Unhandled Exception when domain name cannot be resolved [v4.0.2]

[YetAnotherGeorge]

Hello,

I think this line can throw unhandled exceptions because it is never awaited and it has a throw in its catch block.

Commit

When the server is offline and i attempt a connection, i get:

[2026-03-27 17:01:50.2954] [georg] [Fatal] [WPFApp] Unobserved task exception SocketIOClient.Exceptions.ConnectionException: Cannot connect to server 'SOME_SERVER_URL'
 ---> SocketIOClient.Session.ConnectionFailedException: Failed to connect to the server
 ---> System.Net.Http.HttpRequestException: No such host is known. (SOME_SERVER_URL)
 ---> System.Net.Sockets.SocketException (11001): No such host is known.
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
   at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.InjectNewHttp11ConnectionAsync(QueueItem queueItem)
   at System.Threading.Tasks.TaskCompletionSourceWithCancellation`1.WaitWithCancellationAsync(CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.SocketsHttpHandler.<SendAsync>g__CreateHandlerAndSendAsync|115_0(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
   at SocketIOClient.Protocol.Http.SystemHttpClient.SendAsync(HttpRequest req, CancellationToken cancellationToken)
   at SocketIOClient.Protocol.Http.HttpAdapter.SendAsync(HttpRequest req, CancellationToken cancellationToken)
   at SocketIOClient.Session.Http.HttpSession.ConnectCoreAsync(Uri uri, CancellationToken cancellationToken)
   at SocketIOClient.Session.SessionBase`1.ConnectAsync(CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at SocketIOClient.Session.SessionBase`1.ConnectAsync(CancellationToken cancellationToken)
   at SocketIOClient.SocketIO.TryConnectAsync(ISession session, CancellationToken cancellationToken)
   at SocketIOClient.SocketIO.ConnectCoreAsync(CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at SocketIOClient.SocketIO.ConnectCoreAsync(CancellationToken cancellationToken)
[2026-03-27 17:01:50.2954] [georg] [Fatal] [WPFApp] Unobserved task exception System.InvalidOperationException: An attempt was made to transition a task to a final state when it had already completed.
   at System.Threading.Tasks.TaskCompletionSource`1.SetResult(TResult result)
   at SocketIOClient.SocketIO.<ConnectAsync>b__62_1()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)
[2026-03-27 17:01:50.3093] [georg] [Fatal] [WPFApp] Unobserved task exception System.InvalidOperationException: An attempt was made to transition a task to a final state when it had already completed.
   at System.Threading.Tasks.TaskCompletionSource`1.SetResult(TResult result)
   at SocketIOClient.SocketIO.<ConnectAsync>b__62_1()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)
[2026-03-27 17:01:50.3093] [georg] [Fatal] [WPFApp] Unobserved task exception SocketIOClient.Exceptions.ConnectionException: Cannot connect to server 'SOME_SERVER_URL'
 ---> SocketIOClient.Session.ConnectionFailedException: Failed to connect to the server
 ---> System.Net.Http.HttpRequestException: No such host is known. (SOME_SERVER_URL)
 ---> System.Net.Sockets.SocketException (11001): No such host is known.
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
   at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.InjectNewHttp11ConnectionAsync(QueueItem queueItem)
   at System.Threading.Tasks.TaskCompletionSourceWithCancellation`1.WaitWithCancellationAsync(CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.SocketsHttpHandler.<SendAsync>g__CreateHandlerAndSendAsync|115_0(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
   at SocketIOClient.Protocol.Http.SystemHttpClient.SendAsync(HttpRequest req, CancellationToken cancellationToken)
   at SocketIOClient.Protocol.Http.HttpAdapter.SendAsync(HttpRequest req, CancellationToken cancellationToken)
   at SocketIOClient.Session.Http.HttpSession.ConnectCoreAsync(Uri uri, CancellationToken cancellationToken)
   at SocketIOClient.Session.SessionBase`1.ConnectAsync(CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at SocketIOClient.Session.SessionBase`1.ConnectAsync(CancellationToken cancellationToken)
   at SocketIOClient.SocketIO.TryConnectAsync(ISession session, CancellationToken cancellationToken)
   at SocketIOClient.SocketIO.ConnectCoreAsync(CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at SocketIOClient.SocketIO.ConnectCoreAsync(CancellationToken cancellationToken)

I got this error after registering a callback to TaskScheduler.UnobservedTaskException.

Please let me know if you need a sample application.
The problem is that i want to be able to catch these errors when awaiting socket.ConnectAsync().

Many thanks,
George

[doghappy]

This is by design. Since reconnection logic can be somewhat complex, it is better handled internally within the library.

By default, the library will attempt to reconnect to the server multiple times until it succeeds, so some errors are caught and logged.

If you prefer to handle these details yourself, you can set Reconnection = false . In that case, each call to ConnectAsync() may throw an exception due to network issues, and you can handle it manually.

Not sure if this helps — feel free to reply