逐步解說：在背景執行作業
如果您有要花費較長時間才能完成的作業，但您不想導致使用者介面發生延遲，就可以使用 BackgroundWorker 類別在另一個執行緒上執行該作業。
如需此範例中使用的程式碼完整清單，請參閱操作說明：在背景執行作業。
在背景執行作業
在 Visual Studio 的 Windows Form 設計工具中，將兩個 Button 控制項從 [工具箱] 拖曳至表單，然後根據下表設定按鈕的
Name和 Text 屬性。按鈕 名稱 Text button1startBtn啟動 button2cancelBtn取消 開啟 [工具箱]，按一下 [元件] 索引標籤，然後將 BackgroundWorker 元件拖曳到表單上。
backgroundWorker1元件會出現在 [元件匣] 中。在 [屬性] 視窗中，將 WorkerSupportsCancellation 屬性設定為
true。在 [屬性] 視窗中，按一下 [事件] 按鈕，然後按一下 DoWork 和 RunWorkerCompleted 事件來建立事件處理常式。
將耗時的程式碼插入 DoWork 事件處理常式中。
從 DoWorkEventArgs 參數的 Argument 屬性擷取作業所需的任何參數。
將計算的結果指派給 DoWorkEventArgs 的 Result 屬性。
這可供 RunWorkerCompleted 事件處理常式使用。
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) { // Do not access the form's BackgroundWorker reference directly. // Instead, use the reference provided by the sender parameter. BackgroundWorker bw = sender as BackgroundWorker; // Extract the argument. int arg = (int)e.Argument; // Start the time-consuming operation. e.Result = TimeConsumingOperation(bw, arg); // If the operation was canceled by the user, // set the DoWorkEventArgs.Cancel property to true. if (bw.CancellationPending) { e.Cancel = true; } }Private Sub backgroundWorker1_DoWork( _ sender As Object, e As DoWorkEventArgs) _ Handles backgroundWorker1.DoWork ' Do not access the form's BackgroundWorker reference directly. ' Instead, use the reference provided by the sender parameter. Dim bw As BackgroundWorker = CType( sender, BackgroundWorker ) ' Extract the argument. Dim arg As Integer = Fix(e.Argument) ' Start the time-consuming operation. e.Result = TimeConsumingOperation(bw, arg) ' If the operation was canceled by the user, ' set the DoWorkEventArgs.Cancel property to true. If bw.CancellationPending Then e.Cancel = True End If End Sub插入程式碼，以在 RunWorkerCompleted 事件處理常式中擷取作業的結果。
// This event handler demonstrates how to interpret // the outcome of the asynchronous operation implemented // in the DoWork event handler. private void backgroundWorker1_RunWorkerCompleted( object sender, RunWorkerCompletedEventArgs e) { if (e.Cancelled) { // The user canceled the operation. MessageBox.Show("Operation was canceled"); } else if (e.Error != null) { // There was an error during the operation. string msg = String.Format("An error occurred: {0}", e.Error.Message); MessageBox.Show(msg); } else { // The operation completed normally. string msg = String.Format("Result = {0}", e.Result); MessageBox.Show(msg); } }' This event handler demonstrates how to interpret ' the outcome of the asynchronous operation implemented ' in the DoWork event handler. Private Sub backgroundWorker1_RunWorkerCompleted( _ sender As Object, e As RunWorkerCompletedEventArgs) _ Handles backgroundWorker1.RunWorkerCompleted If e.Cancelled Then ' The user canceled the operation. MessageBox.Show("Operation was canceled") ElseIf (e.Error IsNot Nothing) Then ' There was an error during the operation. Dim msg As String = String.Format("An error occurred: {0}", e.Error.Message) MessageBox.Show(msg) Else ' The operation completed normally. Dim msg As String = String.Format("Result = {0}", e.Result) MessageBox.Show(msg) End If End Sub實作
TimeConsumingOperation方法。// This method models an operation that may take a long time // to run. It can be cancelled, it can raise an exception, // or it can exit normally and return a result. These outcomes // are chosen randomly. private int TimeConsumingOperation( BackgroundWorker bw, int sleepPeriod ) { int result = 0; Random rand = new Random(); while (!bw.CancellationPending) { bool exit = false; switch (rand.Next(3)) { // Raise an exception. case 0: { throw new Exception("An error condition occurred."); break; } // Sleep for the number of milliseconds // specified by the sleepPeriod parameter. case 1: { Thread.Sleep(sleepPeriod); break; } // Exit and return normally. case 2: { result = 23; exit = true; break; } default: { break; } } if( exit ) { break; } } return result; }' This method models an operation that may take a long time ' to run. It can be cancelled, it can raise an exception, ' or it can exit normally and return a result. These outcomes ' are chosen randomly. Private Function TimeConsumingOperation( _ bw As BackgroundWorker, _ sleepPeriod As Integer) As Integer Dim result As Integer = 0 Dim rand As New Random() While Not bw.CancellationPending Dim [exit] As Boolean = False Select Case rand.Next(3) ' Raise an exception. Case 0 Throw New Exception("An error condition occurred.") Exit While ' Sleep for the number of milliseconds ' specified by the sleepPeriod parameter. Case 1 Thread.Sleep(sleepPeriod) Exit While ' Exit and return normally. Case 2 result = 23 [exit] = True Exit While Case Else Exit While End Select If [exit] Then Exit While End If End While Return result End Function在 Windows Form 設計工具中，按一下
startButton以建立 Click 事件處理常式。在
startButton的 Click 事件處理常式中呼叫 RunWorkerAsync 方法。private void startBtn_Click(object sender, EventArgs e) { this.backgroundWorker1.RunWorkerAsync(2000); }Private Sub startButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles startBtn.Click Me.backgroundWorker1.RunWorkerAsync(2000) End Sub在 Windows Form 設計工具中，按一下
cancelButton以建立 Click 事件處理常式。在
cancelButton的 Click 事件處理常式中呼叫 CancelAsync 方法。private void cancelBtn_Click(object sender, EventArgs e) { this.backgroundWorker1.CancelAsync(); }Private Sub cancelButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cancelBtn.Click Me.backgroundWorker1.CancelAsync() End Sub在檔案頂端，匯入 System.ComponentModel 和 System.Threading 命名空間。
using System; using System.ComponentModel; using System.Drawing; using System.Threading; using System.Windows.Forms;Imports System.ComponentModel Imports System.Drawing Imports System.Threading Imports System.Windows.Forms按 F6 建置方案，然後按 Ctrl+F5 以在偵錯工具外部執行應用程式。
注意
如果您按下 F5 以在偵錯工具下執行應用程式，偵錯工具就會攔截
TimeConsumingOperation方法中引發的例外狀況並加以顯示。 當您在偵錯工具外部執行應用程式時，BackgroundWorker 會處理例外狀況，並將其快取在 RunWorkerCompletedEventArgs 的 Error 屬性中。按一下 [開始] 按鈕以執行非同步作業，然後按一下 [取消] 按鈕以停止執行中的非同步作業。
每個作業的結果會顯示於 MessageBox。
下一步
實作以非同步作業方式報告進度的表單。 如需詳細資訊，請參閱操作說明：實作使用背景作業的表單。
實作支援元件非同步模式的類別。 如需詳細資訊，請參閱實作事件架構非同步模式。
