asp.net BackgroundWorker之在后台下载文件

示例：
下面的代码示例演示如何使用 BackgroundWorker 组件从 URL 加载 XML 文件。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。在下载过程中，将禁用该按钮，然后在下载完成后再启用该按钮。MessageBox 将显示文件的内容。

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 dowloadButton; 
private XmlDocument document = null; 
public Form1() 
{ 
InitializeComponent(); 
} 
private void dowloadButton_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.dowloadButton.Enabled = false; 
// Wait for the BackgroundWorker to finish the download. 
while (this.backgroundWorker1.IsBusy) 
{ 
// Keep UI messages moving, so the form remains 
// responsive during the asynchronous operation. 
Application.DoEvents(); 
} 
// The download is done, so enable the button. 
this.dowloadButton.Enabled = true; 
} 
private void backgroundWorker1_DoWork( 
object sender, 
DoWorkEventArgs e) 
{ 
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); 
} 
private void backgroundWorker1_RunWorkerCompleted( 
object sender, 
RunWorkerCompletedEventArgs e) 
{ 
if (e.Error == null) 
{ 
MessageBox.Show(document.InnerXml, "Download Complete"); 
} 
else 
{ 
MessageBox.Show( 
"Failed to download file", 
"Download failed", 
MessageBoxButtons.OK, 
MessageBoxIcon.Error ); 
} 
} 
/// <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.dowloadButton = new System.Windows.Forms.Button(); 
this.SuspendLayout(); 
// 
// backgroundWorker1 
// 
this.backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork); 
this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted); 
// 
// dowloadButton 
// 
this.dowloadButton.Location = new System.Drawing.Point(12, 12); 
this.dowloadButton.Name = "dowloadButton"; 
this.dowloadButton.Size = new System.Drawing.Size(75, 23); 
this.dowloadButton.TabIndex = 0; 
this.dowloadButton.Text = "Download file"; 
this.dowloadButton.UseVisualStyleBackColor = true; 
this.dowloadButton.Click += new System.EventHandler(this.dowloadButton_Click); 
// 
// Form1 
// 
this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F); 
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font; 
this.ClientSize = new System.Drawing.Size(104, 54); 
this.Controls.Add(this.dowloadButton); 
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()); 
} 
}

下载文件：
文件下载在 BackgroundWorker 组件的辅助线程上进行，该线程运行 DoWork 事件处理程序。当代码调用 RunWorkerAsync 方法时，将启动此线程。

private void backgroundWorker1_DoWork( 
object sender, 
DoWorkEventArgs e) 
{ 
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); 
}

等待 BackgroundWorker 完成
dowloadButton_Click 事件处理程序演示如何等待 BackgroundWorker 组件完成它的异步任务。使用 IsBusy 属性可以确定 BackgroundWorker 线程是否仍在运行。如果代码在主 UI 线程上（对于 Click 事件处理程序即是如此），请务必调用 Application.DoEvents 方法以使用户界面能够响应用户操作。

private void dowloadButton_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.dowloadButton.Enabled = false; 
// Wait for the BackgroundWorker to finish the download. 
while (this.backgroundWorker1.IsBusy) 
{ 
// Keep UI messages moving, so the form remains 
// responsive during the asynchronous operation. 
Application.DoEvents(); 
} 
// The download is done, so enable the button. 
this.dowloadButton.Enabled = true; 
}

显示结果
backgroundWorker1_RunWorkerCompleted 方法将处理 RunWorkerCompleted 事件，并在后台操作完成后被调用。它首先检查 AsyncCompletedEventArgs.Error 属性，如果该属性是 null，它将显示文件内容。

private void backgroundWorker1_RunWorkerCompleted( 
object sender, 
RunWorkerCompletedEventArgs e) 
{ 
if (e.Error == null) 
{ 
MessageBox.Show(document.InnerXml, "Download Complete"); 
} 
else 
{ 
MessageBox.Show( 
"Failed to download file", 
"Download failed", 
MessageBoxButtons.OK, 
MessageBoxIcon.Error ); 
} 
}