HttpWebRequest.EndGetResponse在Windows Phone 7中抛出NotSupportedException

Question

在一个Silverlight-Windows Phone 7项目中，我正在创建一个HttpWebRequest，获取RequestStream，在Stream中写入内容并尝试获取响应，但我总是得到一个NotSupportedException：“System.Net.Browser.OHWRAsyncResult.AsyncWaitHandle扔了一个类型'System.NotSupportedException'的异常

我的生产代码要复杂得多，但我可以将其缩小到这一小段代码：

public class HttpUploadHelper
{
    private HttpWebRequest request;
    private RequestState state = new RequestState();

    public HttpUploadHelper(string url)
    {
        this.request = WebRequest.Create(url) as HttpWebRequest;
        state.Request = request;
    }

    public void Execute()
    {
        request.Method = "POST";
        this.request.BeginGetRequestStream(
            new AsyncCallback(BeginRequest), state);
    }

    private void BeginRequest(IAsyncResult ar)
    {
        Stream stream = state.Request.EndGetRequestStream(ar);
        state.Request.BeginGetResponse(
            new AsyncCallback(BeginResponse), state);
    }

    private void BeginResponse(IAsyncResult ar)
    {
        // BOOM: NotSupportedException was unhandled; 
        // {System.Net.Browser.OHWRAsyncResult}
        // AsyncWaitHandle = 'ar.AsyncWaitHandle' threw an 
        // exception of type 'System.NotSupportedException'
        HttpWebResponse response = state.Request.EndGetResponse(ar) as HttpWebResponse;
        Debug.WriteLine(response.StatusCode);
    }
}

public class RequestState
{
    public WebRequest Request;
}

}

有人知道这段代码有什么问题吗？

Answer 1

在调用EndGetResponse之前未关闭请求流时，可能会抛出NotSupportedException 。 当您尝试获取响应时，WebRequest流仍处于打开状态并将数据发送到服务器。 由于stream实现了IDisposable接口，因此一个简单的解决方案是使用using块中的请求流来包装代码：

private void BeginRequest(IAsyncResult ar)
{
    using (Stream stream = request.EndGetRequestStream(ar))
    {
        //write to stream in here.
    }
    state.Request.BeginGetResponse(
        new AsyncCallback(BeginResponse), state);
}

在尝试从Web服务器获取响应之前，using块将确保关闭流。

Answer 2

问题在于您如何处理从BeginGetResponse回调中访问原始请求的方法。

而不是保持对状态的引用，而是通过以下方式获取对原始请求的引用：

var request = (HttpWebRequest)asynchronousResult.AsyncState;

看一下通过向网站发布电子邮件和密码凭证来实现登录的这个非常基本（但有效）的示例。

public static string Email;

public static string Password;


private void LoginClick(object sender, RoutedEventArgs e)
{
    Email = enteredEmailAddress.Text.Trim().ToLower();

    Password = enteredPassword.Password;

    var request = (HttpWebRequest)WebRequest.Create(App.Config.ServerUris.Login);

    request.ContentType = "application/x-www-form-urlencoded";
    request.Method = "POST";
    request.BeginGetRequestStream(ReadCallback, request);
}

private void ReadCallback(IAsyncResult asynchronousResult)
{
    var request = (HttpWebRequest)asynchronousResult.AsyncState;

    using (var postStream = request.EndGetRequestStream(asynchronousResult))
    {
        using (var memStream = new MemoryStream())
        {
            var content = string.Format("Password={0}&Email={1}",
                                        HttpUtility.UrlEncode(Password), 
                                        HttpUtility.UrlEncode(Email));

            var bytes = System.Text.Encoding.UTF8.GetBytes(content);

            memStream.Write(bytes, 0, bytes.Length);

            memStream.Position = 0;
            var tempBuffer = new byte[memStream.Length];
            memStream.Read(tempBuffer, 0, tempBuffer.Length);

            postStream.Write(tempBuffer, 0, tempBuffer.Length);
        }
    }

    request.BeginGetResponse(ResponseCallback, request);
}

private void ResponseCallback(IAsyncResult asynchronousResult)
{
    var request = (HttpWebRequest)asynchronousResult.AsyncState;

    using (var resp = (HttpWebResponse)request.EndGetResponse(asynchronousResult))
    {
        using (var streamResponse = resp.GetResponseStream())
        {
            using (var streamRead = new StreamReader(streamResponse))
            {
                string responseString = streamRead.ReadToEnd();

                // do something with responseString to check if login was successful
            }
        }
    }
}

Answer 3

当string url太长时，也会抛出NotSupportedException 。 我搞砸了并将帖子数据附加到url而不是post body。 当数据很短时，它工作正常，但一旦它变得太大 - EndGetResponse崩溃了。

Answer 4

改变这个：

    state.Request.BeginGetResponse(
        new AsyncCallback(BeginResponse), state);

对此：

    state.Request.BeginGetResponse(BeginResponse, state);