断点续传其实就是在请求流的时候请求中间流，而不是每次重新从头来过 

HttpWebRequest Myrq = (HttpWebRequest)HttpWebRequest.CreateHttp(URL);

Myrq.Headers["Range"] = "bytes=1024-";//设置Range值

这句就是请求1024位置开始到末尾的流 

如果Myrq.Headers["Range"] = "bytes=1024-2048"; 

则表示请求1024到2048位置的数据  

有了这个方法，写断点续传就会简单许多了，错略的方法如下 

 

public

void DownFile(string URL, string Filename, ProgressBar Prog)

{

//打开上次下载的文件或新建文件

long lStartPos = 0;

IsolatedStorageFileStream fs;

if (IsolatedStorageFile.GetUserStoreForApplication().FileExists(Filename))

{

fs = IsolatedStorageFile.GetUserStoreForApplication().OpenFile(Filename, FileMode.Open);

lStartPos = fs.Length;

fs.Seek(lStartPos, System.IO.SeekOrigin.Current); //移动文件流中的当前指针

}

else

{

fs = IsolatedStorageFile.GetUserStoreForApplication().CreateFile(Filename);

lStartPos = 0;

}

try

{

HttpWebRequest Myrq = (HttpWebRequest)HttpWebRequest.CreateHttp(URL);

Myrq.Headers["User-Agent"] = “Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; .NET CLR 1.0.3705)”;

if (lStartPos > 0)

{

Myrq.Headers["Range"] = “bytes=” + lStartPos + “-”;//设置Range值

}

Myrq.BeginGetResponse(new

AsyncCallback((a) =>

{

var myrp = Myrq.EndGetResponse(a);

long totalBytes = myrp.ContentLength + fs.Length;

this.Dispatcher.BeginInvoke(() => { Prog.Maximum = (int)totalBytes; });

Stream st = myrp.GetResponseStream();

//Stream so = new FileStream(Filename, FileMode.Create);

long totalDownloadedByte = fs.Length;

byte[] by = new

byte[1024];

int osize = st.Read(by, 0, (int)by.Length);

while (osize > 0)

{

totalDownloadedByte = osize + totalDownloadedByte;

fs.Write(by, 0, osize);

this.Dispatcher.BeginInvoke(() => { Prog.Value = (int)totalDownloadedByte; });

osize = st.Read(by, 0, (int)by.Length);

}

fs.Close();

st.Close();

MessageBox.Show(“下载完毕!”);

}), null);

}

catch (Exception ex)

{

MessageBox.Show(“提示“ + ex.Message);

}

}