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        private void button3_Click(object sender, RoutedEventArgs e)
        {
            WebClient client = new WebClient();
            client.UploadStringCompleted += new UploadStringCompletedEventHandler(client_UploadStringCompleted);
            client.UploadStringAsync(new Uri("http://lightpaper.xzeer.de/getPost.php"), "POST", "key1=value1&key2=value2");
        }

        void client_UploadStringCompleted(object sender, UploadStringCompletedEventArgs e)
        {
            string result = e.Result;
            textBox3.Text = e.Result;
        }					

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System.Reflection.TargetInvocationException wurde nicht von Benutzercode behandelt.
  Message=Während des Vorgangs ist eine Ausnahme aufgetreten, sodass das Ergebnis ungültig ist. Weitere Ausnahmedetails finden Sie in InnerException.
  StackTrace:
       bei System.ComponentModel.AsyncCompletedEventArgs.RaiseExceptionIfNecessary()
       bei System.Net.UploadStringCompletedEventArgs.get_Result()
       bei XCMSBackend.MainPage.client_UploadStringCompleted(Object sender, UploadStringCompletedEventArgs e)
       bei System.Net.WebClient.OnUploadStringCompleted(UploadStringCompletedEventArgs e)
       bei System.Net.WebClient.UploadStringOperationCompleted(Object arg)
  InnerException: System.Security.SecurityException
       Message=""
       StackTrace:
            bei System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
            bei System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
            bei System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result)
            bei System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result)
       InnerException: System.Security.SecurityException
            Message=Sicherheitsfehler
            StackTrace:
                 bei System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
                 bei System.Net.Browser.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
                 bei System.Net.Browser.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)