Como: Executar uma operação em segundo plano

Se você tiver uma operação que levará muito tempo para ser concluída e não quiser causar atrasos na interface do usuário, poderá usar a classe BackgroundWorker para executar a operação em outro thread.

O exemplo de código a seguir mostra como executar uma operação demorada em segundo plano. O formulário possui botões intitulados Iniciar e Cancelar. Clique no botão Iniciar para executar uma operação assíncrona. Clique no botão Cancelar para interromper uma operação assíncrona em execução. O resultado de cada operação é apresentado num MessageBox.

Há suporte extensivo para essa tarefa no Visual Studio.

Consulte também Passo a passo: Executando uma operação em segundo plano.

Exemplo

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

namespace BackgroundWorkerExample
{
    public class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            // Do not access the form's BackgroundWorker reference directly.
            // Instead, use the reference provided by the sender parameter.
            BackgroundWorker bw = sender as BackgroundWorker;

            // Extract the argument.
            int arg = (int)e.Argument;

            // Start the time-consuming operation.
            e.Result = TimeConsumingOperation(bw, arg);

            // If the operation was canceled by the user,
            // set the DoWorkEventArgs.Cancel property to true.
            if (bw.CancellationPending)
            {
                e.Cancel = true;
            }
        }

        // This event handler demonstrates how to interpret
        // the outcome of the asynchronous operation implemented
        // in the DoWork event handler.
        private void backgroundWorker1_RunWorkerCompleted(
            object sender,
            RunWorkerCompletedEventArgs e)
        {
            if (e.Cancelled)
            {
                // The user canceled the operation.
                MessageBox.Show("Operation was canceled");
            }
            else if (e.Error != null)
            {
                // There was an error during the operation.
                string msg = String.Format("An error occurred: {0}", e.Error.Message);
                MessageBox.Show(msg);
            }
            else
            {
                // The operation completed normally.
                string msg = String.Format("Result = {0}", e.Result);
                MessageBox.Show(msg);
            }
        }

        // This method models an operation that may take a long time
        // to run. It can be cancelled, it can raise an exception,
        // or it can exit normally and return a result. These outcomes
        // are chosen randomly.
        private int TimeConsumingOperation(
            BackgroundWorker bw,
            int sleepPeriod )
        {
            int result = 0;

            Random rand = new Random();

            while (!bw.CancellationPending)
            {
                bool exit = false;

                switch (rand.Next(3))
                {
                    // Raise an exception.
                    case 0:
                    {
                        throw new Exception("An error condition occurred.");
                        break;
                    }

                    // Sleep for the number of milliseconds
                    // specified by the sleepPeriod parameter.
                    case 1:
                    {
                        Thread.Sleep(sleepPeriod);
                        break;
                    }

                    // Exit and return normally.
                    case 2:
                    {
                        result = 23;
                        exit = true;
                        break;
                    }

                    default:
                    {
                        break;
                    }
                }

                if( exit )
                {
                    break;
                }
            }

            return result;
        }

        private void startBtn_Click(object sender, EventArgs e)
        {
            this.backgroundWorker1.RunWorkerAsync(2000);
        }

        private void cancelBtn_Click(object sender, EventArgs e)
        {
            this.backgroundWorker1.CancelAsync();
        }

        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
            this.startBtn = new System.Windows.Forms.Button();
            this.cancelBtn = new System.Windows.Forms.Button();
            this.SuspendLayout();
            //
            // backgroundWorker1
            //
            this.backgroundWorker1.WorkerSupportsCancellation = true;
            this.backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
            this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
            //
            // startBtn
            //
            this.startBtn.Location = new System.Drawing.Point(12, 12);
            this.startBtn.Name = "startBtn";
            this.startBtn.Size = new System.Drawing.Size(75, 23);
            this.startBtn.TabIndex = 0;
            this.startBtn.Text = "Start";
            this.startBtn.Click += new System.EventHandler(this.startBtn_Click);
            //
            // cancelBtn
            //
            this.cancelBtn.Location = new System.Drawing.Point(94, 11);
            this.cancelBtn.Name = "cancelBtn";
            this.cancelBtn.Size = new System.Drawing.Size(75, 23);
            this.cancelBtn.TabIndex = 1;
            this.cancelBtn.Text = "Cancel";
            this.cancelBtn.Click += new System.EventHandler(this.cancelBtn_Click);
            //
            // Form1
            //
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(183, 49);
            this.Controls.Add(this.cancelBtn);
            this.Controls.Add(this.startBtn);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
        }

        #endregion

        private System.ComponentModel.BackgroundWorker backgroundWorker1;
        private System.Windows.Forms.Button startBtn;
        private System.Windows.Forms.Button cancelBtn;
    }

    public class Program
    {
        private Program()
        {
        }

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.Run(new Form1());
        }
    }
}

Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms

Public Class Form1
   Inherits Form
   
   Public Sub New()
      InitializeComponent()
   End Sub
   
   
   Private Sub backgroundWorker1_DoWork( _
   sender As Object, e As DoWorkEventArgs) _
   Handles backgroundWorker1.DoWork

      ' Do not access the form's BackgroundWorker reference directly.
      ' Instead, use the reference provided by the sender parameter.
      Dim bw As BackgroundWorker = CType( sender, BackgroundWorker )
      
      ' Extract the argument.
      Dim arg As Integer = Fix(e.Argument)
      
      ' Start the time-consuming operation.
      e.Result = TimeConsumingOperation(bw, arg)
      
      ' If the operation was canceled by the user, 
      ' set the DoWorkEventArgs.Cancel property to true.
      If bw.CancellationPending Then
         e.Cancel = True
      End If

   End Sub   

   ' This event handler demonstrates how to interpret 
   ' the outcome of the asynchronous operation implemented
   ' in the DoWork event handler.
   Private Sub backgroundWorker1_RunWorkerCompleted( _
   sender As Object, e As RunWorkerCompletedEventArgs) _
   Handles backgroundWorker1.RunWorkerCompleted

      If e.Cancelled Then
         ' The user canceled the operation.
         MessageBox.Show("Operation was canceled")
      ElseIf (e.Error IsNot Nothing) Then
         ' There was an error during the operation.
         Dim msg As String = String.Format("An error occurred: {0}", e.Error.Message)
         MessageBox.Show(msg)
      Else
         ' The operation completed normally.
         Dim msg As String = String.Format("Result = {0}", e.Result)
         MessageBox.Show(msg)
      End If
   End Sub   

   ' This method models an operation that may take a long time 
   ' to run. It can be cancelled, it can raise an exception,
   ' or it can exit normally and return a result. These outcomes
   ' are chosen randomly.
   Private Function TimeConsumingOperation( _
   bw As BackgroundWorker, _
   sleepPeriod As Integer) As Integer

      Dim result As Integer = 0
      
      Dim rand As New Random()
      
        While Not bw.CancellationPending
            Dim [exit] As Boolean = False

            Select Case rand.Next(3)
                ' Raise an exception.
                Case 0
                    Throw New Exception("An error condition occurred.")
                    Exit While

                    ' Sleep for the number of milliseconds
                    ' specified by the sleepPeriod parameter.
                Case 1
                    Thread.Sleep(sleepPeriod)
                    Exit While

                    ' Exit and return normally.
                Case 2
                    result = 23
                    [exit] = True
                    Exit While

                Case Else
                    Exit While
            End Select

            If [exit] Then
                Exit While
            End If
        End While
      
      Return result
   End Function

    Private Sub startButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles startBtn.Click
        Me.backgroundWorker1.RunWorkerAsync(2000)
    End Sub

    Private Sub cancelButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cancelBtn.Click
        Me.backgroundWorker1.CancelAsync()
    End Sub

    ' Required designer variable.
    Private components As System.ComponentModel.IContainer = Nothing


    ' Clean up any resources being used.
    ' <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
    Protected Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing AndAlso (components IsNot Nothing) Then
            components.Dispose()
        End If
        MyBase.Dispose(disposing)
    End Sub

#Region "Windows Form Designer generated code"


    ' Required method for Designer support - do not modify
    ' the contents of this method with the code editor.
    Private Sub InitializeComponent()
        Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
        Me.startBtn = New System.Windows.Forms.Button()
        Me.cancelBtn = New System.Windows.Forms.Button()
        Me.SuspendLayout()
        ' 
        ' backgroundWorker1
        ' 
        Me.backgroundWorker1.WorkerSupportsCancellation = True
        ' 
        ' startButton
        ' 
        Me.startBtn.Location = New System.Drawing.Point(12, 12)
        Me.startBtn.Name = "startButton"
        Me.startBtn.Size = New System.Drawing.Size(75, 23)
        Me.startBtn.TabIndex = 0
        Me.startBtn.Text = "Start"
        ' 
        ' cancelButton
        ' 
        Me.cancelBtn.Location = New System.Drawing.Point(94, 11)
        Me.cancelBtn.Name = "cancelButton"
        Me.cancelBtn.Size = New System.Drawing.Size(75, 23)
        Me.cancelBtn.TabIndex = 1
        Me.cancelBtn.Text = "Cancel"
        ' 
        ' Form1
        ' 
        Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0F, 13.0F)
        Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
        Me.ClientSize = New System.Drawing.Size(183, 49)
        Me.Controls.Add(cancelBtn)
        Me.Controls.Add(startBtn)
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.ResumeLayout(False)
    End Sub

#End Region

    Private WithEvents backgroundWorker1 As System.ComponentModel.BackgroundWorker
    Private WithEvents startBtn As System.Windows.Forms.Button
    Private WithEvents cancelBtn As System.Windows.Forms.Button
End Class 


Public Class Program

    Private Sub New()

    End Sub

    ' The main entry point for the application.
    <STAThread()> _
    Shared Sub Main()
        Application.EnableVisualStyles()
        Application.Run(New Form1())
    End Sub


End Class

Compilando o código

Este exemplo requer:

• Referências aos assemblies System, System.Drawing e System.Windows.Forms.

Ver também

• BackgroundWorker
• DoWorkEventArgs
• Como: Implementar um formulário que usa uma operação em segundo plano
• Componente BackgroundWorker