ASP.NET 大檔案下載程式與思路原理

當我們的網站需要支援下載大檔案時，如果不做控制可能會導致使用者在訪問下載頁面時發生無響應，使得瀏覽器崩潰。可以參考如下代碼來避免這個問題。

 

代碼如下

複製代碼

using System; namespace WebApplication1 {     public partial class DownloadFile : System.Web.UI.Page     {         protected void Page_Load(object sender, EventArgs e)         {             System.IO.Stream iStream = null;             // Buffer to read 10K bytes in chunk:             byte[] buffer = new Byte[10000];             // Length of the file:             int length;             // Total bytes to read.             long dataToRead;             // Identify the file to download including its path.             string filepath = Server.MapPath("/") +"./Files/TextFile1.txt";             // Identify the file name.             string filename = System.IO.Path.GetFileName(filepath);             try             {                 // Open the file.                 iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open,                             System.IO.FileAccess.Read, System.IO.FileShare.Read);                 // Total bytes to read.                 dataToRead = iStream.Length;                 Response.Clear();                 Response.ClearHeaders();                 Response.ClearContent();                 Response.ContentType = "text/plain"; // Set the file type                 Response.AddHeader("Content-Length", dataToRead.ToString());                 Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);                 // Read the bytes.                 while (dataToRead > 0)                 {                     // Verify that the client is connected.                     if (Response.IsClientConnected)                     {                         // Read the data in buffer.                         length = iStream.Read(buffer, 0, 10000);                         // Write the data to the current output stream.                         Response.OutputStream.Write(buffer, 0, length);                         // Flush the data to the HTML output.                         Response.Flush();                         buffer = new Byte[10000];                         dataToRead = dataToRead - length;                     }                     else                     {                         // Prevent infinite loop if user disconnects                         dataToRead = -1;                     }                 }             }             catch (Exception ex)             {                 // Trap the error, if any.                 Response.Write("Error : " + ex.Message);             }             finally             {                 if (iStream != null)                 {                     //Close the file.                     iStream.Close();                 }                 Response.End();             }         }     } }

關於此代碼的幾點說明：

1. 將資料分成較小的部分，然後將其移動到輸出資料流以供下載，從而擷取這些資料。

2. 根據下載的檔案類型來指定 Response.ContentType 。(參考OSChina的這個網址可以找到大部分檔案類型的對照表：http://tool.oschina.net/commons)

3. 在每次寫完response時記得調用 Response.Flush()

4. 在迴圈下載的過程中使用 Response.IsClientConnected 這個判斷可以協助程式儘早發現串連是否正常。若不正常，可以及早的放棄下載，以釋放所佔用的伺服器資源。

5. 在下載結束後，需要調用 Response.End() 來保證當前線程可以在最後被終止掉。