Vorgehensweise: Verwenden eines Hintergrundthreads zur Dateisuche
Obwohl die BackgroundWorker-Komponente den System.Threading-Namespace ersetzt und funktionell erweitert, wird der System.Threading-Namespace sowohl für die Abwärtskompatibilität als auch für Ihre zukünftige Verwendung beibehalten. Weitere Informationen finden Sie unter der Übersicht über die BackgroundWorker-Komponente.
Windows Forms verwendet das STA-Modell (Single-Threaded Apartment), da Windows Forms auf nativen Win32-Fenstern basiert, die von Natur aus eine Komponente mit dem Threadingmodell „Apartment“ sind. Das STA-Modell besagt, dass ein Fenster auf einem beliebigen Thread erstellt werden kann, dass es aber nach der Erstellung nicht den Thread wechseln kann und dass alle Funktionsaufrufe für das Fenster in seinem Erstellungsthread erfolgen müssen. Außerhalb von Windows Forms verwenden die Klassen im .NET Framework das kostenlose Threadingmodell. Informationen zum Threading im .NET Framework finden Sie unter Threading.
Das STA-Modell erfordert, dass alle Methoden eines Steuerelements, die von außerhalb des Erstellungsthreads des Steuerelements aufgerufen werden müssen, an den Erstellungsthread des Steuerelements übergeben (dort ausgeführt) werden müssen. Die Basisklasse Control stellt verschiedene Methoden (Invoke, BeginInvoke und EndInvoke) zu diesem Zweck bereit. Invoke führt synchrone Methodenaufrufe durch; BeginInvoke führt asynchrone Methodenaufrufe durch.
Wenn Sie in Ihrem Steuerelement für ressourcenintensive Aufgaben das Multithreading verwenden, kann die Benutzeroberfläche reaktionsfähig bleiben, während eine ressourcenintensive Berechnung in einem Hintergrundthread ausgeführt wird.
Das folgende Beispiel (DirectorySearcher) zeigt ein Windows Forms-Steuerelement mit Multithreading, das einen Hintergrundthread verwendet, um ein Verzeichnis rekursiv nach Dateien zu durchsuchen, die mit einer angegebenen Suchzeichenfolge übereinstimmen, und dann ein Listenfeld mit dem Suchergebnis auffüllt. Die wichtigsten Konzepte, die durch das Beispiel veranschaulicht werden, sind folgende:
DirectorySearcherstartet einen neuen Thread, um die Suche durchzuführen. Der Thread führt dieThreadProcedure-Methode aus, die wiederum die HilfsmethodeRecurseDirectoryaufruft, um die eigentliche Suche durchzuführen und das Listenfeld aufzufüllen. Das Auffüllen des Listenfelds erfordert jedoch einen threadübergreifenden Aufruf, wie in den nächsten beiden Aufzählungspunkten erläutert.DirectorySearcherdefiniert dieAddFiles-Methode zum Hinzufügen von Dateien zu einem Listenfeld.RecurseDirectorykann jedoch nicht direktAddFilesaufrufen, daAddFilesnur in dem STA-Thread ausgeführt werden kann, derDirectorySearchererstellt hat.Die einzige Möglichkeit für
RecurseDirectory,AddFilesaufzurufen, ist ein threadübergreifender Aufruf, d. h. der Aufruf von Invoke oder BeginInvoke, umAddFilesauf den Erstellungsthread vonDirectorySearcherzu marshallen.RecurseDirectoryverwendet BeginInvoke, sodass der Aufruf asynchron erfolgen kann.Das Marshaling einer Methode erfordert das Äquivalent eines Funktionszeigers oder Rückrufs. Dies wird durch die Verwendung von Delegaten im .NET Framework erreicht. BeginInvoke übernimmt einen Delegaten als Argument.
DirectorySearcherdefiniert daher einen Delegaten (FileListDelegate), bindetAddFilesan eine Instanz vonFileListDelegatein seinem Konstruktor und übergibt diese Delegatinstanz an BeginInvoke.DirectorySearcherdefiniert auch einen Ereignisdelegaten, für den bei Abschluss der Suche das Marshalling durchgeführt wird.
Option Strict
Option Explicit
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});
}
}
}
}
Verwenden des Multithread-Steuerelements in einem Formular
Das folgende Beispiel zeigt, wie das Multithread-Steuerelement DirectorySearcher in einem Formular verwendet werden kann.
Option Explicit
Option Strict
Imports System.Collections
Imports System.ComponentModel
Imports System.Data
Imports System.Drawing
Imports System.Windows.Forms
Imports Microsoft.Samples.DirectorySearcher
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 System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Windows.Forms;
using Microsoft.Samples.DirectorySearcher;
/// <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;
}
}
}
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