System.Net.Http.HttpRequestException: An error occurred while sending the request

Question

I do long task in background , it may take several minutes or hours and I run this task periodically ..., every 30 minute with repeating task :

        NSTimer.CreateRepeatingScheduledTimer(SyncIntervalSeconds, (obj) =>
             {
                 DataOfNextSync = DataOfNextSync.AddSeconds(SyncIntervalSeconds);
                 System.Threading.Tasks.Task.Run(() =>
                        {
                            syncService.FullDataSync();
                        });
             });

inside "fullsync" I use HttpClient and SQLite database, getting some information from server and put it in my database and saving some files on the disk

Generally everything works fine, but in some cases, I don't know why (often when mac is sleeping, not always), task stops with following error message:

System.AggregateException: One or more errors occurred. ---> System.Net.Http.HttpRequestException: An error occurred while sending the request ---> System.Net.WebException: Error: NameResolutionFailure at System.Net.WebConnection+d__16.MoveNext () [0x0002c] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebConnection.cs:137 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable+ConfiguredTaskAwaiter.GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:447 at System.Net.WebConnection+d__19.MoveNext () [0x000cc] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebConnection.cs:259 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:535 at System.Net.WebOperation+<Run>d__57.MoveNext () [0x00052] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebOperation.cs:268 --- End of stack trace from previous location where exception was thrown --- at System.Net.WebCompletionSource1+d__15[T].MoveNext () [0x0008e] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebCompletionSource.cs:111 --- End of stack trace from previous location where exception was thrown --- at System.Net.HttpWebRequest+d__2411[T].MoveNext () [0x000b5] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/HttpWebRequest.cs:952 --- End of stack trace from previous location where exception was thrown --- at System.Net.HttpWebRequest.EndGetResponse (System.IAsyncResult asyncResult) [0x00018] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/HttpWebRequest.cs:1180 at System.Threading.Tasks.TaskFactory1[TResult].FromAsyncCoreLogic (System.IAsyncResult iar, System.Func2[T,TResult] endFunction, System.Action1[T] endAction, System.Threading.Tasks.Task1[TResult] promise, System.Boolean requiresSynchronization) [0x0000f] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/threading/Tasks/FutureFactory.cs:550 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:535 at System.Net.Http.HttpClientHandler+d__64.MoveNext () [0x003d3] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System.Net.Http/System.Net.Http/HttpClientHandler.cs:402 --- End of inner exception stack trace --- at System.Net.Http.HttpClientHandler+d__64.MoveNext () [0x0046c] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System.Net.Http/System.Net.Http/HttpClientHandler.cs:406 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:535 at System.Net.Http.HttpClient+<SendAsyncWorker>d__46.MoveNext () [0x00080] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System.Net.Http/System.Net.Http/HttpClient.cs:276 --- End of inner exception stack trace --- at System.Threading.Tasks.Task.ThrowIfExceptional (System.Boolean includeTaskCanceledExceptions) [0x00013] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/threading/Tasks/Task.cs:2166 at System.Threading.Tasks.Task1[TResult].GetResultCore (System.Boolean waitCompletionNotification) [0x0002b] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/threading/Tasks/Future.cs:562 at System.Threading.Tasks.Task1[TResult].get_Result () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/threading/Tasks/Future.cs:532 at Aptos.Core.APIProvider.Get (System.String url, System.Collections.Generic.Dictionary2[TKey,TValue] headers) [0x00080] in <2162774b92f2416e98dc5d9fc831c8f3>:0 at Aptos.Core.SynchronizationService.DownloadImage (SQLite.SQLiteConnection conn, System.Collections.Generic.Dictionary2[TKey,TValue] headers, Aptos.Core.PatientImage newimage, System.String oldRelPath, System.Boolean isImport) [0x00097] in <2162774b92f2416e98dc5d9fc831c8f3>:0 at Aptos.Core.SynchronizationService.GetAllChanges (System.Collections.Generic.Dictionary2[TKey,TValue] headers) [0x008ba] in <2162774b92f2416e98dc5d9fc831c8f3>:0 ---> (Inner Exception #0) System.Net.Http.HttpRequestException: An error occurred while sending the request ---> System.Net.WebException: Error: NameResolutionFailure at System.Net.WebConnection+d__16.MoveNext () [0x0002c] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebConnection.cs:137 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable+ConfiguredTaskAwaiter.GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:447 at System.Net.WebConnection+d__19.MoveNext () [0x000cc] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebConnection.cs:259 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:535 at System.Net.WebOperation+<Run>d__57.MoveNext () [0x00052] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebOperation.cs:268 --- End of stack trace from previous location where exception was thrown --- at System.Net.WebCompletionSource1+d__15[T].MoveNext () [0x0008e] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/WebCompletionSource.cs:111 --- End of stack trace from previous location where exception was thrown --- at System.Net.HttpWebRequest+d__2411[T].MoveNext () [0x000b5] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/HttpWebRequest.cs:952 --- End of stack trace from previous location where exception was thrown --- at System.Net.HttpWebRequest.EndGetResponse (System.IAsyncResult asyncResult) [0x00018] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System/System.Net/HttpWebRequest.cs:1180 at System.Threading.Tasks.TaskFactory1[TResult].FromAsyncCoreLogic (System.IAsyncResult iar, System.Func2[T,TResult] endFunction, System.Action1[T] endAction, System.Threading.Tasks.Task1[TResult] promise, System.Boolean requiresSynchronization) [0x0000f] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/threading/Tasks/FutureFactory.cs:550 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:535 at System.Net.Http.HttpClientHandler+d__64.MoveNext () [0x003d3] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System.Net.Http/System.Net.Http/HttpClientHandler.cs:402 --- End of inner exception stack trace --- at System.Net.Http.HttpClientHandler+d__64.MoveNext () [0x0046c] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System.Net.Http/System.Net.Http/HttpClientHandler.cs:406 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:535 at System.Net.Http.HttpClient+d__46.MoveNext () [0x00080] in /Library/Frameworks/Xamarin.Mac.framework/Versions/5.0.0.0/src/Xamarin.Mac/mcs/class/System.Net.Http/System.Net.Http/HttpClient.cs:276 <---