Windows 表單編程 如何：使用後台線程搜尋檔案 (線程經典例子)

Windows 表單編程

如何：使用後台線程搜尋檔案

組件替換了 命名空間並添加了功能，但是，也可選擇保留 System.Threading 命名空間以備向後相容和將來使用。有關更多資訊，請參見 BackgroundWorker 組件概述。

“Windows 表單”使用單一執行緒 Apartment (STA) 模型，因為“Windows 表單”基於本機 Win32 視窗，而 Win32 視窗從本質上而言是單元線程。STA 模型意味著可以在任何線程上建立視窗，但視窗一旦建立後就不能切換線程，並且對它的所有函數調用都必須在其建立線程上發生。除了 Windows 表單之外，.NET Framework 中的類使用自由執行緒模式。有關 .NET Framework 中的線程處理的資訊，請參見。

STA 模型要求需從控制項的非建立線程調用的控制項上的任何方法必須被封送到（在其上執行）該控制項的建立線程。為實現此目的，基類 提供了幾種方法（、 和）。 調用同步方法，而 調用非同步方法呼叫。

如果您在控制項中為大量佔用資源的任務使用多線程，則使用者介面可以在背景線程上執行一個大量佔用資源的計算的同時保持可響應。

以下樣本 (DirectorySearcher) 說明一個多線程 Windows 表單控制項，該控制項使用背景線程來遞迴地搜尋一個目錄以找到與特定搜尋字串匹配的檔案，然後用搜尋結果填充列表框。該樣本闡釋的主要概念如下：

• DirectorySearcher 啟動要執行搜尋的新線程。該線程執行 ThreadProcedure 方法，而該方法又調用協助器 RecurseDirectory 方法以執行實際搜尋並填充列表框。但是，填充列表框要求線程間調用，接下來的兩項對此進行了說明。

• DirectorySearcher 定義 AddFiles 方法以將檔案添加到列表框；但是，RecurseDirectory 不能直接調用 AddFiles，因為 AddFiles 只能在建立 DirectorySearcher 的 STA 線程中執行。

• RecurseDirectory 可以 AddFiles 的唯一方式是通過跨線程調用，即通過調用 Invoke 或 BeginInvoke 將 AddFiles 封送到 DirectorySearcher 的建立線程。RecurseDirectory 使用 BeginInvoke 以便可以非同步執行該調用。

• 封送一個方法要求函數指標或回調的等效項。這是通過 .NET Framework 中的委託來實現的。BeginInvoke 採用委託作為參數。DirectorySearcher 因此定義了委託 (FileListDelegate)，在其建構函式中將 AddFiles 綁定到 FileListDelegate 的一個執行個體上，並將此委託執行個體傳遞到 BeginInvoke。當完成搜尋時，DirectorySearcher 也定義了封送的事件委託。

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});
         }
      }
   }
}

在表單上使用多線程式控制件

下面的樣本說明如何在表單上使用多線程 DirectorySearcher 控制項。

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;
      }
   }
}

Reference from :
ms-help://MS.MSDNQTR.v80.chs/MS.MSDN.v80/MS.VisualStudio.v80.chs/dv_fxmclictl/html/7fe3956f-5b8f-4f78-8aae-c9eb0b28f13a.htm