如何：在后台下载文件

下载文件是一项常见任务，在单独的线程上运行这种可能耗时的操作通常很有用。 使用 BackgroundWorker 组件通过很少的代码完成此任务。

例

下面的代码示例演示如何使用 BackgroundWorker 组件从 URL 加载 XML 文件。 当用户单击 “下载”按钮时，Click 事件处理程序将调用 BackgroundWorker 组件的 RunWorkerAsync 方法来启动下载操作。 按钮在下载期间处于禁用状态，然后在下载完成后启用。 MessageBox 用于显示文件的内容。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Threading;
using System.Windows.Forms;
using System.Xml;

public class Form1 : Form
{
    private BackgroundWorker backgroundWorker1;
    private Button downloadButton;
    private ProgressBar progressBar1;
    private XmlDocument document = null;

    public Form1()
    {
        InitializeComponent();

        // Instantiate BackgroundWorker and attach handlers to its
        // DoWork and RunWorkerCompleted events.
        backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
        backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
        backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
    }

    private void downloadButton_Click(object sender, EventArgs e)
    {
        // Start the download operation in the background.
        this.backgroundWorker1.RunWorkerAsync();

        // Disable the button for the duration of the download.
        this.downloadButton.Enabled = false;

        // Once you have started the background thread you
        // can exit the handler and the application will
        // wait until the RunWorkerCompleted event is raised.

        // Or if you want to do something else in the main thread,
        // such as update a progress bar, you can do so in a loop
        // while checking IsBusy to see if the background task is
        // still running.

        while (this.backgroundWorker1.IsBusy)
        {
            progressBar1.Increment(1);
            // Keep UI messages moving, so the form remains
            // responsive during the asynchronous operation.
            Application.DoEvents();
        }
    }

    private void backgroundWorker1_DoWork(
        object sender,
        DoWorkEventArgs e)
    {
        document = new XmlDocument();

        // Uncomment the following line to
        // simulate a noticeable latency.
        //Thread.Sleep(5000);

        // Replace this file name with a valid file name.
        document.Load(@"http://www.tailspintoys.com/sample.xml");
    }

    private void backgroundWorker1_RunWorkerCompleted(
        object sender,
        RunWorkerCompletedEventArgs e)
    {
        // Set progress bar to 100% in case it's not already there.
        progressBar1.Value = 100;

        if (e.Error == null)
        {
            MessageBox.Show(document.InnerXml, "Download Complete");
        }
        else
        {
            MessageBox.Show(
                "Failed to download file",
                "Download failed",
                MessageBoxButtons.OK,
                MessageBoxIcon.Error);
        }

        // Enable the download button and reset the progress bar.
        this.downloadButton.Enabled = true;
        progressBar1.Value = 0;
    }

    #region Windows Form Designer generated code

    /// <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);
    }

    /// <summary>
    /// Required method for Designer support
    /// </summary>
    private void InitializeComponent()
    {
        this.downloadButton = new System.Windows.Forms.Button();
        this.progressBar1 = new System.Windows.Forms.ProgressBar();
        this.SuspendLayout();
        //
        // downloadButton
        //
        this.downloadButton.Location = new System.Drawing.Point(12, 12);
        this.downloadButton.Name = "downloadButton";
        this.downloadButton.Size = new System.Drawing.Size(100, 23);
        this.downloadButton.TabIndex = 0;
        this.downloadButton.Text = "Download file";
        this.downloadButton.UseVisualStyleBackColor = true;
        this.downloadButton.Click += new System.EventHandler(this.downloadButton_Click);
        //
        // progressBar1
        //
        this.progressBar1.Location = new System.Drawing.Point(12, 50);
        this.progressBar1.Name = "progressBar1";
        this.progressBar1.Size = new System.Drawing.Size(100, 26);
        this.progressBar1.TabIndex = 1;
        //
        // Form1
        //
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(133, 104);
        this.Controls.Add(this.progressBar1);
        this.Controls.Add(this.downloadButton);
        this.Name = "Form1";
        this.Text = "Form1";
        this.ResumeLayout(false);
    }

    #endregion
}

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

Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms
Imports System.Xml

Public Class Form1
    Inherits Form

    Private WithEvents downloadButton As Button
    Private WithEvents progressBar1 As ProgressBar
    Private WithEvents backgroundWorker1 As BackgroundWorker
    Private document As XmlDocument = Nothing

    Public Sub New()
        InitializeComponent()
        Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
    End Sub

    Private Sub downloadButton_Click( _
        ByVal sender As Object, _
        ByVal e As EventArgs) _
        Handles downloadButton.Click

        ' Start the download operation in the background.
        Me.backgroundWorker1.RunWorkerAsync()

        ' Disable the button for the duration of the download.
        Me.downloadButton.Enabled = False

        ' Once you have started the background thread you 
        ' can exit the handler and the application will 
        ' wait until the RunWorkerCompleted event is raised.

        ' If you want to do something else in the main thread,
        ' such as update a progress bar, you can do so in a loop 
        ' while checking IsBusy to see if the background task is
        ' still running.
        While Me.backgroundWorker1.IsBusy
            progressBar1.Increment(1)
            ' Keep UI messages moving, so the form remains 
            ' responsive during the asynchronous operation.
            Application.DoEvents()
        End While
    End Sub

    Private Sub backgroundWorker1_DoWork( _
        ByVal sender As Object, _
        ByVal e As DoWorkEventArgs) _
        Handles backgroundWorker1.DoWork

        document = New XmlDocument()

        ' Replace this file name with a valid file name.
        document.Load("http://www.tailspintoys.com/sample.xml")

        ' Uncomment the following line to
        ' simulate a noticeable latency.
        'Thread.Sleep(5000);
    End Sub

    Private Sub backgroundWorker1_RunWorkerCompleted( _
        ByVal sender As Object, _
        ByVal e As RunWorkerCompletedEventArgs) _
        Handles backgroundWorker1.RunWorkerCompleted

        ' Set progress bar to 100% in case it isn't already there.
        progressBar1.Value = 100

        If e.Error Is Nothing Then
            MessageBox.Show(document.InnerXml, "Download Complete")
        Else
            MessageBox.Show("Failed to download file", "Download failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If

        ' Enable the download button and reset the progress bar.
        Me.downloadButton.Enabled = True
        progressBar1.Value = 0
    End Sub

#Region "Windows Form Designer generated code"
    ' 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

    ' Required method for Designer support - do not modify
    ' the contents of this method with the code editor.
    Private Sub InitializeComponent()
        Me.downloadButton = New System.Windows.Forms.Button
        Me.progressBar1 = New System.Windows.Forms.ProgressBar
        Me.SuspendLayout()
        '
        'downloadButton
        '
        Me.downloadButton.Location = New System.Drawing.Point(12, 12)
        Me.downloadButton.Name = "downloadButton"
        Me.downloadButton.Size = New System.Drawing.Size(100, 23)
        Me.downloadButton.TabIndex = 0
        Me.downloadButton.Text = "Download file"
        Me.downloadButton.UseVisualStyleBackColor = True
        '
        'progressBar1
        '
        Me.progressBar1.Location = New System.Drawing.Point(12, 50)
        Me.progressBar1.Name = "progressBar1"
        Me.progressBar1.Size = New System.Drawing.Size(100, 26)
        Me.progressBar1.TabIndex = 1
        '
        'Form1
        '
        Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0!, 13.0!)
        Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
        Me.ClientSize = New System.Drawing.Size(136, 104)
        Me.Controls.Add(Me.downloadButton)
        Me.Controls.Add(Me.progressBar1)
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.ResumeLayout(False)

    End Sub

#End Region
End Class


Public Class Program

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

下载文件

该文件在 BackgroundWorker 组件的辅助线程上下载，该线程运行 DoWork 事件处理程序。 此线程在代码调用 RunWorkerAsync 方法时启动。

private void backgroundWorker1_DoWork(
    object sender,
    DoWorkEventArgs e)
{
    document = new XmlDocument();

    // Uncomment the following line to
    // simulate a noticeable latency.
    //Thread.Sleep(5000);

    // Replace this file name with a valid file name.
    document.Load(@"http://www.tailspintoys.com/sample.xml");
}

Private Sub backgroundWorker1_DoWork( _
    ByVal sender As Object, _
    ByVal e As DoWorkEventArgs) _
    Handles backgroundWorker1.DoWork

    document = New XmlDocument()

    ' Replace this file name with a valid file name.
    document.Load("http://www.tailspintoys.com/sample.xml")

    ' Uncomment the following line to
    ' simulate a noticeable latency.
    'Thread.Sleep(5000);
End Sub

等待 BackgroundWorker 完成

downloadButton_Click 事件处理程序演示如何等待 BackgroundWorker 组件完成其异步任务。

如果只想让应用程序响应事件，并且不希望在等待后台线程完成时在主线程中执行任何工作，只需退出处理程序。

如果要继续在主线程中执行工作，请使用 IsBusy 属性来确定 BackgroundWorker 线程是否仍在运行。 在这个示例中，在下载处理的同时会更新进度条。 请务必调用 Application.DoEvents 方法以使 UI 保持响应。

private void downloadButton_Click(object sender, EventArgs e)
{
    // Start the download operation in the background.
    this.backgroundWorker1.RunWorkerAsync();

    // Disable the button for the duration of the download.
    this.downloadButton.Enabled = false;

    // Once you have started the background thread you
    // can exit the handler and the application will
    // wait until the RunWorkerCompleted event is raised.

    // Or if you want to do something else in the main thread,
    // such as update a progress bar, you can do so in a loop
    // while checking IsBusy to see if the background task is
    // still running.

    while (this.backgroundWorker1.IsBusy)
    {
        progressBar1.Increment(1);
        // Keep UI messages moving, so the form remains
        // responsive during the asynchronous operation.
        Application.DoEvents();
    }
}

Private Sub downloadButton_Click( _
    ByVal sender As Object, _
    ByVal e As EventArgs) _
    Handles downloadButton.Click

    ' Start the download operation in the background.
    Me.backgroundWorker1.RunWorkerAsync()

    ' Disable the button for the duration of the download.
    Me.downloadButton.Enabled = False

    ' Once you have started the background thread you 
    ' can exit the handler and the application will 
    ' wait until the RunWorkerCompleted event is raised.

    ' If you want to do something else in the main thread,
    ' such as update a progress bar, you can do so in a loop 
    ' while checking IsBusy to see if the background task is
    ' still running.
    While Me.backgroundWorker1.IsBusy
        progressBar1.Increment(1)
        ' Keep UI messages moving, so the form remains 
        ' responsive during the asynchronous operation.
        Application.DoEvents()
    End While
End Sub

显示结果

backgroundWorker1_RunWorkerCompleted 方法处理 RunWorkerCompleted 事件，并在后台操作完成时调用。 此方法首先检查 AsyncCompletedEventArgs.Error 属性。 如果 AsyncCompletedEventArgs.Errornull，则此方法将显示文件的内容。 然后，它启用下载按钮，该按钮在下载开始时被禁用，并重置进度栏。

private void backgroundWorker1_RunWorkerCompleted(
    object sender,
    RunWorkerCompletedEventArgs e)
{
    // Set progress bar to 100% in case it's not already there.
    progressBar1.Value = 100;

    if (e.Error == null)
    {
        MessageBox.Show(document.InnerXml, "Download Complete");
    }
    else
    {
        MessageBox.Show(
            "Failed to download file",
            "Download failed",
            MessageBoxButtons.OK,
            MessageBoxIcon.Error);
    }

    // Enable the download button and reset the progress bar.
    this.downloadButton.Enabled = true;
    progressBar1.Value = 0;
}

Private Sub backgroundWorker1_RunWorkerCompleted( _
    ByVal sender As Object, _
    ByVal e As RunWorkerCompletedEventArgs) _
    Handles backgroundWorker1.RunWorkerCompleted

    ' Set progress bar to 100% in case it isn't already there.
    progressBar1.Value = 100

    If e.Error Is Nothing Then
        MessageBox.Show(document.InnerXml, "Download Complete")
    Else
        MessageBox.Show("Failed to download file", "Download failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End If

    ' Enable the download button and reset the progress bar.
    Me.downloadButton.Enabled = True
    progressBar1.Value = 0
End Sub

编译代码

此示例需要：

• 对 System.Drawing、System.Windows.Forms 和 System.Xml 程序集的引用。

可靠的编程

在尝试访问 RunWorkerCompletedEventArgs.Result 属性或可能受 DoWork 事件处理程序影响的任何其他对象之前，请始终检查 RunWorkerCompleted 事件处理程序中的 AsyncCompletedEventArgs.Error 属性。

另请参阅

• BackgroundWorker
• 操作方法：在后台运行操作
• 如何：实施一个使用后台操作的窗体