逐步解說：在背景執行作業

發行項
08/12/2011

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。

如需在此範例中使用的完整程式碼清單，請參閱 HOW TO：在背景執行作業。

注意事項
根據您目前使用的設定或版本，您所看到的對話方塊與功能表指令可能會與 [說明] 中描述的不同。若要變更設定，請從 [工具] 功能表中選取 [匯入和匯出設定]。如需詳細資訊，請參閱使用設定。

若要在背景執行作業

使您的表單成為 [Windows Form 設計工具] 中的現用表單，從 [工具箱] 拖曳兩個 Button 控制項到表單中，然後根據下表設定按鈕的 Name 和 Text 屬性。

Button

Name

Text

button1

startBtn

Start

button2

cancelBtn

Cancel
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 元件隨即出現在 [元件匣] 上。
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，再按兩下 DoWork 和 RunWorkerCompleted 事件以建立事件處理常式。
在 DoWork 事件處理常式中插入會耗費許多時間的程式碼。
從 DoWorkEventArgs 參數的 Argument 屬性中擷取這項作業所需的任何參數。

Button	Name	Text
button1	startBtn	Start
button2	cancelBtn	Cancel

將計算的結果指派至 DoWorkEventArgs 的 Result 屬性。

RunWorkerCompleted 事件處理常式將可以使用這個結果。

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

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;
    }
}

在 RunWorkerCompleted 事件處理常式中插入擷取結果的程式碼。

' 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

// 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);
    }
}

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 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

// 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;
}

在 [Windows Form 設計工具] 中按兩下 startButton 以建立 Click 事件處理常式。

為 startButton 呼叫 Click 事件處理常式中的 RunWorkerAsync 方法。

Private Sub startButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles startBtn.Click
    Me.backgroundWorker1.RunWorkerAsync(2000)
End Sub

private void startBtn_Click(object sender, EventArgs e)
{
    this.backgroundWorker1.RunWorkerAsync(2000);
}

在 [Windows Form 設計工具] 中按兩下 cancelButton 以建立 Click 事件處理常式。

為 cancelButton 呼叫 Click 事件處理常式中的 CancelAsync 方法。

Private Sub cancelButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cancelBtn.Click
    Me.backgroundWorker1.CancelAsync()
End Sub

private void cancelBtn_Click(object sender, EventArgs e)
{
    this.backgroundWorker1.CancelAsync();
}

在檔案的頂端，匯入 System.ComponentModel 和 System.Threading 命名空間。

Imports System
Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms

using System;
using System.ComponentModel;
using System.Drawing;
using System.Threading;
using System.Windows.Forms;

按 F6 建置方案，然後按 CTRL-F5 在偵錯工具外部執行應用程式。

注意事項
如果您按 F5 在偵錯工具內部執行應用程式，偵錯工具會攔截並顯示在 TimeConsumingOperation 方法中所引發的例外狀況。當您在偵錯工具內部執行應用程式時，BackgroundWorker 會處理例外狀況，並在 RunWorkerCompletedEventArgs 的 Error 屬性中將其攔截。

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。

後續步驟

實作一個可以回報非同步作業進度的表單。如需詳細資訊，請參閱 HOW TO：實作使用背景作業的表單。
實作一個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。

請參閱

共用方式為

逐步解說：在背景執行作業

若要在背景執行作業

後續步驟

請參閱

工作

參考

其他資源

其他資源