ASP.NET 大文件下载的实现思路及代码

尊重原创：http://www.admin10000.com/document/5999.html

当我们的网站需要支持下载大文件时，如果不做控制可能会导致用户在访问下载页面时发生无响应，使得浏览器崩溃。可以参考如下代码来避免这个问题。

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.
            <span id="1_nwp" style=" auto; height: auto; float: none;"><a id="1_nwl" href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&app_id=0&c=news&cf=1001&ch=0&di=128&fv=18&is_app=0&jk=9be248377319fba5&k=long&k0=long&kdi0=0&luki=1&n=10&p=baidu&q=06011078_cpr&rb=0&rs=1&seller_id=1&sid=a5fb19733748e29b&ssp2=1&stid=0&t=tpclicked3_hc&td=1922429&tu=u1922429&u=http%3A%2F%2Fwww%2Eadmin10000%2Ecom%2Fdocument%2F5999%2Ehtml&urlid=0" target="_blank" mpid="1" style="text-decoration: none;"><span style="color:#0000ff;font-size:14px;auto;height:auto;float:none;">long</span></a></span> <span id="2_nwp" style=" auto; height: auto; float: none;"><a id="2_nwl" href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&app_id=0&c=news&cf=1001&ch=0&di=128&fv=18&is_app=0&jk=9be248377319fba5&k=data&k0=data&kdi0=0&luki=4&n=10&p=baidu&q=06011078_cpr&rb=0&rs=1&seller_id=1&sid=a5fb19733748e29b&ssp2=1&stid=0&t=tpclicked3_hc&td=1922429&tu=u1922429&u=http%3A%2F%2Fwww%2Eadmin10000%2Ecom%2Fdocument%2F5999%2Ehtml&urlid=0" target="_blank" mpid="2" style="text-decoration: none;"><span style="color:#0000ff;font-size:14px;auto;height:auto;float:none;">data</span></a></span>ToRead;
            // 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.
                <span id="3_nwp" style=" auto; height: auto; float: none;"><a id="3_nwl" href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&app_id=0&c=news&cf=1001&ch=0&di=128&fv=18&is_app=0&jk=9be248377319fba5&k=data&k0=data&kdi0=0&luki=4&n=10&p=baidu&q=06011078_cpr&rb=0&rs=1&seller_id=1&sid=a5fb19733748e29b&ssp2=1&stid=0&t=tpclicked3_hc&td=1922429&tu=u1922429&u=http%3A%2F%2Fwww%2Eadmin10000%2Ecom%2Fdocument%2F5999%2Ehtml&urlid=0" target="_blank" mpid="3" style="text-decoration: none;"><span style="color:#0000ff;font-size:14px;auto;height:auto;float:none;">data</span></a></span>ToRead = 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 (<span id="4_nwp" style=" auto; height: auto; float: none;"><a id="4_nwl" href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&app_id=0&c=news&cf=1001&ch=0&di=128&fv=18&is_app=0&jk=9be248377319fba5&k=data&k0=data&kdi0=0&luki=4&n=10&p=baidu&q=06011078_cpr&rb=0&rs=1&seller_id=1&sid=a5fb19733748e29b&ssp2=1&stid=0&t=tpclicked3_hc&td=1922429&tu=u1922429&u=http%3A%2F%2Fwww%2Eadmin10000%2Ecom%2Fdocument%2F5999%2Ehtml&urlid=0" target="_blank" mpid="4" style="text-decoration: none;"><span style="color:#0000ff;font-size:14px;auto;height:auto;float:none;">data</span></a></span>ToRead > 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();
            }
        }
    }
}

关于此代码的几点说明：

　　1. 将数据分成较小的部分，然后将其移动到输出流以供下载，从而获取这些数据。

　　2. 根据下载的文件类型来指定 Response.ContentType 。（参考OSChina的这个网址可以找到大部分文件类型的对照表：http://tool.oschina.net/commons）

　　3. 在每次写完response时记得调用 Response.Flush()

　　4. 在循环下载的过程中使用 Response.IsClientConnected 这个判断可以帮助程序尽早发现连接是否正常。若不正常，可以及早的放弃下载，以释放所占用的服务器资源。

　　5. 在下载结束后，需要调用 Response.End() 来保证当前线程可以在最后被终止掉。