asp.net 文件下载实现代码

public static bool DownloadFile(HttpContext httpContext, string filePath, long speed)
{
bool ret = true;
try
{
#region--验证：HttpMethod，请求的文件是否存在
switch (httpContext.Request.HttpMethod.ToUpper())
{ //目前只支持GET和HEAD方法
case "GET":
case "HEAD":
break;
default:
httpContext.Response.StatusCode = 501;
return false;
}
if (!File.Exists(filePath))
{
httpContext.Response.StatusCode = 404;
return false;
}
#endregion
#region 定义局部变量
long startBytes = 0;
int packSize = 1024 * 10; //分块读取，每块10K bytes
string fileName = Path.GetFileName(filePath);
FileStream myFile = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
BinaryReader br = new BinaryReader(myFile);
long fileLength = myFile.Length;
int sleep = (int)Math.Ceiling(1000.0 * packSize / speed);//毫秒数：读取下一数据块的时间间隔
string lastUpdateTiemStr = File.GetLastWriteTimeUtc(filePath).ToString("r");
string eTag = HttpUtility.UrlEncode(fileName, Encoding.UTF8) + lastUpdateTiemStr;//便于恢复下载时提取请求头;
#endregion
#region--验证：文件是否太大，是否是续传，且在上次被请求的日期之后是否被修
if (myFile.Length > Int32.MaxValue)
{//-------文件太大了-------
httpContext.Response.StatusCode = 413;//请求实体太大
return false;
}

if (httpContext.Request.Headers["If-Range"] != null)//对应响应头ETag：文件名+文件最后修改时间
{
//----------上次被请求的日期之后被修改过--------------
if (httpContext.Request.Headers["If-Range"].Replace("\"", "") != eTag)
{//文件修改过
httpContext.Response.StatusCode = 412;//预处理失败
return false;
}
}
#endregion
try
{
#region -------添加重要响应头、解析请求头、相关验证-------------------
httpContext.Response.Clear();
httpContext.Response.Buffer = false;
httpContext.Response.AddHeader("Content-MD5", GetMD5Hash(myFile));//用于验证文件
httpContext.Response.AddHeader("Accept-Ranges", "bytes");//重要：续传必须
httpContext.Response.AppendHeader("ETag", "\"" + eTag + "\"");//重要：续传必须
httpContext.Response.AppendHeader("Last-Modified", lastUpdateTiemStr);//把最后修改日期写入响应
httpContext.Response.ContentType = "application/octet-stream";//MIME类型：匹配任意文件类型
httpContext.Response.AddHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(fileName, Encoding.UTF8).Replace("+", "%20"));
httpContext.Response.AddHeader("Content-Length", (fileLength - startBytes).ToString());
httpContext.Response.AddHeader("Connection", "Keep-Alive");
httpContext.Response.ContentEncoding = Encoding.UTF8;
if (httpContext.Request.Headers["Range"] != null)
{//------如果是续传请求，则获取续传的起始位置，即已经下载到客户端的字节数------
httpContext.Response.StatusCode = 206;//重要：续传必须，表示局部范围响应。初始下载时默认为200
string[] range = httpContext.Request.Headers["Range"].Split(new char[] { '=', '-' });//"bytes=1474560-"
startBytes = Convert.ToInt64(range[1]);//已经下载的字节数，即本次下载的开始位置
if (startBytes < 0 || startBytes >= fileLength)
{//无效的起始位置
return false;
}
}
if (startBytes > 0)
{//------如果是续传请求，告诉客户端本次的开始字节数，总长度，以便客户端将续传数据追加到startBytes位置后----------
httpContext.Response.AddHeader("Content-Range", string.Format(" bytes {0}-{1}/{2}", startBytes, fileLength - 1, fileLength));
}
#endregion
#region -------向客户端发送数据块-------------------
br.BaseStream.Seek(startBytes, SeekOrigin.Begin);
int maxCount = (int)Math.Ceiling((fileLength - startBytes + 0.0) / packSize);//分块下载，剩余部分可分成的块数
for (int i = 0; i < maxCount && httpContext.Response.IsClientConnected; i++)
{//客户端中断连接，则暂停
httpContext.Response.BinaryWrite(br.ReadBytes(packSize));
httpContext.Response.Flush();
if (sleep > 1) Thread.Sleep(sleep);
}
#endregion
}
catch
{
ret = false;
}
finally
{
br.Close();
myFile.Close();
}
}
catch
{
ret = false;
}
return ret;
}