代碼如下:

public static bool DownloadFile(HttpContext httpContext, string filePath, long speed) 
{ 
bool ret = true; 
try 
{ 
#region--驗證：HttpMethod，請求的文件是否存在 
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 定義局部變量 
long startBytes = 0; 
int packSize = 1024 * 10; //分塊讀取，每塊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);//毫秒數：讀取下一數據塊的時間間隔 
string lastUpdateTiemStr = File.GetLastWriteTimeUtc(filePath).ToString("r"); 
string eTag = HttpUtility.UrlEncode(fileName, Encoding.UTF8) + lastUpdateTiemStr;//便于恢復下載時提取請求頭; 
#endregion 
#region--驗證：文件是否太大，是否是續傳，且在上次被請求的日期之后是否被修 
if (myFile.Length > Int32.MaxValue) 
{//-------文件太大了------- 
httpContext.Response.StatusCode = 413;//請求實體太大 
return false; 
} 

if (httpContext.Request.Headers["If-Range"] != null)//對應響應頭ETag：文件名+文件最后修改時間 
{ 
//----------上次被請求的日期之后被修改過-------------- 
if (httpContext.Request.Headers["If-Range"].Replace("\"", "") != eTag) 
{//文件修改過 
httpContext.Response.StatusCode = 412;//預處理失敗 
return false; 
} 
} 
#endregion 
try 
{ 
#region -------添加重要響應頭、解析請求頭、相關驗證------------------- 
httpContext.Response.Clear(); 
httpContext.Response.Buffer = false; 
httpContext.Response.AddHeader("Content-MD5", GetMD5Hash(myFile));//用于驗證文件 
httpContext.Response.AddHeader("Accept-Ranges", "bytes");//重要：續傳必須 
httpContext.Response.AppendHeader("ETag", "\"" + eTag + "\"");//重要：續傳必須 
httpContext.Response.AppendHeader("Last-Modified", lastUpdateTiemStr);//把最后修改日期寫入響應 
httpContext.Response.ContentType = "application/octet-stream";//MIME類型：匹配任意文件類型 
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) 
{//------如果是續傳請求，則獲取續傳的起始位置，即已經下載到客戶端的字節數------ 
httpContext.Response.StatusCode = 206;//重要：續傳必須，表示局部范圍響應。初始下載時默認為200 
string[] range = httpContext.Request.Headers["Range"].Split(new char[] { '=', '-' });//"bytes=1474560-" 
startBytes = Convert.ToInt64(range[1]);//已經下載的字節數，即本次下載的開始位置 
if (startBytes < 0 || startBytes >= fileLength) 
{//無效的起始位置 
return false; 
} 
} 
if (startBytes > 0) 
{//------如果是續傳請求，告訴客戶端本次的開始字節數，總長度，以便客戶端將續傳數據追加到startBytes位置后---------- 
httpContext.Response.AddHeader("Content-Range", string.Format(" bytes {0}-{1}/{2}", startBytes, fileLength - 1, fileLength)); 
} 
#endregion 
#region -------向客戶端發送數據塊------------------- 
br.BaseStream.Seek(startBytes, SeekOrigin.Begin); 
int maxCount = (int)Math.Ceiling((fileLength - startBytes + 0.0) / packSize);//分塊下載，剩余部分可分成的塊數 
for (int i = 0; i < maxCount && httpContext.Response.IsClientConnected; i++) 
{//客戶端中斷連接，則暫停 
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; 
}