注意：此类在 .NET Framework 2.0 版中是新增的。
在单独的线程上执行操作。
命名空间:System.ComponentModel
程序集:System（在 system.dll 中）
语法 public class BackgroundWorker : Component
public ref class BackgroundWorker : public Component
备注
BackgroundWorker 类允许您在单独的专用线程上运行操作。耗时的操作（如下载和数据库事务）在长时间运行时可能会导致用户界面 (UI) 似乎处于停止响应状态。如果您需要能进行响应的用户界面，而且面临与这类操作相关的长时间延迟，则可以使用 BackgroundWorker 类方便地解决问题。
若要在后台执行耗时的操作，请创建一个 BackgroundWorker，侦听那些报告操作进度并在操作完成时发出信号的事件。可以通过编程方式创建 BackgroundWorker，也可以将它从“工具箱”的“组件”选项卡中拖到窗体上。如果在 Windows 窗体设计器中创建 BackgroundWorker，则它会出现在组件栏中，而且它的属性会显示在“属性”窗口中。
若要设置后台操作，请为 DoWork 事件添加一个事件处理程序。在此事件处理程序中调用耗时的操作。若要启动该操作，请调用 RunWorkerAsync。若要收到进度更新通知，请对 ProgressChanged 事件进行处理。若要在操作完成时收到通知，请对 RunWorkerCompleted 事件进行处理。
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您必须非常小心，确保在 DoWork 事件处理程序中不操作任何用户界面对象。而应该通过 ProgressChanged 和 RunWorkerCompleted 事件与用户界面进行通信。 BackgroundWorker 事件不跨 AppDomain 边界进行封送处理。请不要使用 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，请在调用 RunWorkerAsync 时给出参数。在 DoWork 事件处理程序内部，可以从 DoWorkEventArgs.Argument 属性中提取该参数。
有关 BackgroundWorker 的更多信息，请参见 如何：在后台运行操作。
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应用于此类的 HostProtectionAttribute 属性 (Attribute) 具有以下 Resources 属性 (Property) 值：SharedState。HostProtectionAttribute 不影响桌面应用程序（这些应用程序通常通过双击图标、键入命令或在浏览器中输入 URL 来启动）。有关更多信息，请参见 HostProtectionAttribute 类或 SQL Server 编程和宿主保护属性。 |
示例 下面的代码示例演示如何使用 BackgroundWorker 类异步执行耗时的操作。该操作计算选定的斐波纳契数，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(); InitializeBackgoundWorker();
} // Set up the BackgroundWorker object by // attaching event handlers. private void InitializeBackgoundWorker() ...{ 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 ...