ASP.NET实现大文件下载

https://support.microsoft.com/zh-cn/kb/812406

http://www.cnblogs.com/luisliu/p/4253815.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。

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 。（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。

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 = "application/octet-stream"; // 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();
            }
        }
    }
}

重要说明：当您在 ASP.NET 应用程序的 Web.config 文件中将编译元素的 debug 属性值设置为 false 时，必须针对要下载的文件大小将Server.ScriptTimeout 属性设置为适当的值。默认情况下，Server.ScriptTimeout 值被设置为 90 秒。但是，当 debug 属性被设置为 true时，Server.ScriptTimeout 值将被设置为一个非常大的值（30,000,000 秒）。作为一名开发人员，您必须知道这可能会对您的 ASP.NET Web 应用程序的行为造成的影响。

VB。NET代码如下：

Dim iStream As System.IO.Stream

      ' Buffer to read 10K bytes in chunk:
      Dim buffer(10000) As Byte

      ' Length of the file:
      Dim length As Integer

      ' Total bytes to read:
      Dim dataToRead As Long

      ' Identify the file to download including its path.
      Dim filepath As String = "DownloadFileName"

      ' Identify the file name.
      Dim filename As String = System.IO.Path.GetFileName(filepath)

      Try
         ' Open the file.
         iStream = New System.IO.FileStream(filepath, System.IO.FileMode.Open, _
                                                IO.FileAccess.Read, IO.FileShare.Read)

         ' Total bytes to read:
         dataToRead = iStream.Length
         
         Response.ContentType = "application/octet-stream"
         Response.AddHeader("Content-Disposition", "attachment; filename=" & filename)

         ' Read the bytes.
            While dataToRead > 0
                ' Verify that the client is connected.
                If Response.IsClientConnected Then
                    ' 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()

                    ReDim buffer(10000) ' Clear the buffer
                    dataToRead = dataToRead - length
                Else
                    'prevent infinite loop if user disconnects
                    dataToRead = -1
                End If
            End While

      Catch ex As Exception
         ' Trap the error, if any.
         Response.Write("Error : " & ex.Message)
      Finally
         If IsNothing(iStream) = False Then
            ' Close the file.
            iStream.Close()
         End If
      End Try