[DeviceClient IoT Hub] Exception handling in callback

Andirly 16

Hello,

I'm trying hard to figure out a way to do proper exception handling using the DeviceClient API to connect to IoT hub.

I've been using the API for a while and still I can't get my head around the design of the callback implementation

E.g.

public void SetConnectionStatusChangesHandler(ConnectionStatusChangesHandler statusChangesHandler);

forces me to not use any async method inside the handler as I can't propagate any errors outside. A typical use case is fetching all upstream desired properties and upload reported properties after having checked for changes. But any error occuring during these functions (which are all async and therefore force me to have an async void callback) won't be fetched and basically crashes my application. All I can do is logging but that's all. What is the idea behind this design? Is there a workaround?

Thank you
Andy

QuantumCache 20,271 Reputation points

2022-09-02T05:33:38.083+00:00

Hello @Andirly thanks for reaching out on this forum, do you have your sample code on Github? so that we can have a look and help you in a better way/

Andirly 16

Thank you for your reply. I'll try to post the relevant passages of my code. Let me know if you need more info.

 // BAD: async void -> how to handle errors?  
        private async void OnConnectionStatusChange(ConnectionStatus status, ConnectionStatusChangeReason reason)  
        {  
            // Async void FIXME:: Handle errors within this method!  
            if (status == ConnectionStatus.Connected && reason == ConnectionStatusChangeReason.Connection_Ok)  
            {  
                // Enforce reading device twin to fetch desired properties  
                await RetrieveDeviceTwin();  
            }  
        }  
  

        public async Task RetrieveDeviceTwin()  
        {  
            Twin deviceTwin = await _deviceClient.GetTwinAsync();  
  
            TwinCollection desiredProperties = deviceTwin.Properties.Desired;  
            long serverWritablePropertiesVersion = desiredProperties.Version;  
  
            await OnDesiredPropertyChanged(desiredProperties, null);  
        }  
  
        private async Task OnDesiredPropertyChanged(TwinCollection desiredProperties, object userContext)  
        {  

           if (_onUpdatePropertiesCallback != null)  
            {  
                TwinCollection reportedProperties = _onUpdatePropertiesCallback(desiredProperties);  
                await _deviceClient.UpdateReportedPropertiesAsync(reportedProperties);  
            }  
        }

How can I implement proper error handling here? Is there another option to update device properties?

(Could not reply to your answer directly)

Andirly 16 Reputation points

2022-09-02T09:10:07.107+00:00

Sorry, for some reason I cannot reply here, so I wrote an answer.
QuantumCache 20,271 Reputation points

2022-09-03T01:28:26.247+00:00

Now worries we are looking into this.

1 answer

David R. Williamson [MSFT] 26 Reputation points Microsoft Employee

2022-09-06T15:48:16.237+00:00

Even though the signature returns void, the method can still be declared as 'async'. Note that 'async' is not part of a method definition.

Although async void methods are generally discouraged, they are common with callbacks where the caller (in this case, the SDK) does not await the call.
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1 person found this answer helpful.
Andirly 16 Reputation points

2022-09-15T10:48:40.913+00:00

Ok thank you! So it is propably advisable to catch any error (and at least log int) within the delegate because it can't be propagated?
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