BackgroundWorker 类

定义

命名空间:

System.ComponentModel

程序集:

System.ComponentModel.EventBasedAsync.dll

Source:

BackgroundWorker.cs

在单独的线程上执行操作。

C#

public class BackgroundWorker : System.ComponentModel.Component

继承

Object

MarshalByRefObject

Component

BackgroundWorker

示例

下面的代码示例演示 类的 BackgroundWorker 基础知识，这些基础知识用于异步执行耗时的操作。 下图显示了输出的示例。

若要尝试此代码，请创建Windows 窗体应用程序。 添加名为 Label 的resultLabel控件，并添加两Button个名为 和 cancelAsyncButton的startAsyncButton控件。 为这两个按钮创建 Click 事件处理程序。 在工具箱的“ 组件 ”选项卡中，添加名为 BackgroundWorker 的 backgroundWorker1组件。 为 BackgroundWorker创建 DoWork、 ProgressChanged和 RunWorkerCompleted 事件处理程序。 在窗体的代码中，将现有代码替换为以下代码。

C#

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace BackgroundWorkerSimple
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            backgroundWorker1.WorkerReportsProgress = true;
            backgroundWorker1.WorkerSupportsCancellation = true;
        }

        private void startAsyncButton_Click(object sender, EventArgs e)
        {
            if (backgroundWorker1.IsBusy != true)
            {
                // Start the asynchronous operation.
                backgroundWorker1.RunWorkerAsync();
            }
        }

        private void cancelAsyncButton_Click(object sender, EventArgs e)
        {
            if (backgroundWorker1.WorkerSupportsCancellation == true)
            {
                // Cancel the asynchronous operation.
                backgroundWorker1.CancelAsync();
            }
        }

        // This event handler is where the time-consuming work is done.
        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = sender as BackgroundWorker;

            for (int i = 1; i <= 10; i++)
            {
                if (worker.CancellationPending == true)
                {
                    e.Cancel = true;
                    break;
                }
                else
                {
                    // Perform a time consuming operation and report progress.
                    System.Threading.Thread.Sleep(500);
                    worker.ReportProgress(i * 10);
                }
            }
        }

        // This event handler updates the progress.
        private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            resultLabel.Text = (e.ProgressPercentage.ToString() + "%");
        }

        // This event handler deals with the results of the background operation.
        private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if (e.Cancelled == true)
            {
                resultLabel.Text = "Canceled!";
            }
            else if (e.Error != null)
            {
                resultLabel.Text = "Error: " + e.Error.Message;
            }
            else
            {
                resultLabel.Text = "Done!";
            }
        }
    }
}

下面的代码示例演示如何使用 BackgroundWorker 类异步执行耗时的操作。 下图显示了输出的示例。

该操作计算所选的斐波纳契数，在计算过程中报告进度更新，并允许取消挂起的计算。

C#

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

namespace BackgroundWorkerExample
{	
    public class FibonacciForm : System.Windows.Forms.Form
    {	
        private int numberToCompute = 0;
        private int highestPercentageReached = 0;

        private System.Windows.Forms.NumericUpDown numericUpDown1;
        private System.Windows.Forms.Button startAsyncButton;
        private System.Windows.Forms.Button cancelAsyncButton;
        private System.Windows.Forms.ProgressBar progressBar1;
        private System.Windows.Forms.Label resultLabel;
        private System.ComponentModel.BackgroundWorker backgroundWorker1;

        public FibonacciForm()
        {	
            InitializeComponent();

            InitializeBackgroundWorker();
        }

        // Set up the BackgroundWorker object by 
        // attaching event handlers. 
        private void InitializeBackgroundWorker()
        {
            backgroundWorker1.DoWork += 
                new DoWorkEventHandler(backgroundWorker1_DoWork);
            backgroundWorker1.RunWorkerCompleted += 
                new RunWorkerCompletedEventHandler(
            backgroundWorker1_RunWorkerCompleted);
            backgroundWorker1.ProgressChanged += 
                new ProgressChangedEventHandler(
            backgroundWorker1_ProgressChanged);
        }
    
        private void startAsyncButton_Click(System.Object sender, 
            System.EventArgs e)
        {
            // Reset the text in the result label.
            resultLabel.Text = String.Empty;

            // Disable the UpDown control until 
            // the asynchronous operation is done.
            this.numericUpDown1.Enabled = false;

            // Disable the Start button until 
            // the asynchronous operation is done.
            this.startAsyncButton.Enabled = false;

            // Enable the Cancel button while 
            // the asynchronous operation runs.
            this.cancelAsyncButton.Enabled = true;

            // Get the value from the UpDown control.
            numberToCompute = (int)numericUpDown1.Value;

            // Reset the variable for percentage tracking.
            highestPercentageReached = 0;

            // Start the asynchronous operation.
            backgroundWorker1.RunWorkerAsync(numberToCompute);
        }

        private void cancelAsyncButton_Click(System.Object sender, 
            System.EventArgs e)
        {   
            // Cancel the asynchronous operation.
            this.backgroundWorker1.CancelAsync();

            // Disable the Cancel button.
            cancelAsyncButton.Enabled = false;
        }

        // This event handler is where the actual,
        // potentially time-consuming work is done.
        private void backgroundWorker1_DoWork(object sender, 
            DoWorkEventArgs e)
        {   
            // Get the BackgroundWorker that raised this event.
            BackgroundWorker worker = sender as BackgroundWorker;

            // Assign the result of the computation
            // to the Result property of the DoWorkEventArgs
            // object. This is will be available to the 
            // RunWorkerCompleted eventhandler.
            e.Result = ComputeFibonacci((int)e.Argument, worker, e);
        }

        // This event handler deals with the results of the
        // background operation.
        private void backgroundWorker1_RunWorkerCompleted(
            object sender, RunWorkerCompletedEventArgs e)
        {
            // First, handle the case where an exception was thrown.
            if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message);
            }
            else if (e.Cancelled)
            {
                // Next, handle the case where the user canceled 
                // the operation.
                // Note that due to a race condition in 
                // the DoWork event handler, the Cancelled
                // flag may not have been set, even though
                // CancelAsync was called.
                resultLabel.Text = "Canceled";
            }
            else
            {
                // Finally, handle the case where the operation 
                // succeeded.
                resultLabel.Text = e.Result.ToString();
            }

            // Enable the UpDown control.
            this.numericUpDown1.Enabled = true;

            // Enable the Start button.
            startAsyncButton.Enabled = true;

            // Disable the Cancel button.
            cancelAsyncButton.Enabled = false;
        }

        // This event handler updates the progress bar.
        private void backgroundWorker1_ProgressChanged(object sender,
            ProgressChangedEventArgs e)
        {
            this.progressBar1.Value = e.ProgressPercentage;
        }

        // This is the method that does the actual work. For this
        // example, it computes a Fibonacci number and
        // reports progress as it does its work.
        long ComputeFibonacci(int n, BackgroundWorker worker, DoWorkEventArgs e)
        {
            // The parameter n must be >= 0 and <= 91.
            // Fib(n), with n > 91, overflows a long.
            if ((n < 0) || (n > 91))
            {
                throw new ArgumentException(
                    "value must be >= 0 and <= 91", "n");
            }

            long result = 0;

            // Abort the operation if the user has canceled.
            // Note that a call to CancelAsync may have set 
            // CancellationPending to true just after the
            // last invocation of this method exits, so this 
            // code will not have the opportunity to set the 
            // DoWorkEventArgs.Cancel flag to true. This means
            // that RunWorkerCompletedEventArgs.Cancelled will
            // not be set to true in your RunWorkerCompleted
            // event handler. This is a race condition.

            if (worker.CancellationPending)
            {   
                e.Cancel = true;
            }
            else
            {   
                if (n < 2)
                {   
                    result = 1;
                }
                else
                {   
                    result = ComputeFibonacci(n - 1, worker, e) + 
                             ComputeFibonacci(n - 2, worker, e);
                }

                // Report progress as a percentage of the total task.
                int percentComplete = 
                    (int)((float)n / (float)numberToCompute * 100);
                if (percentComplete > highestPercentageReached)
                {
                    highestPercentageReached = percentComplete;
                    worker.ReportProgress(percentComplete);
                }
            }

            return result;
        }

        #region Windows Form Designer generated code
        
        private void InitializeComponent()
        {
            this.numericUpDown1 = new System.Windows.Forms.NumericUpDown();
            this.startAsyncButton = new System.Windows.Forms.Button();
            this.cancelAsyncButton = new System.Windows.Forms.Button();
            this.resultLabel = new System.Windows.Forms.Label();
            this.progressBar1 = new System.Windows.Forms.ProgressBar();
            this.backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
            ((System.ComponentModel.ISupportInitialize)(this.numericUpDown1)).BeginInit();
            this.SuspendLayout();
            // 
            // numericUpDown1
            // 
            this.numericUpDown1.Location = new System.Drawing.Point(16, 16);
            this.numericUpDown1.Maximum = new System.Decimal(new int[] {
            91,
            0,
            0,
            0});
            this.numericUpDown1.Minimum = new System.Decimal(new int[] {
            1,
            0,
            0,
            0});
            this.numericUpDown1.Name = "numericUpDown1";
            this.numericUpDown1.Size = new System.Drawing.Size(80, 20);
            this.numericUpDown1.TabIndex = 0;
            this.numericUpDown1.Value = new System.Decimal(new int[] {
            1,
            0,
            0,
            0});
            // 
            // startAsyncButton
            // 
            this.startAsyncButton.Location = new System.Drawing.Point(16, 72);
            this.startAsyncButton.Name = "startAsyncButton";
            this.startAsyncButton.Size = new System.Drawing.Size(120, 23);
            this.startAsyncButton.TabIndex = 1;
            this.startAsyncButton.Text = "Start Async";
            this.startAsyncButton.Click += new System.EventHandler(this.startAsyncButton_Click);
            // 
            // cancelAsyncButton
            // 
            this.cancelAsyncButton.Enabled = false;
            this.cancelAsyncButton.Location = new System.Drawing.Point(153, 72);
            this.cancelAsyncButton.Name = "cancelAsyncButton";
            this.cancelAsyncButton.Size = new System.Drawing.Size(119, 23);
            this.cancelAsyncButton.TabIndex = 2;
            this.cancelAsyncButton.Text = "Cancel Async";
            this.cancelAsyncButton.Click += new System.EventHandler(this.cancelAsyncButton_Click);
            // 
            // resultLabel
            // 
            this.resultLabel.BorderStyle = System.Windows.Forms.BorderStyle.Fixed3D;
            this.resultLabel.Location = new System.Drawing.Point(112, 16);
            this.resultLabel.Name = "resultLabel";
            this.resultLabel.Size = new System.Drawing.Size(160, 23);
            this.resultLabel.TabIndex = 3;
            this.resultLabel.Text = "(no result)";
            this.resultLabel.TextAlign = System.Drawing.ContentAlignment.MiddleCenter;
            // 
            // progressBar1
            // 
            this.progressBar1.Location = new System.Drawing.Point(18, 48);
            this.progressBar1.Name = "progressBar1";
            this.progressBar1.Size = new System.Drawing.Size(256, 8);
            this.progressBar1.Step = 2;
            this.progressBar1.TabIndex = 4;
            // 
            // backgroundWorker1
            // 
            this.backgroundWorker1.WorkerReportsProgress = true;
            this.backgroundWorker1.WorkerSupportsCancellation = true;
            // 
            // FibonacciForm
            // 
            this.ClientSize = new System.Drawing.Size(292, 118);
            this.Controls.Add(this.progressBar1);
            this.Controls.Add(this.resultLabel);
            this.Controls.Add(this.cancelAsyncButton);
            this.Controls.Add(this.startAsyncButton);
            this.Controls.Add(this.numericUpDown1);
            this.Name = "FibonacciForm";
            this.Text = "Fibonacci Calculator";
            ((System.ComponentModel.ISupportInitialize)(this.numericUpDown1)).EndInit();
            this.ResumeLayout(false);
        }
        #endregion

        [STAThread]
        static void Main()
        {
            Application.Run(new FibonacciForm());
        }
    }
}

注解

类 BackgroundWorker 允许在单独的专用线程上运行操作。 下载和数据库事务等耗时的操作可能会导致用户界面 (UI) 在运行时似乎已停止响应。 当你需要响应式 UI 并且遇到与此类操作相关的长时间延迟时，类 BackgroundWorker 提供了一个方便的解决方案。

若要在后台执行耗时的操作，请创建 并 BackgroundWorker 侦听报告操作进度的事件，并在操作完成时发出信号。 可以通过编程方式创建 ，BackgroundWorker也可以从工具箱的“组件”选项卡将其拖到窗体上。 如果在Windows 窗体 Designer中创建 BackgroundWorker ，它将出现在组件托盘中，其属性将显示在属性窗口中。

若要设置后台操作，请为 DoWork 事件添加事件处理程序。 在此事件处理程序中调用耗时的操作。 若要启动操作，请调用 RunWorkerAsync。 若要接收进度更新通知，请 ProgressChanged 处理 事件。 若要在操作完成时接收通知，请 RunWorkerCompleted 处理 事件。

备注

必须小心不要在事件处理程序中 DoWork 操作任何用户界面对象。 而是通过 ProgressChanged 和 RunWorkerCompleted 事件与用户界面通信。

BackgroundWorker 事件不会跨 AppDomain 边界封送。 不要使用 BackgroundWorker 组件在多个 AppDomain中执行多线程操作。

如果后台操作需要参数，请使用参数调用 RunWorkerAsync 。 在事件处理程序中 DoWork ，可以从 属性中提取 参数 DoWorkEventArgs.Argument 。

有关 BackgroundWorker 的详细信息，请参阅如何：在后台运行操作。

构造函数

BackgroundWorker()

初始化 BackgroundWorker 类的新实例。

属性

CancellationPending	获取一个值，指示应用程序是否已请求取消后台操作。
CanRaiseEvents	获取一个指示组件是否可以引发事件的值。 (继承自 Component)
Container	获取包含 IContainer 的 Component。 (继承自 Component)
DesignMode	获取一个值，用以指示 Component 当前是否处于设计模式。 (继承自 Component)
Events	获取附加到此 Component 的事件处理程序的列表。 (继承自 Component)
IsBusy	获取一个值，指示 BackgroundWorker 是否正在运行异步操作。
Site	获取或设置 Component 的 ISite。 (继承自 Component)
WorkerReportsProgress	获取或设置一个值，该值指示 BackgroundWorker 能否报告进度更新。
WorkerSupportsCancellation	获取或设置一个值，该值指示 BackgroundWorker 是否支持异步取消。

方法

CancelAsync()	请求取消挂起的后台操作。
CreateObjRef(Type)	创建一个对象，该对象包含生成用于与远程对象进行通信的代理所需的全部相关信息。 (继承自 MarshalByRefObject)
Dispose()	释放由 Component 使用的所有资源。 (继承自 Component)
Dispose(Boolean)	此方法不执行任何操作。
Equals(Object)	确定指定对象是否等于当前对象。 (继承自 Object)
GetHashCode()	作为默认哈希函数。 (继承自 Object)
GetLifetimeService()	已过时. 检索控制此实例的生存期策略的当前生存期服务对象。 (继承自 MarshalByRefObject)
GetService(Type)	返回一个对象，该对象表示由 Component 或它的 Container 提供的服务。 (继承自 Component)
GetType()	获取当前实例的 Type。 (继承自 Object)
InitializeLifetimeService()	已过时. 获取生存期服务对象来控制此实例的生存期策略。 (继承自 MarshalByRefObject)
MemberwiseClone()	创建当前 Object 的浅表副本。 (继承自 Object)
MemberwiseClone(Boolean)	创建当前 MarshalByRefObject 对象的浅表副本。 (继承自 MarshalByRefObject)
OnDoWork(DoWorkEventArgs)	引发 DoWork 事件。
OnProgressChanged(ProgressChangedEventArgs)	引发 ProgressChanged 事件。
OnRunWorkerCompleted(RunWorkerCompletedEventArgs)	引发 RunWorkerCompleted 事件。
ReportProgress(Int32)	引发 ProgressChanged 事件。
ReportProgress(Int32, Object)	引发 ProgressChanged 事件。
RunWorkerAsync()	开始执行后台操作。
RunWorkerAsync(Object)	开始执行后台操作。
ToString()	返回包含 Component 的名称的 String（如果有）。 不应重写此方法。 (继承自 Component)

事件

Disposed	在通过调用 Dispose() 方法释放组件时发生。 (继承自 Component)
DoWork	调用 RunWorkerAsync() 时发生。
ProgressChanged	调用 ReportProgress(Int32) 时发生。
RunWorkerCompleted	当后台操作已完成、被取消或引发异常时发生。

适用于

另请参阅

• 如何：在后台运行操作
• 托管线程处理的最佳做法
• 如何：在后台下载文件