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演练：在后台运行操作

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更新：2007 年 11 月

如果有一个需要很长时间才能完成的操作，而且不希望用户界面中出现延迟，则可以使用 BackgroundWorker 类来在另一个线程上运行该操作。

有关此示例中所用代码的完整清单，请参见 如何：在后台运行操作。

说明：

显示的对话框和菜单命令可能会与“帮助”中的描述不同，具体取决于您的当前设置或版本。若要更改设置，请在“工具”菜单上选择“导入和导出设置”。有关更多信息，请参见 Visual Studio 设置。

在后台运行操作

  1. 当窗体在 Windows 窗体设计器中处于活动状态时，将两个 Button 控件从“工具箱”拖动到窗体中，然后根据下表设置这两个按钮的 Name 和 Text 属性。

    按钮

    名称

    文本

    button1

    startBtn

    启动

    button2

    cancelBtn

    取消

  2. 打开“工具箱”，单击“组件”选项卡，然后将 BackgroundWorker 组件拖动到窗体上。

    backgroundWorker1 组件显示在“组件栏”中。

  3. 在“属性”窗口中，单击“事件”按钮，然后双击 DoWork 和 RunWorkerCompleted 事件以创建事件处理程序。

  4. 将耗时的代码插入到 DoWork 事件处理程序中。

  5. 从 DoWorkEventArgs 参数的 Argument 属性中提取该操作所需的所有参数。

  6. 将计算的结果赋给 DoWorkEventArgs 的 Result 属性。

    计算结果可供 RunWorkerCompleted 事件处理程序使用。

    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;
    }
}

  7. 在 RunWorkerCompleted 事件处理程序中，插入用来检索操作结果的代码。

    ' 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);
    }
}

  8. 实现 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;
}

  9. 在 Windows 窗体设计器中双击 startButton，创建新的 Click 事件处理程序。

  10. 在 startButton 的 Click 事件处理程序中，调用 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);
}

  11. 在 Windows 窗体设计器中双击 cancelButton，创建新的 Click 事件处理程序。

  12. 在 cancelButton 的 Click 事件处理程序中，调用 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();
}

  13. 在文件的顶部，导入 System.ComponentModel 和 System.Threading 命名空间。

    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;

  14. 按 F6 生成解决方案，然后按 Ctrl-F5 在调试器外部运行该应用程序。

说明：

如果按 F5 在调试器中运行应用程序，则调试器将捕捉和显示在 TimeConsumingOperation 方法中引发的异常。当在调试器外部运行应用程序时，BackgroundWorker 将处理该异常并将其缓存在 RunWorkerCompletedEventArgs 的 Error 属性中。

  1. 单击“启动”按钮运行异步操作，然后单击“取消”按钮停止正在运行的异步操作。

    每个操作的结果均在 MessageBox 中显示。

后续步骤

请参见

任务

如何：实现使用后台操作的窗体

如何：在后台运行操作

参考

BackgroundWorker

DoWorkEventArgs

其他资源

BackgroundWorker 组件