BackgroundWorker Class


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Executes an operation on a separate thread.

Namespace:   System.ComponentModel
Assembly:  System (in System.dll)


[HostProtectionAttribute(SecurityAction::LinkDemand, SharedState = true)]
public ref class BackgroundWorker : Component


Initializes a new instance of the BackgroundWorker class.


Gets a value indicating whether the application has requested cancellation of a background operation.


Gets a value indicating whether the component can raise an event.(Inherited from Component.)


Gets the IContainer that contains the Component.(Inherited from Component.)


Gets a value that indicates whether the Component is currently in design mode.(Inherited from Component.)


Gets the list of event handlers that are attached to this Component.(Inherited from Component.)


Gets a value indicating whether the BackgroundWorker is running an asynchronous operation.


Gets or sets the ISite of the Component.(Inherited from Component.)


Gets or sets a value indicating whether the BackgroundWorker can report progress updates.


Gets or sets a value indicating whether the BackgroundWorker supports asynchronous cancellation.


Requests cancellation of a pending background operation.


Creates an object that contains all the relevant information required to generate a proxy used to communicate with a remote object.(Inherited from MarshalByRefObject.)


Releases all resources used by the Component.(Inherited from Component.)


Releases the unmanaged resources used by the Component and optionally releases the managed resources.(Inherited from Component.)


Determines whether the specified object is equal to the current object.(Inherited from Object.)


Releases unmanaged resources and performs other cleanup operations before the Component is reclaimed by garbage collection.(Inherited from Component.)


Serves as the default hash function. (Inherited from Object.)


Retrieves the current lifetime service object that controls the lifetime policy for this instance.(Inherited from MarshalByRefObject.)


Returns an object that represents a service provided by the Component or by its Container.(Inherited from Component.)


Gets the Type of the current instance.(Inherited from Object.)


Obtains a lifetime service object to control the lifetime policy for this instance.(Inherited from MarshalByRefObject.)


Creates a shallow copy of the current Object.(Inherited from Object.)


Creates a shallow copy of the current MarshalByRefObject object.(Inherited from MarshalByRefObject.)


Raises the DoWork event.


Raises the ProgressChanged event.


Raises the RunWorkerCompleted event.


Raises the ProgressChanged event.

System_CAPS_pubmethodReportProgress(Int32, Object^)

Raises the ProgressChanged event.


Starts execution of a background operation.


Starts execution of a background operation.


Returns a String containing the name of the Component, if any. This method should not be overridden.(Inherited from Component.)


Occurs when the component is disposed by a call to the Dispose method. (Inherited from Component.)


Occurs when RunWorkerAsync is called.


Occurs when ReportProgress is called.


Occurs when the background operation has completed, has been canceled, or has raised an exception.

The BackgroundWorker class allows you to run an operation on a separate, dedicated thread. Time-consuming operations like downloads and database transactions can cause your user interface (UI) to seem as though it has stopped responding while they are running. When you want a responsive UI and you are faced with long delays associated with such operations, the BackgroundWorker class provides a convenient solution.

To execute a time-consuming operation in the background, create a BackgroundWorker and listen for events that report the progress of your operation and signal when your operation is finished. You can create the BackgroundWorker programmatically or you can drag it onto your form from the Components tab of the Toolbox. If you create the BackgroundWorker in the Windows Forms Designer, it will appear in the Component Tray, and its properties will be displayed in the Properties window.

To set up for a background operation, add an event handler for the DoWork event. Call your time-consuming operation in this event handler. To start the operation, call RunWorkerAsync. To receive notifications of progress updates, handle the ProgressChanged event. To receive a notification when the operation is completed, handle the RunWorkerCompleted event.


You must be careful not to manipulate any user-interface objects in your DoWork event handler. Instead, communicate to the user interface through the ProgressChanged and RunWorkerCompleted events.

BackgroundWorker events are not marshaled across AppDomain boundaries. Do not use a BackgroundWorker component to perform multithreaded operations in more than one AppDomain.

If your background operation requires a parameter, call RunWorkerAsync with your parameter. Inside the DoWork event handler, you can extract the parameter from the DoWorkEventArgs::Argument property.

For more information about BackgroundWorker, see How to: Run an Operation in the Background.

The following code example demonstrates the basics of the BackgroundWorker class for executing a time-consuming operation asynchronously. The following illustration shows an example of the output.

BackgroundWorker simple example

To try this code, create a Windows Forms application. Add a Label control named resultLabel and add two Button controls named startAsyncButton and cancelAsyncButton. Create Click event handlers for both buttons. From the Components tab of the Toolbox, add a BackgroundWorker component named backgroundWorker1. Create DoWork, ProgressChanged, and RunWorkerCompleted event handlers for the BackgroundWorker. In the code for the form, replace the existing code with the following code.

No code example is currently available or this language may not be supported.

The following code example demonstrates the use of the BackgroundWorker class for executing a time-consuming operation asynchronously. The following illustration shows an example of the output.

BackgroundWorker Fibonacci example

The operation computes the selected Fibonacci number, reports progress updates as the calculation proceeds, and permits a pending calculation to be canceled.

#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

   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;

      numberToCompute = highestPercentageReached = 0;


   // 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.

      // 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 );
      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";
         // 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;
         if ( n < 2 )
            result = 1;
            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;

      // 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";
      this->ResumeLayout( false );

int main()
   Application::Run( gcnew FibonacciForm );

Universal Windows Platform
Available since 10
.NET Framework
Available since 2.0
Available since 2.0
Windows Phone Silverlight
Available since 7.0

Any public static ( Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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