C# 带有BeginGetRequestStream的InvalidOperationException

我遇到以下错误：

System.InvalidOperationException was unhandled by user code
  HResult=-2146233079
  Message=Operation is not valid due to the current state of the object.
  Source=System.Windows
  StackTrace:
       at System.Net.Browser.ClientHttpWebRequest.InternalEndGetRequestStream(IAsyncResult asyncResult)
       at System.Net.Browser.ClientHttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult)
       at SophosMobileControl.MainPage.<>c__DisplayClass5.<PushTestButton_Click>b__3(IAsyncResult ar)
       at System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClass18.<InvokeGetRequestStreamCallback>b__16(Object state2)
  InnerException: 

用户代码未处理System.InvalidOperationException HResult=-2146233079 Message=由于对象的当前状态，操作无效。 Source=System.Windows 堆栈跟踪： 位于System.Net.Browser.ClientHttpWebRequest.InternalEndGetRequestStream（IAsyncResult asyncResult） 位于System.Net.Browser.ClientHttpWebRequest.EndGetRequestStream（IAsyncResult asyncResult） 在SophosMobileControl.MainPage.c__显示Class5.b___3（IAsyncResult ar） 在System.Net.Browser.ClientHttpWebRequest.c__中显示Class18.b__16（对象状态2） 内部异常： 使用以下代码：

// Get the URI that the Microsoft Push Notification Service returns to the push client when creating a notification channel.
            // Normally, a web service would listen for URIs coming from the web client and maintain a list of URIs to send
            // notifications out to.
            string subscriptionUri = (App.Current.Resources["Locator"] as ViewModelLocator).Main.PushUri;

            HttpWebRequest sendNotificationRequest = (HttpWebRequest)WebRequest.Create(subscriptionUri);

            // Create an HTTPWebRequest that posts the toast notification to the Microsoft Push Notification Service.
            // HTTP POST is the only method allowed to send the notification.
            sendNotificationRequest.Method = "POST";

            // Create the toast message.
            string toastMessage = "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
            "<wp:Notification xmlns:wp=\"WPNotification\">" +
               "<wp:Toast>" +
                    "<wp:Text1>" + "Title" + "</wp:Text1>" +
                    "<wp:Text2>" + "Subtitle" + "</wp:Text2>" +                        
               "</wp:Toast> " +
            "</wp:Notification>";

            // Set the notification payload to send.
            byte[] notificationMessage = Encoding.UTF8.GetBytes(toastMessage);

            // Set the web request content length.
            sendNotificationRequest.ContentLength = notificationMessage.Length;
            sendNotificationRequest.ContentType = "text/xml";

            //sendNotificationRequest.Headers["X-WindowsPhone-Target"] = "toast";
            //sendNotificationRequest.Headers["X-NotificationClass"] = "2";

            sendNotificationRequest.BeginGetRequestStream(ar =>
            {
                using (Stream postStream = sendNotificationRequest.EndGetRequestStream(ar))
                {
                    postStream.Write(notificationMessage, 0, notificationMessage.Length);
                }
            }, sendNotificationRequest);

        sendNotificationRequest.BeginGetRequestStream(ar =>
        {
            using (Stream postStream = sendNotificationRequest.EndGetRequestStream(ar))
            {
                postStream.Write(notificationMessage, 0, notificationMessage.Length);
            }
        }, sendNotificationRequest);

//获取Microsoft推送通知服务在创建通知通道时返回给推送客户端的URI。
//通常，web服务将侦听来自web客户端的URI，并维护要发送的URI列表
//通知发送到。
字符串subscriptionUri=（App.Current.Resources[“Locator”]作为ViewModelLocator）；
HttpWebRequest sendNotificationRequest=（HttpWebRequest）WebRequest.Create（subscriptionUri）；
//创建一个HTTPWebRequest，将toast通知发布到Microsoft推送通知服务。
//HTTP POST是唯一允许发送通知的方法。
sendNotificationRequest.Method=“POST”；
//创建toast消息。
字符串toastMessage=“”+
"" +
"" +
“+”标题“+”+
“+”副标题“+”
" " +
"";
//设置要发送的通知有效负载。
byte[]notificationMessage=Encoding.UTF8.GetBytes（toastMessage）；
//设置web请求内容长度。
sendNotificationRequest.ContentLength=notificationMessage.Length；
sendNotificationRequest.ContentType=“text/xml”；
//sendNotificationRequest.Headers[“X-WindowsPhone-Target”]=“toast”；
//sendNotificationRequest.Headers[“X-NotificationClass”]=“2”；
sendNotificationRequest.BeginGetRequestStream（ar=>
{
使用（Stream postStream=sendNotificationRequest.EndGetRequestStream（ar））
{
Write（notificationMessage，0，notificationMessage.Length）；
}
}，发送通知请求）；

我的解决方案是更改以下代码：

// Get the URI that the Microsoft Push Notification Service returns to the push client when creating a notification channel.
            // Normally, a web service would listen for URIs coming from the web client and maintain a list of URIs to send
            // notifications out to.
            string subscriptionUri = (App.Current.Resources["Locator"] as ViewModelLocator).Main.PushUri;

            HttpWebRequest sendNotificationRequest = (HttpWebRequest)WebRequest.Create(subscriptionUri);

            // Create an HTTPWebRequest that posts the toast notification to the Microsoft Push Notification Service.
            // HTTP POST is the only method allowed to send the notification.
            sendNotificationRequest.Method = "POST";

            // Create the toast message.
            string toastMessage = "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
            "<wp:Notification xmlns:wp=\"WPNotification\">" +
               "<wp:Toast>" +
                    "<wp:Text1>" + "Title" + "</wp:Text1>" +
                    "<wp:Text2>" + "Subtitle" + "</wp:Text2>" +                        
               "</wp:Toast> " +
            "</wp:Notification>";

            // Set the notification payload to send.
            byte[] notificationMessage = Encoding.UTF8.GetBytes(toastMessage);

            // Set the web request content length.
            sendNotificationRequest.ContentLength = notificationMessage.Length;
            sendNotificationRequest.ContentType = "text/xml";

            //sendNotificationRequest.Headers["X-WindowsPhone-Target"] = "toast";
            //sendNotificationRequest.Headers["X-NotificationClass"] = "2";

            sendNotificationRequest.BeginGetRequestStream(ar =>
            {
                using (Stream postStream = sendNotificationRequest.EndGetRequestStream(ar))
                {
                    postStream.Write(notificationMessage, 0, notificationMessage.Length);
                }
            }, sendNotificationRequest);

        sendNotificationRequest.BeginGetRequestStream(ar =>
        {
            using (Stream postStream = sendNotificationRequest.EndGetRequestStream(ar))
            {
                postStream.Write(notificationMessage, 0, notificationMessage.Length);
            }
        }, sendNotificationRequest);

通过以下方式：

    sendNotificationRequest.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), sendNotificationRequest);

    void GetRequestStreamCallback(IAsyncResult callbackResult)
    {
        HttpWebRequest myRequest = (HttpWebRequest)callbackResult.AsyncState;
        // End the stream request operation
        Stream postStream = myRequest.EndGetRequestStream(callbackResult);

        // Add the post data to the web request
        postStream.Write(notificationMessage, 0, notificationMessage.Length);
        postStream.Close();
    }