Resolving Native HttpClient Exceptions - Android

Question

Hello, I'd like to ask if there is a way to catch unhandled native exceptions on android in a shared code... Because currently I made a class which is making http requests using HttpClient and everything was working perfectly until I tried to use my app without an internet connection, that's when my android app started crashing so I debugged the exception and it seems like it throws a native Java.Net.UnknownHostException which my try{} catch(){} is unable to handle.

public static async Task<string> Get(string Url, Dictionary<string, object> Arguments, CancellationToken Token)
        {
            try
            {
                HttpClient client = new HttpClient();
                string finalUrl = Url;
                if (Arguments != null && Arguments.Count > 0)
                {
                    finalUrl = UrlCompiler(Url, Arguments);
                }
                var response = client.GetAsync(finalUrl, HttpCompletionOption.ResponseContentRead, Token).GetAwaiter().GetResult();
                return await response.Content.ReadAsStringAsync();
            }
            catch(Exception er)
            {
                Console.WriteLine("Error thrown: " + er.ToString());
                return null;
            }
        }

And whenever I call this function with no connection to the internet my app crashes on android with this stack trace.

[monodroid] Not wrapping exception of type Java.Net.UnknownHostException from method `SendAsync`. This will change in a future release.
[InputMethodManager] HSIFW - flag : 0
Error thrown: Java.Net.UnknownHostException: Unable to resolve host "google.com": No address associated with hostname ---> Java.Lang.RuntimeException: android_getaddrinfo failed: EAI_NODATA (No address associated with hostname)
   --- End of inner exception stack trace ---
  at Java.Interop.JniEnvironment+InstanceMethods.CallVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x0006e] in <00c315a988634383b446eff646084784>:0 
  at Java.Interop.JniPeerMembers+JniInstanceMethods.InvokeAbstractVoidMethod (System.String encodedMember, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x00014] in <00c315a988634383b446eff646084784>:0 
  at Javax.Net.Ssl.HttpsURLConnectionInvoker.Connect () [0x00000] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/obj/Release/monoandroid10/android-30/mcw/Javax.Net.Ssl.HttpsURLConnection.cs:433 
  at Xamarin.Android.Net.AndroidClientHandler+<>c__DisplayClass44_0.<ConnectAsync>b__0 () [0x0007d] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.Legacy.cs:356 
  at System.Threading.Tasks.Task.InnerInvoke () [0x0000f] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corert/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs:2476 
  at System.Threading.Tasks.Task.Execute () [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corert/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs:2319 
--- End of stack trace from previous location where exception was thrown ---
  at Xamarin.Android.Net.AndroidClientHandler.DoProcessRequest (System.Net.Http.HttpRequestMessage request, Java.Net.URL javaUrl, Java.Net.HttpURLConnection httpConnection, System.Threading.CancellationToken cancellationToken, Xamarin.Android.Net.AndroidClientHandler+RequestRedirectionState redirectState) [0x000e4] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.Legacy.cs:405 
  at Xamarin.Android.Net.AndroidClientHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) [0x00286] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.Legacy.cs:287 
  at System.Net.Http.HttpClient.FinishSendAsyncBuffered (System.Threading.Tasks.Task`1[TResult] sendTask, System.Net.Http.HttpRequestMessage request, System.Threading.CancellationTokenSource cts, System.Boolean disposeCts) [0x0017e] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.Net.Http/src/System/Net/Http/HttpClient.cs:506 
  at Mobile_App.Modules.Request.Get (System.String Url, System.Collections.Generic.Dictionary`2[TKey,TValue] Arguments, System.Threading.CancellationToken Token) [0x00092] in repos\Mobile App\Mobile App\Mobile App\Modules\Request.cs:190 
  --- End of managed Java.Net.UnknownHostException stack trace ---

Obviously, I can't just let this be and wait for a future releases that may change this, so I searched the internet for the most efficient workaround and found one guy saying that you could create a custom AndroidClientHandler and catch all the exceptions we want. I tried doing so and extended the Handler yet I don't know how to use it in my shared code since it is in my android project.

namespace Mobile_App.Droid
{
    public class CustomAndroidClientHandler : AndroidClientHandler
    {
        public CustomAndroidClientHandler()
        {
            this.AutomaticDecompression = System.Net.DecompressionMethods.GZip | System.Net.DecompressionMethods.Deflate; 
        }

        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            // The native Android HTTP client handler bubbles up Java exceptions in contrast to the iOS native handler.
            // See for instance https://github.com/xamarin/xamarin-android/issues/3216.
            // We see this behavior on a Samsung 9 with Android 9.
            // Remedy is to explicitly catch expected native network exceptions.
            try
            {
                return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
            }
            catch (Java.Net.UnknownHostException ex)
            {
                throw new NoConnectionException($"Native UnknownHostException on HTTP request: {ex.Message}", ex);
            }
            catch (Java.Net.SocketException ex)
            {
                throw new WebException($"Native SocketException on HTTP request: {ex.Message}", ex);
            }
            catch (Java.IO.IOException ex)
            {
                throw new WebException($"Native IOException on HTTP request: {ex.Message}", ex);
            }
            catch (Java.Lang.Exception ex)
            {
                throw new WebException($"Native Exception on HTTP request: {ex.Message}", ex);
            }
        }
    }
}

Answer 1

Hello,​

Welcome to our Microsoft Q&A platform!

I tried doing so and extended the Handler yet I don't know how to use it in my shared code since it is in my android project.

You can create a static object in the App.xaml.cs like this code.

   public static object HttpClientHandler;  

Then open your MainActivity.cs to initialize CustomAndroidClientHandler in the OnCreate method, put it before the LoadApplication(new App()); line

   protected override void OnCreate(Bundle savedInstanceState)  
           {  
               TabLayoutResource = Resource.Layout.Tabbar;  
               ToolbarResource = Resource.Layout.Toolbar;  
     
               base.OnCreate(savedInstanceState);  
     
               Xamarin.Essentials.Platform.Init(this, savedInstanceState);  
               global::Xamarin.Forms.Forms.Init(this, savedInstanceState);  
     
               var androidClientHandler = new CustomAndroidClientHandler();  
               App.HttpClientHandler = androidClientHandler;  
     
               LoadApplication(new App());  
           }  

Then you can use App.HttpClientHandler in your shared code when you new HttpClient().

   new HttpClient((HttpMessageHandler)App.HttpClientHandler)  
               {  
     
               };  

Best Regards,

Leon Lu

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