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Comment : utiliser un thread d'arrière-plan pour rechercher des fichiers

Le composant BackgroundWorker remplace l'espace de noms System.Threading et lui ajoute des fonctionnalités ; toutefois, l'espace de noms System.Threading est conservé pour la compatibilité descendante et une utilisation ultérieure, si tel est votre choix. Pour plus d'informations, consultez Vue d'ensemble du composant BackgroundWorker.

Windows Forms se sert du modèle à thread cloisonné, (STA, Single-Threaded Apartment), car il s'appuie sur des fenêtres Win32 natives qui, de façon inhérente, sont à threads cloisonnés. Selon le modèle STA, une fenêtre peut être créée sur n'importe quel thread, mais une fois la fenêtre créée, le thread utilisé ne peut pas être changé ; par ailleurs, tous les appels de fonction qui lui sont adressés doivent se produire sur son thread de création. En dehors de Windows Forms, les classes du .NET Framework utilisent le modèle libre de thread. Pour plus d'informations sur le threading dans le .NET Framework, consultez Threading managé.

Le modèle STA requiert que toutes les méthodes sur un contrôle devant être appelées en dehors du thread de création du contrôle soient marshalées (exécutées) sur le thread de création du contrôle. La classe de base Control fournit plusieurs méthodes (Invoke, BeginInvoke et EndInvoke) à cette fin. Invoke effectue des appels de méthode synchrones et BeginInvoke effectue des appels de méthode asynchrones.

Si vous utilisez le multithreading dans votre contrôle pour les tâches consommant beaucoup de ressources, l'interface utilisateur peut rester réactive tandis qu'un calcul consommant beaucoup de ressources s'exécute sur un thread d'arrière-plan.

L'exemple suivant (DirectorySearcher) montre un contrôle Windows Forms multithread qui utilise un thread d'arrière-plan pour rechercher dans un répertoire, de manière récursive, tous les fichiers correspondant à la chaîne recherchée spécifiée, puis qui remplit une zone de liste avec les résultats de la recherche. Les concepts clés illustrés par l'exemple sont les suivants :

  • DirectorySearcher démarre un nouveau thread pour effectuer la recherche. Le thread exécute la méthode ThreadProcedure qui, à son tour, appelle la méthode d'assistance RecurseDirectory pour effectuer la recherche réelle et remplir la zone de liste. Toutefois, le remplissage de la zone de liste requiert un appel inter-threads, comme l'expliquent les deux éléments suivants de la liste à puce.

  • DirectorySearcher définit la méthode AddFiles pour ajouter des fichiers à une zone de liste ; cependant, RecurseDirectory ne peut pas appeler directement AddFiles car AddFiles s'exécute uniquement dans le thread STA qui a créé DirectorySearcher.

  • Le seul moyen dont RecurseDirectory peut appeler AddFiles est via un appel inter-threads, c'est-à-dire en appelant Invoke ou BeginInvoke pour marshaler AddFiles en un thread de création de DirectorySearcher. RecurseDirectory utilise BeginInvoke afin que l'appel puisse être effectué de façon asynchrone.

  • Le marshaling d'une méthode requiert l'équivalent d'un pointeur fonction ou d'un rappel de fonction. Pour cela, vous devez utiliser les délégués du .NET Framework. BeginInvoke prend un délégué comme argument. DirectorySearcher définit alors un délégué (FileListDelegate), lie AddFiles à une instance de FileListDelegate dans son constructeur et passe cette instance de délégué à BeginInvoke. DirectorySearcher définit également un délégué d'événement marshalé une fois la recherche effectuée.

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
         
         Controls.Add(_listBox)
         
         _fileListDelegate = New FileListDelegate(AddressOf AddFiles)
         _onSearchComplete = New EventHandler(AddressOf OnSearchComplete)
      End Sub
      
      Public Property SearchCriteria() As String
         Get
            Return _searchCriteria
         End Get
         Set
            ' If currently searching, abort
            ' the search and restart it after
            ' setting the new criteria.
            '
            Dim wasSearching As Boolean = Searching
            
            If wasSearching Then
               StopSearch()
            End If
            
            _listBox.Items.Clear()
            _searchCriteria = value
            
            If wasSearching Then
               BeginSearch()
            End If
         End Set
      End Property
      
      Public ReadOnly Property Searching() As Boolean
         Get
            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
            Return
         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
            _searchThread.Start()
         Else
            _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
            StopSearch()
         End If
         MyBase.OnHandleDestroyed(e)
      End Sub
      
      Protected Overrides Sub OnHandleCreated(e As EventArgs)
         MyBase.OnHandleCreated(e)
         If _deferSearch Then
            _deferSearch = False
            BeginSearch()
         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
            Return
         End If
         
         If _searchThread.IsAlive Then
            _searchThread.Abort()
            _searchThread.Join()
         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
            Return
         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.
         Try
            files = Directory.GetFiles(directoryPath, search)
         Catch e As UnauthorizedAccessException
            Return
         Catch e As DirectoryNotFoundException
            Return
         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.
         Try
            Dim localSearch As String = SearchCriteria
            
            ' Now, search the file system.
            '
            RecurseDirectory(localSearch)
         Finally
            ' 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
namespace Microsoft.Samples.DirectorySearcher
{
   using System;
   using System.IO;
   using System.Threading;
   using System.Windows.Forms;

   /// <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 : 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.
      private delegate void FileListDelegate(string[] files, int startIndex, int count);

      private ListBox listBox;
      private string  searchCriteria;
      private bool searching;
      private bool deferSearch;
      private Thread searchThread;
      private FileListDelegate fileListDelegate;
      private EventHandler onSearchComplete;

      public DirectorySearcher()
      {
         listBox = new ListBox();
         listBox.Dock = DockStyle.Fill;

         Controls.Add(listBox);

         fileListDelegate = new FileListDelegate(AddFiles);
         onSearchComplete = new EventHandler(OnSearchComplete);
      }

      public string SearchCriteria 
      {
         get 
         {
            return searchCriteria;
         }
         set 
         {
            // If currently searching, abort
            // the search and restart it after
            // setting the new criteria.
            //
            bool wasSearching = Searching;
            
            if (wasSearching)
            {
               StopSearch();
            }

            listBox.Items.Clear();
            searchCriteria = value;

            if (wasSearching)
            {
               BeginSearch();
            }
         }
      }

      public bool Searching 
      {
         get 
         {
            return searching;
         }
      }

      public event EventHandler SearchComplete;

      /// <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 void AddFiles(string[] files, int startIndex, int count)
      {
         while(count-- > 0)
         {
            listBox.Items.Add(files[startIndex + count]);
         }
      }

      public void BeginSearch() 
      {
         // Create the search thread, which 
         // will begin the search.
         // If already searching, do nothing.
         //
         if (Searching)
         {
            return;
         }

         // Start the search if the handle has
         // been created. Otherwise, defer it until the
         // handle has been created.
         if (IsHandleCreated)
         {
            searchThread = new Thread(new ThreadStart(ThreadProcedure));
            searching = true;
            searchThread.Start();
         }
         else
         {
            deferSearch = true;
         }
      }

      protected override void OnHandleDestroyed(EventArgs e)
      {
         // If the handle is being destroyed and you are not
         // recreating it, then abort the search.
         if (!RecreatingHandle)
         {
            StopSearch();
         }
         base.OnHandleDestroyed(e);
      }

      protected override void OnHandleCreated(EventArgs e) 
      {
         base.OnHandleCreated(e);
         if (deferSearch)
         {
            deferSearch = false;
            BeginSearch();
         }
      }

      /// <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 void OnSearchComplete(object sender, EventArgs e)
      {
         if (SearchComplete != null)
         {
            SearchComplete(sender, e);
         }
      }

      public void StopSearch()
      {
         if (!searching)
         {
            return;
         }

         if (searchThread.IsAlive)
         {
            searchThread.Abort();
            searchThread.Join();
         }

         searchThread = null;
         searching = false;
      }

      /// <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 void RecurseDirectory(string searchPath)
      {
         // Split searchPath into a directory and a wildcard specification.
         //
         string directory = Path.GetDirectoryName(searchPath);
         string search = Path.GetFileName(searchPath);

         // If a directory or search criteria are not specified, then return.
         //
         if (directory == null || search == null)
         {
            return;
         }

         string[] files;
         
         // File systems like NTFS that have
         // access permissions might result in exceptions
         // when looking into directories without permission.
         // Catch those exceptions and return.
         try 
         {
            files = Directory.GetFiles(directory, search);
         }
         catch(UnauthorizedAccessException)
         {
            return;
         }
         catch(DirectoryNotFoundException)
         {
            return;
         }

         // 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.
         int startingIndex = 0;

         while(startingIndex < files.Length)
         {
            // Batch up 20 files at once, unless at the
            // end.
            //
            int count = 20;
            if (count + startingIndex >= files.Length)
            {
               count = files.Length - startingIndex;
            }

            // 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.
            IAsyncResult r = BeginInvoke(fileListDelegate, new object[] {files, startingIndex, count});
            startingIndex += count;
         }

         // Now that you have finished the files in this directory, recurse for
         // each subdirectory.
         string[] directories = Directory.GetDirectories(directory);
         foreach(string d in directories)
         {
            RecurseDirectory(Path.Combine(d, search));
         }
      }

      /// <summary>
      /// This is the actual thread procedure. This method runs in a background
      /// thread to scan directories. When finished, it simply exits.
      /// </summary>
      private void 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.
         try 
         {
            string localSearch = SearchCriteria;

            // Now, search the file system.
            //
            RecurseDirectory(localSearch);
         }
         finally
         {
            // 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[] {this, EventArgs.Empty});
         }
      }
   }
}

Utilisation du contrôle multithread sur un formulaire

L'exemple suivant montre l'utilisation du contrôle DirectorySearcher multithread sur un formulaire.

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.
         '
         InitializeComponent()
         '
         ' 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..."
         directorySearcher.BeginSearch()
      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
namespace SampleUsage
{
   using Microsoft.Samples.DirectorySearcher;
   using System;
   using System.Drawing;
   using System.Collections;
   using System.ComponentModel;
   using System.Windows.Forms;
   using System.Data;

   /// <summary>
   ///      Summary description for Form1.
   /// </summary>
   public class Form1 : System.Windows.Forms.Form
   {
      private DirectorySearcher directorySearcher;
      private System.Windows.Forms.TextBox searchText;
      private System.Windows.Forms.Label searchLabel;
      private System.Windows.Forms.Button searchButton;

      public Form1()
      {
         //
         // Required for Windows Forms designer support.
         //
         InitializeComponent();

         //
         // Add any constructor code after InitializeComponent call here.
         //
      }

      #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 void InitializeComponent()
      {
         this.directorySearcher = new Microsoft.Samples.DirectorySearcher.DirectorySearcher();
         this.searchButton = new System.Windows.Forms.Button();
         this.searchText = new System.Windows.Forms.TextBox();
         this.searchLabel = new System.Windows.Forms.Label();
         this.directorySearcher.Anchor = (((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Bottom) 
            | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right);
         this.directorySearcher.Location = new System.Drawing.Point(8, 72);
         this.directorySearcher.SearchCriteria = null;
         this.directorySearcher.Size = new System.Drawing.Size(271, 173);
         this.directorySearcher.TabIndex = 2;
         this.directorySearcher.SearchComplete += new System.EventHandler(this.directorySearcher_SearchComplete);
         this.searchButton.Location = new System.Drawing.Point(8, 16);
         this.searchButton.Size = new System.Drawing.Size(88, 40);
         this.searchButton.TabIndex = 0;
         this.searchButton.Text = "&Search";
         this.searchButton.Click += new System.EventHandler(this.searchButton_Click);
         this.searchText.Anchor = ((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right);
         this.searchText.Location = new System.Drawing.Point(104, 24);
         this.searchText.Size = new System.Drawing.Size(175, 20);
         this.searchText.TabIndex = 1;
         this.searchText.Text = "c:\\*.cs";
         this.searchLabel.ForeColor = System.Drawing.Color.Red;
         this.searchLabel.Location = new System.Drawing.Point(104, 48);
         this.searchLabel.Size = new System.Drawing.Size(176, 16);
         this.searchLabel.TabIndex = 3;
         this.ClientSize = new System.Drawing.Size(291, 264);
         this.Controls.AddRange(new System.Windows.Forms.Control[] {this.searchLabel,
                                                        this.directorySearcher,
                                                        this.searchText,
                                                        this.searchButton});
         this.Text = "Search Directories";

      }
      #endregion

      /// <summary>
      ///    The main entry point for the application.
      /// </summary>
      [STAThread]
      static void Main() 
      {
         Application.Run(new Form1());
      }

      private void searchButton_Click(object sender, System.EventArgs e)
      {
         directorySearcher.SearchCriteria = searchText.Text;
         searchLabel.Text = "Searching...";
         directorySearcher.BeginSearch();
      }

      private void directorySearcher_SearchComplete(object sender, System.EventArgs e)
      {
         searchLabel.Text = string.Empty;
      }
   }
}

Voir aussi

Référence

BackgroundWorker

Concepts

Vue d'ensemble du modèle asynchrone basé sur des événements

Autres ressources

Développement de contrôles Windows Forms personnalisés avec le .NET Framework