如何：在后台运行操作
如果某项操作需要很长时间才能完成，而你不希望造成用户界面的延迟，则可以使用 BackgroundWorker 类在另一个线程上运行此操作。
以下代码示例显示如何在后台运行耗时的操作。 此窗体具有“启动”和“取消”按钮。 单击“启动”按钮运行异步操作。 单击“取消”按钮停止运行异步操作。 每个操作的结果显示在 MessageBox 中。
Visual Studio 中对此任务提供广泛支持。
另请参阅演练：在后台运行操作。
示例
using System;
using System.ComponentModel;
using System.Drawing;
using System.Threading;
using System.Windows.Forms;
namespace BackgroundWorkerExample
{
public class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
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;
}
}
// 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 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;
}
private void startBtn_Click(object sender, EventArgs e)
{
this.backgroundWorker1.RunWorkerAsync(2000);
}
private void cancelBtn_Click(object sender, EventArgs e)
{
this.backgroundWorker1.CancelAsync();
}
/// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.IContainer components = null;
/// <summary>
/// Clean up any resources being used.
/// </summary>
/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
protected override void Dispose(bool disposing)
{
if (disposing && (components != null))
{
components.Dispose();
}
base.Dispose(disposing);
}
#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.backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
this.startBtn = new System.Windows.Forms.Button();
this.cancelBtn = new System.Windows.Forms.Button();
this.SuspendLayout();
//
// backgroundWorker1
//
this.backgroundWorker1.WorkerSupportsCancellation = true;
this.backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
//
// startBtn
//
this.startBtn.Location = new System.Drawing.Point(12, 12);
this.startBtn.Name = "startBtn";
this.startBtn.Size = new System.Drawing.Size(75, 23);
this.startBtn.TabIndex = 0;
this.startBtn.Text = "Start";
this.startBtn.Click += new System.EventHandler(this.startBtn_Click);
//
// cancelBtn
//
this.cancelBtn.Location = new System.Drawing.Point(94, 11);
this.cancelBtn.Name = "cancelBtn";
this.cancelBtn.Size = new System.Drawing.Size(75, 23);
this.cancelBtn.TabIndex = 1;
this.cancelBtn.Text = "Cancel";
this.cancelBtn.Click += new System.EventHandler(this.cancelBtn_Click);
//
// Form1
//
this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
this.ClientSize = new System.Drawing.Size(183, 49);
this.Controls.Add(this.cancelBtn);
this.Controls.Add(this.startBtn);
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);
}
#endregion
private System.ComponentModel.BackgroundWorker backgroundWorker1;
private System.Windows.Forms.Button startBtn;
private System.Windows.Forms.Button cancelBtn;
}
public class Program
{
private Program()
{
}
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.EnableVisualStyles();
Application.Run(new Form1());
}
}
}
Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms
Public Class Form1
Inherits Form
Public Sub New()
InitializeComponent()
End Sub
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
' 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 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
Private Sub startButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles startBtn.Click
Me.backgroundWorker1.RunWorkerAsync(2000)
End Sub
Private Sub cancelButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cancelBtn.Click
Me.backgroundWorker1.CancelAsync()
End Sub
' Required designer variable.
Private components As System.ComponentModel.IContainer = Nothing
' Clean up any resources being used.
' <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
Protected Overrides Sub Dispose(ByVal disposing As Boolean)
If disposing AndAlso (components IsNot Nothing) Then
components.Dispose()
End If
MyBase.Dispose(disposing)
End Sub
#Region "Windows Form Designer generated code"
' Required method for Designer support - do not modify
' the contents of this method with the code editor.
Private Sub InitializeComponent()
Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
Me.startBtn = New System.Windows.Forms.Button()
Me.cancelBtn = New System.Windows.Forms.Button()
Me.SuspendLayout()
'
' backgroundWorker1
'
Me.backgroundWorker1.WorkerSupportsCancellation = True
'
' startButton
'
Me.startBtn.Location = New System.Drawing.Point(12, 12)
Me.startBtn.Name = "startButton"
Me.startBtn.Size = New System.Drawing.Size(75, 23)
Me.startBtn.TabIndex = 0
Me.startBtn.Text = "Start"
'
' cancelButton
'
Me.cancelBtn.Location = New System.Drawing.Point(94, 11)
Me.cancelBtn.Name = "cancelButton"
Me.cancelBtn.Size = New System.Drawing.Size(75, 23)
Me.cancelBtn.TabIndex = 1
Me.cancelBtn.Text = "Cancel"
'
' Form1
'
Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0F, 13.0F)
Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
Me.ClientSize = New System.Drawing.Size(183, 49)
Me.Controls.Add(cancelBtn)
Me.Controls.Add(startBtn)
Me.Name = "Form1"
Me.Text = "Form1"
Me.ResumeLayout(False)
End Sub
#End Region
Private WithEvents backgroundWorker1 As System.ComponentModel.BackgroundWorker
Private WithEvents startBtn As System.Windows.Forms.Button
Private WithEvents cancelBtn As System.Windows.Forms.Button
End Class
Public Class Program
Private Sub New()
End Sub
' The main entry point for the application.
<STAThread()> _
Shared Sub Main()
Application.EnableVisualStyles()
Application.Run(New Form1())
End Sub
End Class
编译代码
此示例需要：
- 对 System、System.Drawing 和 System.Windows.Forms 程序集的引用。
