BackgroundWorker 类

BackgroundWorker 类

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 C#

public class BackgroundWorker : Component

C++

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。

注意

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​​。

注意

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

            ...{   

                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;

        }

        Windows Form Designer generated code#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());

        }

    }

}

C++

#

using   < System.Drawing.dll >

#

using   < System.dll >

#

using   < System.Windows.Forms.dll >

using   namespace  System;

using   namespace  System::Collections;

using   namespace  System::ComponentModel;

using   namespace  System::Drawing;

using   namespace  System::Threading;

using   namespace  System::Windows::Forms;

public   ref   class  FibonacciForm:  public  System::Windows::Forms::Form

... {

private:

   int numberToCompute;

   int highestPercentageReached;

   System::Windows::Forms::NumericUpDown^ numericUpDown1;

   System::Windows::Forms::Button^ startAsyncButton;

   System::Windows::Forms::Button^ cancelAsyncButton;

   System::Windows::Forms::ProgressBar^ progressBar1;

   System::Windows::Forms::Label ^ resultLabel;

   System::ComponentModel::BackgroundWorker^ backgroundWorker1;

public:

   FibonacciForm()

   ...{

      InitializeComponent();

      numberToCompute = highestPercentageReached = 0;

      InitializeBackgoundWorker();

   }

private:

   // Set up the BackgroundWorker object by 

   // attaching event handlers. 

   void InitializeBackgoundWorker()

   ...{

      backgroundWorker1->DoWork += gcnew DoWorkEventHandler( this, &FibonacciForm::backgroundWorker1_DoWork );

      backgroundWorker1->RunWorkerCompleted += gcnew RunWorkerCompletedEventHandler( this, &FibonacciForm::backgroundWorker1_RunWorkerCompleted );

      backgroundWorker1->ProgressChanged += gcnew ProgressChangedEventHandler( this, &FibonacciForm::backgroundWorker1_ProgressChanged );

   }

   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 );

   }

   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.

   void backgroundWorker1_DoWork( Object^ sender, DoWorkEventArgs^ e )

   ...{

      // Get the BackgroundWorker that raised this event.

      BackgroundWorker^ worker = dynamic_cast<BackgroundWorker^>(sender);

      // 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( safe_cast<Int32>(e->Argument), worker, e );

   }

   // This event handler deals with the results of the

   // background operation.

   void backgroundWorker1_RunWorkerCompleted( Object^ /**//*sender*/, RunWorkerCompletedEventArgs^ e )

   ...{

      // First, handle the case where an exception was thrown.

      if ( e->Error != nullptr )

      ...{

         MessageBox::Show( e->Error->Message );

      }

      else

      if ( e->Cancelled )

      ...{

         // Next, handle the case where the user cancelled 

         // 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 = "Cancelled";

      }

      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.

   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 gcnew ArgumentException( "value must be >= 0 and <= 91","n" );

      }

      long result = 0;

      

      // Abort the operation if the user has cancelled.

      // 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;

   }

   void InitializeComponent()

   ...{

      this->numericUpDown1 = gcnew System::Windows::Forms::NumericUpDown;

      this->startAsyncButton = gcnew System::Windows::Forms::Button;

      this->cancelAsyncButton = gcnew System::Windows::Forms::Button;

      this->resultLabel = gcnew System::Windows::Forms::Label;

      this->progressBar1 = gcnew System::Windows::Forms::ProgressBar;

      this->backgroundWorker1 = gcnew System::ComponentModel::BackgroundWorker;

      (dynamic_cast<System::ComponentModel::ISupportInitialize^>(this->numericUpDown1))->BeginInit();

      this->SuspendLayout();

      

      // 

      // numericUpDown1

      // 

      this->numericUpDown1->Location = System::Drawing::Point( 16, 16 );

      array<Int32>^temp0 = ...{91,0,0,0};

      this->numericUpDown1->Maximum = System::Decimal( temp0 );

      array<Int32>^temp1 = ...{1,0,0,0};

      this->numericUpDown1->Minimum = System::Decimal( temp1 );

      this->numericUpDown1->Name = "numericUpDown1";

      this->numericUpDown1->Size = System::Drawing::Size( 80, 20 );

      this->numericUpDown1->TabIndex = 0;

      array<Int32>^temp2 = ...{1,0,0,0};

      this->numericUpDown1->Value = System::Decimal( temp2 );

      // 

      // startAsyncButton

      // 

      this->startAsyncButton->Location = System::Drawing::Point( 16, 72 );

      this->startAsyncButton->Name = "startAsyncButton";

      this->startAsyncButton->Size = System::Drawing::Size( 120, 23 );

      this->startAsyncButton->TabIndex = 1;

      this->startAsyncButton->Text = "Start Async";

      this->startAsyncButton->Click += gcnew System::EventHandler( this, &FibonacciForm::startAsyncButton_Click );

      // 

      // cancelAsyncButton

      // 

      this->cancelAsyncButton->Enabled = false;

      this->cancelAsyncButton->Location = System::Drawing::Point( 153, 72 );

      this->cancelAsyncButton->Name = "cancelAsyncButton";

      this->cancelAsyncButton->Size = System::Drawing::Size( 119, 23 );

      this->cancelAsyncButton->TabIndex = 2;

      this->cancelAsyncButton->Text = "Cancel Async";

      this->cancelAsyncButton->Click += gcnew System::EventHandler( this, &FibonacciForm::cancelAsyncButton_Click );

      // 

      // resultLabel

      // 

      this->resultLabel->BorderStyle = System::Windows::Forms::BorderStyle::Fixed3D;

      this->resultLabel->Location = System::Drawing::Point( 112, 16 );

      this->resultLabel->Name = "resultLabel";

      this->resultLabel->Size = 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 = System::Drawing::Point( 18, 48 );

      this->progressBar1->Name = "progressBar1";

      this->progressBar1->Size = 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 = 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";

      (dynamic_cast<System::ComponentModel::ISupportInitialize^>(this->numericUpDown1))->EndInit();

      this->ResumeLayout( false );

   }

}

;

[STAThread]

int  main()

... {

   Application::Run( gcnew FibonacciForm );

}

 继承层次结构

​​System.Object​​

​​System.MarshalByRefObject​​

​​System.ComponentModel.Component​​

​System.ComponentModel.BackgroundWorker​

 线程安全

此类型的任何公共静态（Visual Basic 中的 ​Shared​）成员都是线程安全的，但不保证所有实例成员都是线程安全的。