How to reliably catch exceptions?

Question

How to reliably catch exceptions?

Rainer 121

My app communicates with my server through a number of different endpoint, but they all go through the following method. I'm trying to catch exceptions when communicating to the server so I can convert them to an error response instead. When I run with a debug build this works fine, but when I run with a release build the catch is never run. This isn't true for all exception handling in my code, but is try for this block. Does anyone know why this happens or have suggestions on how to fix it?

private async Task<HttpResponseMessage> PostAsync(string path, IMessage message)
        {
            var content = new StringContent(JsonConvert.SerializeObject(message), Encoding.UTF8, "application/json");
            content.Headers.Add("x-functions-key", authKey);
            try {
                Console.WriteLine($"########## PostAsync A {path}");
                var result = await client.PostAsync($"{ServiceUriBase}/{path}", content);
                Console.WriteLine($"########## PostAsync B {path}");
                return result;
            } catch(Exception ex) {
                Console.WriteLine($"########## {ex}");
                return new HttpResponseMessage(System.Net.HttpStatusCode.BadRequest);
            }
        }

To clarify, with a release build, the PostAsync A debug message is printed, but no other debug messages after that. So the code is not continuing successfully, and also not catching the exception. I do see a message in the console saying Unhandled managed exception..., but that's my point; why is this unhandled? I'm trying to catch it.

Rainer 121

Here's some more info... I'm using AppCenter to handle and report crashes. I do see the following reports:

mono_handle_exception_internal
SIGABRT: Error Domain=NSURLErrorDomain Code=-1004 "サーバに接続できませんでした。" UserInfo={_kCFStreamErrorCodeKey=61, NSUnderlyingError=0x283ce6070 {Error Domain=kCFErrorDomainCFNetwork Code=-1004 "(null)" UserInfo={_NSURLErrorNWPathKey=satisfied (Path is satisfied), viable, interface: en0, ipv4, dns, _kCFStreamErrorCodeKey=61, _kCFStreamErrorDomainKey=1}}, _NSURLErrorFailingURLSessionTaskErrorKey=LocalDataTask <C61F3B9B-3881-4BE0-88C3-2F2B8E8E3238>.<1>, _NSURLErrorRelatedURLSessionTaskErrorKey=( "LocalDataTask <C61F3B9B-3881-4BE0-88C3-2F2B8E8E3238>.<1>" ), NSLocalizedDescription=サーバに接続できませんでした。, NSErrorFailingURLStringKey=http://192.168.86.38:7071/api/notifications/UpdateInstallation, NSErrorFailingURLKey=http://192.168.86.38:7071/api/notifications/UpdateInstallation, _kCFStreamErrorDomainKey=1}

and this stack trace (the full trace is too long, this is just the end of it):

NSUrlSessionHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
Dictionary`2[TKey,TValue].Resize (System.Int32 newSize, System.Boolean forceNewHashCodes)
AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext (System.Object stateMachine)
ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx)

JarvanZhang 23,866 Reputation points

2022-02-07T09:49:31.047+00:00
Hi, Rainer-M. This may be caused by an unhandled exception. To vertify this, try to execute an error code in the try-catch block as below:

try { int a = 0; int b = 10; var c = b / a; } catch (Exception ex) { Console.WriteLine(ex.Message); }
Rainer 121 Reputation points

2022-02-07T10:50:38.317+00:00

@JarvanZhang , thanks for the reply.

Your try/catch block looks exactly the same as mine. As I said, I already have catch(Exception ex) { ... } but it is not catching the exception that is thrown.
JarvanZhang 23,866 Reputation points

2022-02-08T09:02:56.027+00:00

I already have catch(Exception ex) { ... } but it is not catching the exception that is thrown

Sorry for the misleading information. I mean using the above code to check if the 'client.PostAsync' raises an unhandled exception. If the Console.WriteLine code of catch block is also not called, the problem is due to the try-catch. If not, the issue may be related to 'client.PostAsync' command.
Rainer 121 Reputation points

2022-02-08T09:07:10.347+00:00

The problem is with client.PostAsync, other try/catch blocks work fine.
JarvanZhang 23,866 Reputation points

2022-02-09T06:23:26.23+00:00
mono_handle_exception_internal SIGABRT: Error Domain=NSURLErrorDomai

From the stack log, the issue is related to the internet. Do you use the same net for test in debug mode and release mode? Try adding the key for the local address as below.

<key>NSAppTransportSecurity</key> <dict> <key>NSAllowsLocalNetworking</key> <true/> </dict>

And what is the linking option of the release mode? Please use Link Framework SDKs Only behavior, Link all assemblies option links everything and may modify user code that causes the break.
Rainer 121 Reputation points

2022-02-09T06:33:50.687+00:00

Yes, I know exactly what the problem is, but that is why I'm trying to catch this error. Basically I don't want my app to crash if the network is down, so I have the try/catch. But in this case the catch is not working so the app crashes.
JarvanZhang 23,866 Reputation points

2022-02-09T09:55:28.82+00:00

In this case, the crash is not caused by 'client.PostAsync'. The try-catch cannot catch the exetpion.
Rainer 121 Reputation points

2022-02-09T11:22:11.79+00:00

Sorry, I don't understand. The crash is definitely caused by the client.PostAsync. In my original post you can see I include Console.WriteLine statements before and after the client.PostAsync. The WriteLine before it is executed, but neither the WriteLine after it, nor the WriteLine in the catch is executed. This clearly means the client.PostAsync is run, but fails and causes the crash. My question is, why isn't the exception caught?
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 17,121 Reputation points Microsoft Vendor

2022-02-10T09:28:02.06+00:00

and this stack trace (the full trace is too long, this is just the end of it):

NSUrlSessionHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)

I didn't see the detail Xamarin Exception Stack, but it thrown by the NSUrlSessionHandler on iOS. There is a similar issue on github, you could try to add the exception handling to your custom NsUrlSessionHandler class, and then register that class with DependencyService .

In addition, you said it only exists on release mode, may I ask if you archive an ipa and install it on your device? Would mind sharing the detail exception information?

Rainer 121 Reputation points

2022-02-06T04:34:01.777+00:00

Here's some more info... I'm using AppCenter to handle and report crashes. I do see the following reports:

mono_handle_exception_internal SIGABRT: Error Domain=NSURLErrorDomain Code=-1004 "サーバに接続できませんでした。" UserInfo={_kCFStreamErrorCodeKey=61, NSUnderlyingError=0x283ce6070 {Error Domain=kCFErrorDomainCFNetwork Code=-1004 "(null)" UserInfo={_NSURLErrorNWPathKey=satisfied (Path is satisfied), viable, interface: en0, ipv4, dns, _kCFStreamErrorCodeKey=61, _kCFStreamErrorDomainKey=1}}, _NSURLErrorFailingURLSessionTaskErrorKey=LocalDataTask <C61F3B9B-3881-4BE0-88C3-2F2B8E8E3238>.<1>, _NSURLErrorRelatedURLSessionTaskErrorKey=( "LocalDataTask <C61F3B9B-3881-4BE0-88C3-2F2B8E8E3238>.<1>" ), NSLocalizedDescription=サーバに接続できませんでした。, NSErrorFailingURLStringKey=http://192.168.86.38:7071/api/notifications/UpdateInstallation, NSErrorFailingURLKey=http://192.168.86.38:7071/api/notifications/UpdateInstallation, _kCFStreamErrorDomainKey=1}

and this stack trace (the full trace is too long, this is just the end of it):

NSUrlSessionHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) Dictionary`2[TKey,TValue].Resize (System.Int32 newSize, System.Boolean forceNewHashCodes) AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext (System.Object stateMachine) ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx)
JarvanZhang 23,866 Reputation points

2022-02-07T09:49:31.047+00:00

Hi, Rainer-M. This may be caused by an unhandled exception. To vertify this, try to execute an error code in the try-catch block as below:

try { int a = 0; int b = 10; var c = b / a; } catch (Exception ex) { Console.WriteLine(ex.Message); }
Rainer 121 Reputation points

2022-02-07T10:50:38.317+00:00

@JarvanZhang , thanks for the reply.

Your try/catch block looks exactly the same as mine. As I said, I already have catch(Exception ex) { ... } but it is not catching the exception that is thrown.
JarvanZhang 23,866 Reputation points

2022-02-08T09:02:56.027+00:00

I already have catch(Exception ex) { ... } but it is not catching the exception that is thrown

Sorry for the misleading information. I mean using the above code to check if the 'client.PostAsync' raises an unhandled exception. If the Console.WriteLine code of catch block is also not called, the problem is due to the try-catch. If not, the issue may be related to 'client.PostAsync' command.
Rainer 121 Reputation points

2022-02-08T09:07:10.347+00:00

The problem is with client.PostAsync, other try/catch blocks work fine.
JarvanZhang 23,866 Reputation points

2022-02-09T06:23:26.23+00:00

mono_handle_exception_internal SIGABRT: Error Domain=NSURLErrorDomai

From the stack log, the issue is related to the internet. Do you use the same net for test in debug mode and release mode? Try adding the key for the local address as below.

<key>NSAppTransportSecurity</key> <dict> <key>NSAllowsLocalNetworking</key> <true/> </dict>

And what is the linking option of the release mode? Please use Link Framework SDKs Only behavior, Link all assemblies option links everything and may modify user code that causes the break.
Rainer 121 Reputation points

2022-02-09T06:33:50.687+00:00

Yes, I know exactly what the problem is, but that is why I'm trying to catch this error. Basically I don't want my app to crash if the network is down, so I have the try/catch. But in this case the catch is not working so the app crashes.
JarvanZhang 23,866 Reputation points

2022-02-09T09:55:28.82+00:00

In this case, the crash is not caused by 'client.PostAsync'. The try-catch cannot catch the exetpion.
Rainer 121 Reputation points

2022-02-09T11:22:11.79+00:00

Sorry, I don't understand. The crash is definitely caused by the client.PostAsync. In my original post you can see I include Console.WriteLine statements before and after the client.PostAsync. The WriteLine before it is executed, but neither the WriteLine after it, nor the WriteLine in the catch is executed. This clearly means the client.PostAsync is run, but fails and causes the crash. My question is, why isn't the exception caught?
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 17,121 Reputation points Microsoft Vendor

2022-02-10T09:28:02.06+00:00

and this stack trace (the full trace is too long, this is just the end of it):

NSUrlSessionHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)

I didn't see the detail Xamarin Exception Stack, but it thrown by the NSUrlSessionHandler on iOS. There is a similar issue on github, you could try to add the exception handling to your custom NsUrlSessionHandler class, and then register that class with DependencyService .

In addition, you said it only exists on release mode, may I ask if you archive an ipa and install it on your device? Would mind sharing the detail exception information?

How to reliably catch exceptions?

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