How to catch an exception from Azure Mobile App Backend? - azure

Question

In my Azure Mobile App backend, I have a validation for the data in a post method. In certains circunstances, the backend server throw an exception, but, in the app side, I can't get catch this exception. How can I do that?
That's my method
// POST tables/Paciente
public async Task<IHttpActionResult> PostPaciente(Paciente novoPaciente)
{
//other things
if (paciente != null)
{
var responseMessage = new HttpResponseMessage(HttpStatusCode.BadRequest)
{
Content = new StringContent("Já existe um paciente com esse token cadastrado.")
};
//throw new HttpResponseException(responseMessage);
return InternalServerError(new Exception("Já existe um paciente com esse token cadastrado."));
}
}
I tried throw HttpResponseException and return InternalServerException, but none works.

Answer 1

You need to check the status code of the response to your http call (there should be a IsSuccesStatusCode property to check).

Answer 2

I recommend using EnsureSuccessStatusCode() which exists in the HttpResponseMessage class. This method will throw an exception if the StatusCode is not OK (or some variant of a 200-level status code).
Below is a generic class, BaseHttpClientServices, that I use for making REST API requests in all of my projects. It follows all of the best practices for using HttpClient like Reusing HttpClient, Deserializing JSON using Stream, and using ConfigureAwait(false).
Sending A Post Request in Xamarin.Forms
public abstract class HttpClientServices : BaseHttpClientServices
{
const string apiUrl = "your api url";
public static void PostPaciente(Paciente novoPaciente)
{
try
{
var response = await PostObjectToAPI(apiUrl, novoPaciente);
response.EnsureSuccessStatusCode();
}
catch(Exception e)
{
//Handle Exception
}
}
}
Generic HttpClient Implementation
using System;
using System.IO;
using System.Net;
using System.Text;
using System.Net.Http;
using System.Threading.Tasks;
using System.Net.Http.Headers;
using System.Runtime.CompilerServices;
using Newtonsoft.Json;
using Xamarin.Forms;
namespace NameSpace
{
public abstract class BaseHttpClientService
{
#region Constant Fields
static readonly Lazy<JsonSerializer> _serializerHolder = new Lazy<JsonSerializer>();
static readonly Lazy<HttpClient> _clientHolder = new Lazy<HttpClient>(() => CreateHttpClient(TimeSpan.FromSeconds(60)));
#endregion
#region Fields
static int _networkIndicatorCount = 0;
#endregion
#region Properties
static HttpClient Client => _clientHolder.Value;
static JsonSerializer Serializer => _serializerHolder.Value;
#endregion
#region Methods
protected static async Task<T> GetObjectFromAPI<T>(string apiUrl)
{
using (var responseMessage = await GetObjectFromAPI(apiUrl).ConfigureAwait(false))
return await DeserializeResponse<T>(responseMessage).ConfigureAwait(false);
}
protected static async Task<HttpResponseMessage> GetObjectFromAPI(string apiUrl)
{
try
{
UpdateActivityIndicatorStatus(true);
return await Client.GetAsync(apiUrl).ConfigureAwait(false);
}
catch (Exception e)
{
Report(e);
throw;
}
finally
{
UpdateActivityIndicatorStatus(false);
}
}
protected static async Task<TResponse> PostObjectToAPI<TResponse, TRequest>(string apiUrl, TRequest requestData)
{
using (var responseMessage = await PostObjectToAPI(apiUrl, requestData).ConfigureAwait(false))
return await DeserializeResponse<TResponse>(responseMessage).ConfigureAwait(false);
}
protected static Task<HttpResponseMessage> PostObjectToAPI<T>(string apiUrl, T requestData) => SendAsync(HttpMethod.Post, apiUrl, requestData);
protected static async Task<TResponse> PutObjectToAPI<TResponse, TRequest>(string apiUrl, TRequest requestData)
{
using (var responseMessage = await PutObjectToAPI(apiUrl, requestData).ConfigureAwait(false))
return await DeserializeResponse<TResponse>(responseMessage).ConfigureAwait(false);
}
protected static Task<HttpResponseMessage> PutObjectToAPI<T>(string apiUrl, T requestData) => SendAsync(HttpMethod.Put, apiUrl, requestData);
protected static async Task<TResponse> PatchObjectToAPI<TResponse, TRequest>(string apiUrl, TRequest requestData)
{
using (var responseMessage = await PatchObjectToAPI(apiUrl, requestData).ConfigureAwait(false))
return await DeserializeResponse<TResponse>(responseMessage).ConfigureAwait(false);
}
protected static Task<HttpResponseMessage> PatchObjectToAPI<T>(string apiUrl, T requestData) => SendAsync(new HttpMethod("PATCH"), apiUrl, requestData);
protected static async Task<TResponse> DeleteObjectFromAPI<TResponse>(string apiUrl)
{
using (var responseMessage = await DeleteObjectFromAPI(apiUrl).ConfigureAwait(false))
return await DeserializeResponse<TResponse>(responseMessage).ConfigureAwait(false);
}
protected static Task<HttpResponseMessage> DeleteObjectFromAPI(string apiUrl) => SendAsync<object>(HttpMethod.Delete, apiUrl);
static HttpClient CreateHttpClient(TimeSpan timeout)
{
HttpClient client;
switch (Device.RuntimePlatform)
{
case Device.iOS:
case Device.Android:
client = new HttpClient();
break;
default:
client = new HttpClient(new HttpClientHandler { AutomaticDecompression = DecompressionMethods.GZip });
break;
}
client.Timeout = timeout;
client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("gzip"));
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
return client;
}
static async Task<HttpResponseMessage> SendAsync<T>(HttpMethod httpMethod, string apiUrl, T requestData = default)
{
using (var httpRequestMessage = await GetHttpRequestMessage(httpMethod, apiUrl, requestData).ConfigureAwait(false))
{
try
{
UpdateActivityIndicatorStatus(true);
return await Client.SendAsync(httpRequestMessage).ConfigureAwait(false);
}
catch (Exception e)
{
Report(e);
throw;
}
finally
{
UpdateActivityIndicatorStatus(false);
}
}
}
static void UpdateActivityIndicatorStatus(bool isActivityIndicatorDisplayed)
{
if (isActivityIndicatorDisplayed)
{
Device.BeginInvokeOnMainThread(() => Application.Current.MainPage.IsBusy = true);
_networkIndicatorCount++;
}
else if (--_networkIndicatorCount <= 0)
{
Device.BeginInvokeOnMainThread(() => Application.Current.MainPage.IsBusy = false);
_networkIndicatorCount = 0;
}
}
static async ValueTask<HttpRequestMessage> GetHttpRequestMessage<T>(HttpMethod method, string apiUrl, T requestData = default)
{
var httpRequestMessage = new HttpRequestMessage(method, apiUrl);
switch (requestData)
{
case T data when data.Equals(default(T)):
break;
case Stream stream:
httpRequestMessage.Content = new StreamContent(stream);
httpRequestMessage.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
break;
default:
var stringPayload = await Task.Run(() => JsonConvert.SerializeObject(requestData)).ConfigureAwait(false);
httpRequestMessage.Content = new StringContent(stringPayload, Encoding.UTF8, "application/json");
break;
}
return httpRequestMessage;
}
static async Task<T> DeserializeResponse<T>(HttpResponseMessage httpResponseMessage)
{
httpResponseMessage.EnsureSuccessStatusCode();
try
{
using (var contentStream = await httpResponseMessage.Content.ReadAsStreamAsync().ConfigureAwait(false))
using (var reader = new StreamReader(contentStream))
using (var json = new JsonTextReader(reader))
{
if (json is null)
return default;
return await Task.Run(() => Serializer.Deserialize<T>(json)).ConfigureAwait(false);
}
}
catch (Exception e)
{
Report(e);
throw;
}
}
static void Report(Exception e, [CallerMemberName]string callerMemberName = "") => Debug.WriteLine(e.Message);
#endregion
}
}