WP7下载大文件时,文件大小溢出的问题.(按Uri下载)

public const string URL = "http://media.ch9.ms/ch9/ca4a/1e246d1d-4e3f-44e6-9d62-9ee0015cca4a/TWOC9051311_2MB_ch9.wmv";

WebClient client = new WebClient();
client.OpenReadCompleted += new OpenReadCompletedEventHandler(client_OpenReadCompleted);
client.OpenReadAsync(new Uri(URL));

void client_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
{
    MemoryStream stream = new MemoryStream();

    byte[] data = new byte[16 * 1024];
    int read;

    while ((read = e.Result.Read(data, 0, data.Length)) > 0)
    {
        stream.Write(data, 0, read);
    }
    stream.Close();
}

在WP7中MemoryStream有16M的使用上限,所以读取这个70M的文件必然是失败的.当然,如果是连续的Stream的话,可以通过设置AllowReadStreamBuffering为false来解决问题,但是并没有多少是连续的Stream.

void client_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
{
    IsolatedStorageFile file = IsolatedStorageFile.GetUserStoreForApplication();

    using (IsolatedStorageFileStream stream = new IsolatedStorageFileStream("movie.wmv", FileMode.Create, file))
    {
        byte[] data = new byte[16 * 1024];
        int read;

        while ((read = e.Result.Read(data, 0, data.Length)) > 0)
        {
            stream.Write(data, 0, read);
        }
    }
}

如果是这段代码使用隔离存储,必然是成功的,但是非常影响效率.

小文件还好,如果是大文件,就等死吧.所以不要相信WebClient,当你需要传输大文件时,所以以下代码使用了HttpWebRequest.

IsolatedStorageFile file = IsolatedStorageFile.GetUserStoreForApplication();
IsolatedStorageFileStream streamToWriteTo = new IsolatedStorageFileStream("movie.wmv", FileMode.Create, file);
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);
request.AllowReadStreamBuffering = false;
request.BeginGetResponse(new AsyncCallback(GetData), request);

private void GetData(IAsyncResult result)
{
    HttpWebRequest request = (HttpWebRequest)result.AsyncState;
    HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(result);

    Stream str = response.GetResponseStream();

    byte[] data = new byte[16* 1024];
    int read;

    long totalValue = response.ContentLength;
    while ((read = str.Read(data, 0, data.Length)) > 0)
    {
        if (streamToWriteTo.Length != 0)
            Debug.WriteLine((int)((streamToWriteTo.Length * 100) / totalValue));

        streamToWriteTo.Write(data, 0, read);
    }
    streamToWriteTo.Close();
    Debug.WriteLine("COMPLETED");
}

More:

引用： 如果 AllowReadStreamBuffering 属性为 true，则在收到整个响应流并将其缓冲到内存中后将引发OpenReadCompleted 事件

因此如果文件过大，内存不够在完成之前就可能抛出内存不足异常。请设置为false，这样就不会在内存中缓存全部的文件数据。

Blog:

http://dotnet.dzone.com/articles/2-things-you-should-consider