ASP.NET实现大文件下载

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

/article/5413824.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