Unhandled exception handling in iOS and Android with Xamarin.

Unhandled exceptions are hard to catch and log, and you must do it to be able to handle the errors in your app. One approach is to use Xamarin.Insights but you always want to be able to just log into a file and then access that file locally.

The code below is what we use right now and it has helped us a lot, especially the unobserved task exceptions that are just silent, i.e. they do not crash the app, but you still want to handle the exceptions.

 

  1 // In MainActivity
  2 protected override void OnCreate(Bundle bundle)
  3 {
  4     base.OnCreate(bundle);  
  5      
  6     AppDomain.CurrentDomain.UnhandledException += CurrentDomainOnUnhandledException;
  7     TaskScheduler.UnobservedTaskException += TaskSchedulerOnUnobservedTaskException;  
  8      
  9     Xamarin.Forms.Forms.Init(this, bundle);  
 10     DisplayCrashReport();  
 11      
 12     var app = new App();  
 13     LoadApplication(app);
 14 }  
 15  
 16 ‪#‎region‬ Error handling
 17 private static void TaskSchedulerOnUnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs unobservedTaskExceptionEventArgs)
 18 {
 19     var newExc = new Exception("TaskSchedulerOnUnobservedTaskException", unobservedTaskExceptionEventArgs.Exception);
 20     LogUnhandledException(newExc);
 21 }  
 22  
 23 private static void CurrentDomainOnUnhandledException(object sender, UnhandledExceptionEventArgs unhandledExceptionEventArgs)
 24 {
 25     var newExc = new Exception("CurrentDomainOnUnhandledException", unhandledExceptionEventArgs.ExceptionObject as Exception);
 26     LogUnhandledException(newExc);
 27 }  
 28  
 29 internal static void LogUnhandledException(Exception exception)
 30 {
 31     try
 32     {
 33         const string errorFileName = "Fatal.log";
 34         var libraryPath = Environment.GetFolderPath(Environment.SpecialFolder.Personal); // iOS: Environment.SpecialFolder.Resources
 35         var errorFilePath = Path.Combine(libraryPath, errorFileName);  
 36         var errorMessage = String.Format("Time: {0}\r\nError: Unhandled Exception\r\n{1}",
 37         DateTime.Now, exception.ToString());
 38         File.WriteAllText(errorFilePath, errorMessage);  
 39          
 40         // Log to Android Device Logging.
 41         Android.Util.Log.Error("Crash Report", errorMessage);
 42     }
 43     catch
 44     {
 45         // just suppress any error logging exceptions
 46     }
 47 }  
 48  
 49 /// <summary>
 50 // If there is an unhandled exception, the exception information is diplayed 
 51 // on screen the next time the app is started (only in debug configuration)
 52 /// </summary>
 53 [Conditional("DEBUG")]
 54 private void DisplayCrashReport()
 55 {
 56     const string errorFilename = "Fatal.log";
 57     var libraryPath = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
 58     var errorFilePath = Path.Combine(libraryPath, errorFilename);
 59  
 60     if (!File.Exists(errorFilePath))
 61     {
 62         return; 
 63     }
 64      
 65     var errorText = File.ReadAllText(errorFilePath);
 66     new AlertDialog.Builder(this)
 67         .SetPositiveButton("Clear", (sender, args) =>
 68         {
 69             File.Delete(errorFilePath);
 70         })
 71         .SetNegativeButton("Close", (sender, args) =>
 72         {
 73             // User pressed Close.
 74         })
 75         .SetMessage(errorText)
 76         .SetTitle("Crash Report")
 77         .Show();
 78 } 
 79  
 80 ‪#‎endregion‬  
 81  
 82 //iOS: Different than Android. Must be in FinishedLaunching, not in Main.
 83 // In AppDelegate
 84 public override bool FinishedLaunching(UIApplication uiApplication, NSDictionary options)
 85 {
 86     AppDomain.CurrentDomain.UnhandledException += CurrentDomainOnUnhandledException;
 87     TaskScheduler.UnobservedTaskException += TaskSchedulerOnUnobservedTaskException;  
 88     ...
 89 }  
 90  
 91 /// <summary>
 92 // If there is an unhandled exception, the exception information is diplayed 
 93 // on screen the next time the app is started (only in debug configuration)
 94 /// </summary>
 95 [Conditional("DEBUG")]
 96 private static void DisplayCrashReport()
 97 {
 98     const string errorFilename = "Fatal.log";
 99     var libraryPath = Environment.GetFolderPath(Environment.SpecialFolder.Resources);
100     var errorFilePath = Path.Combine(libraryPath, errorFilename);
101  
102     if (!File.Exists(errorFilePath))
103     {
104         return;
105     }
106      
107     var errorText = File.ReadAllText(errorFilePath);
108     var alertView = new UIAlertView("Crash Report", errorText, null, "Close", "Clear") { UserInteractionEnabled = true };
109     alertView.Clicked += (sender, args) =>
110     {
111         if (args.ButtonIndex != 0)
112         {
113             File.Delete(errorFilePath);
114         }
115     };
116     alertView.Show();
117 }