Net.Web Exception while fetching Data from API

Question

Hi Friends, 

I am making windows phone 8 application and in that i am trying to fetch data from server using API. API returns JSON and that JSON I am mapping to C# class. I am calling api with asynchronous httpwebrequest..

But  the code I am using sometimes suddenly crashes my application and nothing works...and I am not able to figure out why it is crashing sometime... How to catch Exception in asynchronous? it is tedious . 

Code for Getting Data and posting is here... Both are crashing sometimes and Excetion is comming at line  asyncResult => request.EndGetResponse(asyncResult)

Exception:'System.Net.WebException' occurred in System.Windows.ni.dll 

Is anybody have solution? , please reply

Thank U 

       

Code for Get Data


   class ApiConnection<T> where T : class //T is C# class For JSON data
    {
        private readonly string _url;
        CookieContainer cookieJar = new CookieContainer();

        public ApiConnection(string url)
        {
            _url = url;
        }


        public async Task<T> GetResult()
        {

            var response = await MakeAsyncRequest(_url);
            var result = JsonConvert.DeserializeObject<T>(response);
            return result;
        }

        public Task<string> MakeAsyncRequest(string url)
        {
            var request = (HttpWebRequest)WebRequest.Create(url);
            if (PostDataAPIConnection.isCookie == 1)
            {
                request.CookieContainer = cookieJar;
                foreach (Cookie cookie in PostDataAPIConnection.responseCookies)
                {
                    System.Diagnostics.Debugger.Log(1, "", "" + cookie.Name);
                    //request.CookieContainer.Add(new Uri(cookie.Name), cookie);
                }
            }

            try
            {

                Task<WebResponse> task = Task.Factory.FromAsync(
                    request.BeginGetResponse,
                    asyncResult => request.EndGetResponse(asyncResult),
                    null);
                return task.ContinueWith(t => ReadStreamFromResponse(t.Result));
            }
            catch (WebException ex)
            {
                System.Diagnostics.Debugger.Log(1, "", "Network problem :" + ex.Response);
                throw ex;
                // return "";

            }


        }

        private static string ReadStreamFromResponse(WebResponse response)
        {
            using (var responseStream = response.GetResponseStream())
            using (var reader = new StreamReader(responseStream))
            {
                var content = reader.ReadToEnd();
                return content;
            }
        }
    }
}
        

Code for Post Data
class PostDataAPIConnection
    {

        static string data;
        static string url;
        public static int isCookie;
        public static CookieContainer cookieJar = new CookieContainer();
        public static CookieCollection responseCookies = new CookieCollection();

        public PostDataAPIConnection()
        {
            data = "";
            url = "";
            isCookie = 0;
        }

        public static ManualResetEvent allDone = new ManualResetEvent(false);
        public async Task<string> PostData(string data1, string url1)
        {
            data = data1;
            url = url1;

            var response = await MakeAsyncRequest();
            var result = response as string;
            return result;
	}

        public Task<string> MakeAsyncRequest()
        {
	   //Create a new HttpWebRequest object.
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            request.CookieContainer = cookieJar;
            // Set the ContentType property. 
            request.ContentType = "application/json";
            // Set the Method property to 'POST' to post data to the URI.
            request.Method = "POST";
            // Start the asynchronous operation.    
            request.BeginGetRequestStream(new AsyncCallback(ReadCallback), request);
            allDone.WaitOne();

            Task<WebResponse> task = Task.Factory.FromAsync(
                request.BeginGetResponse,
                asyncResult => request.EndGetResponse(asyncResult),
                null); // Exception is coming Here...

            try
            {
                if (task.IsFaulted)
                    throw new Exception();

            }
            catch (Exception e)
            {
                System.Diagnostics.Debugger.Log(1, "", "Error Occured----->>>>" + e);
            }


            Task<string> response = task.ContinueWith(t =>
               {
                   string str = ReadStreamFromResponse(t.Result);

                   var webResponse = t.Result as HttpWebResponse;

                   if (webResponse.Cookies != null && webResponse.Cookies.Count != 0)
                   {
                       responseCookies = webResponse.Cookies;
                     //  cookieJar.Add(webResponse.Cookies);
                       isCookie = 1;

                   }

                   return str;
               }
          );



            return response;


        }
        private static string ReadStreamFromResponse(WebResponse response)
        {
            using (var responseStream = response.GetResponseStream())
            using (var reader = new StreamReader(responseStream))
            {
                var content = reader.ReadToEnd();
                return content;
            }
        }


        private static void ReadCallback(IAsyncResult asynchronousResult)
        {
            HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;
            // End the operation.
            Stream postStream = request.EndGetRequestStream(asynchronousResult);

            string postData = data;

            // Convert the string into a byte array.
            byte[] byteArray = Encoding.UTF8.GetBytes(postData);
            // Write to the request stream.
            postStream.Write(byteArray, 0, postData.Length);
            postStream.Close();
            allDone.Set();
        }
    }

Answer 1

You need try/catch around the code which is executing asynchronously.  (try/catch around the code which calls the asynchronous operation will only catch exception thrown setting up the operation.)

---

-Eric.

Answer 2

Thanks for reply,

but even I used try catch surrounding asynchronous method calling  i.e. 

1) Task.Factory.FromAsync(.... and

2)response = await MakeAsyncRequest(_url);

but Exception Comes and App crashes .. It doesnt catch 

Answer 3

The exception is occurring here because the EndGetResponse isn't successful:

 asyncResult => request.EndGetResponse(asyncResult),

You'll have to add exception handling there.  In this case, the lambda isn't helping you at all - you'll have to actually write a method that gets the response and checks the value (and catches exceptions)

---

Darin R.