim r As IAsyncResult = BeginInvoke(_fileListDelegate, New Object() {files, startingIndex, count})
            startingIndex += count
         End While
         ' Now that you have finished the files in this directory, recurse
         ' for each subdirectory.
         Dim directories As String() = Directory.GetDirectories(directoryPath)
         Dim d As String
         For Each d In  directories
            RecurseDirectory(Path.Combine(d, search))
         Next d
      End Sub
      '/ <summary>
      '/ This is the actual thread procedure. This method runs in a background
      '/ thread to scan directories. When finished, it simply exits.
      '/ </summary>
      Private Sub ThreadProcedure()
         ' Get the search string. Individual 
         ' field assigns are atomic in .NET, so you do not
         ' need to use any thread synchronization to grab
         ' the string value here.
            Dim localSearch As String = SearchCriteria
            ' Now, search the file system.
            ' You are done with the search, so update.
            _searching = False
            ' Raise an event that notifies the user that
            ' the search has terminated.  
            ' You do not have to do this through a marshaled call, but
            ' marshaling is recommended for the following reason:
            ' Users of this control do not know that it is
            ' multithreaded, so they expect its events to 
            ' come back on the same thread as the control.
            BeginInvoke(_onSearchComplete, New Object() {Me, EventArgs.Empty})
         End Try
      End Sub
   End Class
End Namespace

The following example shows how the multithreaded DirectorySearcher control can be used on a form.

Option Explicit
Option Strict

Imports Microsoft.Samples.DirectorySearcher
Imports System
Imports System.Drawing
Imports System.Collections
Imports System.ComponentModel
Imports System.Windows.Forms
Imports System.Data

Namespace SampleUsage

   ' <summary>
   '      Summary description for Form1.
   ' </summary>
   Public Class Form1
      Inherits System.Windows.Forms.Form
      Private WithEvents directorySearcher As DirectorySearcher
      Private searchText As System.Windows.Forms.TextBox
      Private searchLabel As System.Windows.Forms.Label
      Private WithEvents searchButton As System.Windows.Forms.Button
      Public Sub New()
         ' Required for Windows Forms designer support.
         ' Add any constructor code after InitializeComponent call here.
      End Sub

      #Region "Windows Form Designer generated code"
      ' <summary>
      '      Required method for designer support. Do not modify
      '      the contents of this method with the code editor.
      ' </summary>
      Private Sub InitializeComponent()
         Me.directorySearcher = New Microsoft.Samples.DirectorySearcher.DirectorySearcher()
         Me.searchButton = New System.Windows.Forms.Button()
         Me.searchText = New System.Windows.Forms.TextBox()
         Me.searchLabel = New System.Windows.Forms.Label()
         Me.directorySearcher.Anchor = System.Windows.Forms.AnchorStyles.Top Or System.Windows.Forms.AnchorStyles.Bottom Or System.Windows.Forms.AnchorStyles.Left Or System.Windows.Forms.AnchorStyles.Right
         Me.directorySearcher.Location = New System.Drawing.Point(8, 72)
         Me.directorySearcher.SearchCriteria = Nothing
         Me.directorySearcher.Size = New System.Drawing.Size(271, 173)
         Me.directorySearcher.TabIndex = 2
         Me.searchButton.Location = New System.Drawing.Point(8, 16)
         Me.searchButton.Size = New System.Drawing.Size(88, 40)
         Me.searchButton.TabIndex = 0
         Me.searchButton.Text = "&Search"
         Me.searchText.Anchor = System.Windows.Forms.AnchorStyles.Top Or System.Windows.Forms.AnchorStyles.Left Or System.Windows.Forms.AnchorStyles.Right
         Me.searchText.Location = New System.Drawing.Point(104, 24)
         Me.searchText.Size = New System.Drawing.Size(175, 20)
         Me.searchText.TabIndex = 1
         Me.searchText.Text = "c:\*.cs"
         Me.searchLabel.ForeColor = System.Drawing.Color.Red
         Me.searchLabel.Location = New System.Drawing.Point(104, 48)
         Me.searchLabel.Size = New System.Drawing.Size(176, 16)
         Me.searchLabel.TabIndex = 3
         Me.ClientSize = New System.Drawing.Size(291, 264)
         Me.Controls.AddRange(New System.Windows.Forms.Control() {Me.searchLabel, Me.directorySearcher, Me.searchText, Me.searchButton})
         Me.Text = "Search Directories"
      End Sub
      #End Region
      ' <summary>
      '    The main entry point for the application.
      ' </summary>
      <STAThread()> _
      Shared Sub Main()
         Application.Run(New Form1())
      End Sub
      Private Sub searchButton_Click(sender As Object, e As System.EventArgs) Handles searchButton.Click
         directorySearcher.SearchCriteria = searchText.Text
         searchLabel.Text = "Searching..."
      End Sub
      Private Sub directorySearcher_SearchComplete(sender As Object, e As System.EventArgs) Handles directorySearcher.SearchComplete
         searchLabel.Text = String.Empty
      End Sub
   End Class
End Namespace
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