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Classe BackgroundWorker

Aggiornamento: novembre 2007

Esegue un'operazione in un thread separato.

Spazio dei nomi:  System.ComponentModel
Assembly:  System (in System.dll)

[HostProtectionAttribute(SecurityAction.LinkDemand, SharedState = true)]
public class BackgroundWorker : Component
/** @attribute HostProtectionAttribute(SecurityAction.LinkDemand, SharedState = true) */
public class BackgroundWorker extends Component
public class BackgroundWorker extends Component

Nota:

L'attributo HostProtectionAttribute applicato a questo tipo di membro dispone del seguente valore per la proprietà Resources: SharedState. L'oggetto HostProtectionAttribute non influisce sulle applicazioni desktop, che in genere vengono avviate facendo doppio clic sull'icona, digitando un comando oppure immettendo un URL in un browser. Per ulteriori informazioni, vedere la classe HostProtectionAttribute o Programmazione per SQL Server e attributi di protezione host.

La classe BackgroundWorker consente di eseguire un'operazione in un thread separato e dedicato. Durante l'esecuzione di operazioni impegnative come download e transazioni di database, è possibile che l'interfaccia utente sembri non rispondere. Quando si desidera un'interfaccia utente reattiva malgrado i notevoli ritardi associati a questo tipo di operazioni, la classe BackgroundWorker offre una soluzione adeguata.

Per eseguire un'operazione impegnativa in background, creare un oggetto BackgroundWorker e restare in attesa degli eventi che segnalano l'avanzamento e il completamento dell'operazione. È possibile creare la classe BackgroundWorker a livello di codice oppure trascinarla sul form in uso dalla scheda Componenti della Casella degli strumenti. Se si crea la classe BackgroundWorker in Progettazione Windows Form, essa verrà visualizzata nella Barra dei componenti e le sue proprietà nella finestra Proprietà.

Per impostare un'operazione in background, aggiungere un gestore eventi per l'evento DoWork. Chiamare l'operazione desiderata nel gestore eventi. Per avviare l'operazione, chiamare il metodo RunWorkerAsync. Per ricevere notifiche relative agli aggiornamenti dell'avanzamento, gestire l'evento ProgressChanged. Per ricevere una notifica quando l'operazione viene completata, gestire l'evento RunWorkerCompleted.

Nota:

Evitare di modificare oggetti dell'interfaccia utente nel gestore eventi DoWork. Comunicare invece con l'interfaccia utente tramite gli eventi ProgressChanged e RunWorkerCompleted.

Gli eventi BackgroundWorker non sono sottoposti a marshaling sui limiti di AppDomain. Non è possibile utilizzare un componente BackgroundWorker per eseguire operazioni multithread in più di un oggetto AppDomain.

Se l'operazione in background richiede un parametro, chiamare il metodo RunWorkerAsync tramite il parametro. All'interno del gestore eventi di DoWork, è possibile estrarre il parametro dalla proprietà DoWorkEventArgs.Argument.

Per ulteriori informazioni sull'oggetto BackgroundWorker, vedere Procedura: eseguire un'operazione in background.

Nell'esempio di codice seguente viene illustrato l'utilizzo della classe BackgroundWorker per l'esecuzione di un'operazione impegnativa in modo asincrono. L'operazione calcola il numero di Fibonacci selezionato, segnala gli aggiornamenti dell'avanzamento durante il calcolo e consente l'annullamento di un calcolo in sospeso.

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;
        }


		#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());
        }
    }
}



import System.* ;
import System.Collections.* ;
import System.ComponentModel.* ;
import System.Drawing.* ;
import System.Threading.* ;
import System.Windows.Forms.* ;

public class FibonacciForm extends 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();
    } //FibonacciForm

    // Set up the BackgroundWorker object by 
    // attaching event handlers. 
    private void InitializeBackgoundWorker()
    {
        backgroundWorker1.add_DoWork(new DoWorkEventHandler(
            backgroundWorker1_DoWork));
        backgroundWorker1.add_RunWorkerCompleted(
            new RunWorkerCompletedEventHandler(
            backgroundWorker1_RunWorkerCompleted));
        backgroundWorker1.add_ProgressChanged(
            new ProgressChangedEventHandler(backgroundWorker1_ProgressChanged));
    } //InitializeBackgoundWorker

    private void startAsyncButton_Click(Object sender, EventArgs e)
    {
        // Reset the text in the result label.
        resultLabel.set_Text(String.Empty);

        // Disable the UpDown control until 
        // the asynchronous operation is done.
        this.numericUpDown1.set_Enabled(false);

        // Disable the Start button until 
        // the asynchronous operation is done.
        this.startAsyncButton.set_Enabled(false);

        // Enable the Cancel button while 
        // the asynchronous operation runs.
        this.cancelAsyncButton.set_Enabled(true);

        // Get the value from the UpDown control.
        numberToCompute = System.Convert.ToInt32((numericUpDown1.get_Value()));

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

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

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

        // Disable the Cancel button.
        cancelAsyncButton.set_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 = (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.set_Result(new Long(ComputeFibonacci(System.Convert.ToInt32
            (e.get_Argument()), worker, e)));
        //e.Result = ComputeFibonacci((int)e.Argument, worker, e); 
    } //backgroundWorker1_DoWork

    // 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.get_Error() != null) {

            MessageBox.Show(e.get_Error().get_Message());
        }
        else {

            if (e.get_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.set_Text("Cancelled");
            }
            else {

                // Finally, handle the case where the operation 
                // succeeded.
                resultLabel.set_Text(e.get_Result().ToString());
            }
        }

        // Enable the UpDown control.
        this.numericUpDown1.set_Enabled(true);

        // Enable the Start button.
        startAsyncButton.set_Enabled(true);

        // Disable the Cancel button.
        cancelAsyncButton.set_Enabled(false);
    } //backgroundWorker1_RunWorkerCompleted

    // This event handler updates the progress bar.
    private void backgroundWorker1_ProgressChanged(Object sender,
        ProgressChangedEventArgs e)
    {
        this.progressBar1.set_Value(e.get_ProgressPercentage());
    } //backgroundWorker1_ProgressChanged

    // 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 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.get_CancellationPending()) {

            e.set_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.set_Location(new System.Drawing.Point(16, 16));
        this.numericUpDown1.set_Maximum(new System.Decimal(
            new int[] { 91, 0, 0, 0 }));
        this.numericUpDown1.set_Minimum(new System.Decimal(
            new int[] { 1, 0, 0, 0 }));
        this.numericUpDown1.set_Name("numericUpDown1");
        this.numericUpDown1.set_Size(new System.Drawing.Size(80, 20));
        this.numericUpDown1.set_TabIndex(0);
        this.numericUpDown1.set_Value(new System.Decimal(
            new int[] { 1, 0, 0, 0 }));

        // 
        // startAsyncButton
        // 
        this.startAsyncButton.set_Location(new System.Drawing.Point(16, 72));
        this.startAsyncButton.set_Name("startAsyncButton");
        this.startAsyncButton.set_Size(new System.Drawing.Size(120, 23));
        this.startAsyncButton.set_TabIndex(1);
        this.startAsyncButton.set_Text("Start Async");
        this.startAsyncButton.add_Click(
            new System.EventHandler(this.startAsyncButton_Click));

        // 
        // cancelAsyncButton
        // 
        this.cancelAsyncButton.set_Enabled(false);
        this.cancelAsyncButton.set_Location(new System.Drawing.Point(153, 72));
        this.cancelAsyncButton.set_Name("cancelAsyncButton");
        this.cancelAsyncButton.set_Size(new System.Drawing.Size(119, 23));
        this.cancelAsyncButton.set_TabIndex(2);
        this.cancelAsyncButton.set_Text("Cancel Async");
        this.cancelAsyncButton.add_Click(
            new System.EventHandler(this.cancelAsyncButton_Click));

        // 
        // resultLabel
        // 
        this.resultLabel.set_BorderStyle(System.Windows.Forms.BorderStyle.Fixed3D);
        this.resultLabel.set_Location(new System.Drawing.Point(112, 16));
        this.resultLabel.set_Name("resultLabel");
        this.resultLabel.set_Size(new System.Drawing.Size(160, 23));
        this.resultLabel.set_TabIndex(3);
        this.resultLabel.set_Text("(no result)");
        this.resultLabel.set_TextAlign(System.Drawing.ContentAlignment.MiddleCenter);

        // 
        // progressBar1
        // 
        this.progressBar1.set_Location(new System.Drawing.Point(18, 48));
        this.progressBar1.set_Name("progressBar1");
        this.progressBar1.set_Size(new System.Drawing.Size(256, 8));
        this.progressBar1.set_Step(2);
        this.progressBar1.set_TabIndex(4);

        // 
        // backgroundWorker1
        // 
        this.backgroundWorker1.set_WorkerReportsProgress(true);
        this.backgroundWorker1.set_WorkerSupportsCancellation(true);

        // 
        // FibonacciForm
        // 
        this.set_ClientSize(new System.Drawing.Size(292, 118));
        this.get_Controls().Add(this.progressBar1);
        this.get_Controls().Add(this.resultLabel);
        this.get_Controls().Add(this.cancelAsyncButton);
        this.get_Controls().Add(this.startAsyncButton);
        this.get_Controls().Add(this.numericUpDown1);
        this.set_Name("FibonacciForm");
        this.set_Text("Fibonacci Calculator");
        ((System.ComponentModel.ISupportInitialize)(this.numericUpDown1)).
            EndInit();
        this.ResumeLayout(false);
    } //InitializeComponent
    #endregion

    /** @attribute STAThread()
     */
    public static void main(String[] args)
    {
        Application.Run(new FibonacciForm());
    } //main
} //FibonacciForm



Qualsiasi membro static (Shared in Visual Basic) pubblico di questo tipo è thread-safe. I membri di istanza non sono garantiti come thread-safe.

Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition , Windows XP Starter Edition, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

.NET Framework e .NET Compact Framework non supportano tutte le versioni di ciascuna piattaforma. Per un elenco delle versioni supportate, vedere Requisiti di sistema di .NET Framework.

.NET Framework

Supportato in: 3.5, 3.0, 2.0

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