asp.net BackgroundWorker之在后臺下載文件

更新時間：2011年12月23日 00:14:25 作者：

下載文件是常見任務，通常情況下，最好以單獨的線程來運行這項可能很耗時的操作。使用 BackgroundWorker 組件可以用非常少的代碼完成此任務

示例：
下面的代碼示例演示如何使用 BackgroundWorker 組件從 URL 加載 XML 文件。用戶單擊“下載”按鈕時，Click 事件處理程序將調用 BackgroundWorker 組件的 RunWorkerAsync 方法來啟動下載操作。在下載過程中，將禁用該按鈕，然后在下載完成后再啟用該按鈕。MessageBox 將顯示文件的內容。

復制代碼代碼如下:

 
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()); 
} 
} 

下載文件：
文件下載在 BackgroundWorker 組件的輔助線程上進行，該線程運行 DoWork 事件處理程序。當代碼調用 RunWorkerAsync 方法時，將啟動此線程。

復制代碼代碼如下:

 
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 事件處理程序演示如何等待 BackgroundWorker 組件完成它的異步任務。使用 IsBusy 屬性可以確定 BackgroundWorker 線程是否仍在運行。如果代碼在主 UI 線程上（對于 Click 事件處理程序即是如此），請務必調用 Application.DoEvents 方法以使用戶界面能夠響應用戶操作。

復制代碼代碼如下:

 
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; 
} 

顯示結果
backgroundWorker1_RunWorkerCompleted 方法將處理 RunWorkerCompleted 事件，并在后臺操作完成后被調用。它首先檢查 AsyncCompletedEventArgs.Error 屬性，如果該屬性是 null，它將顯示文件內容。

復制代碼代碼如下:

 
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 ); 
} 
} 